postfix-3.4.29-smuggling-patch

2025-08-29 13:18:12 +00:00 · 2024-01-22 00:00:00 -05:00 · 2024-01-22 00:00:00 -05:00 · 484594a81c
commit 484594a81c
parent 4c7a731f08
21 changed files with 895 additions and 9 deletions
--- a/postfix/HISTORY
+++ b/postfix/HISTORY
@ -24915,3 +24915,37 @@ Apologies for any names omitted.
 	return "not found" instead of "error" during the time that
 	all MySQL server connections were turned down after error.
 	Found during code maintenance. File: global/dict_mysql.c.
+
+20240109
+
+	Security (outbound SMTP smuggling): with the default setting
+	"cleanup_replace_stray_cr_lf = yes" Postfix will replace
+	stray <CR> or <LF> characters in message content with a
+	space character. This prevents Postfix from enabling
+	outbound (remote) SMTP smuggling, and it also makes evaluation
+	of Postfix-added DKIM etc. signatures independent from how
+	a remote mail server handles stray <CR> or <LF> characters.
+	Files: global/mail_params.h, cleanup/cleanup.c,
+	cleanup/cleanup_message.c, mantools/postlink, proto/postconf.proto.
+
+20240112
+
+	Security (inbound SMTP smuggling): with "smtpd_forbid_bare_newline
+	= normalize" (default "no" for Postfix < 3.9), the Postfix
+	SMTP server requires the standard End-of-DATA sequence
+	<CR><LF>.<CR><LF>, and otherwise allows command or message
+	content lines ending in the non-standard <LF>, processing
+	them as if the client sent the standard <CR><LF>.
+
+	The alternative setting, "smtpd_forbid_bare_newline = reject"
+	will reject any command or message that contains a bare
+	<LF>, and is more likely to cause problems with legitimate
+	clients.
+
+	For backwards compatibility, local clients are excluded by
+	default with "smtpd_forbid_bare_newline_exclusions =
+	$mynetworks".
+
+	Files: mantools/postlink, proto/postconf.proto,
+	global/mail_params.h, global/smtp_stream.c, global/smtp_stream.h,
+	smtpd/smtpd.c, smtpd/smtpd_check.[hc].
--- a/postfix/RELEASE_NOTES
+++ b/postfix/RELEASE_NOTES
@ -16,6 +16,104 @@ specifies the release date of a stable release or snapshot release.
 If you upgrade from Postfix 3.2 or earlier, read RELEASE_NOTES-3.3
 before proceeding.

+Incompatibility with Postfix 3.8.5, 3.7.10, 3.6.14, and 3.5.24
+==============================================================
+
+Improvements for outbound SMTP smuggling defense:
+
+- With "cleanup_replace_stray_cr_lf = yes" (the default), the cleanup
+  daemon replaces each stray <CR> or <LF> character in message
+  content with a space character. The replacement happens before
+  any other content management (header/body_checks, Milters, etc).
+
+  This prevents outbound SMTP smuggling, where an attacker uses
+  Postfix to send email containing a non-standard End-of-DATA
+  sequence, to exploit inbound SMTP smuggling at a vulnerable remote
+  SMTP server.
+
+  This also improves the remote evaluation of Postfix-added DKIM
+  and other signatures, as the evaluation result will not depend
+  on how a remote email server handles stray <CR> or <LF> characters.
+
+This feature applies to all email that Postfix locally or remotely
+sends out. It is not allowlisted based on client identity.
+
+Major changes with Postfix 3.8.5, 3.7.10, 3.6.14, and 3.5.24
+============================================================
+
+Improvements for inbound SMTP smuggling defense:
+
+- Better compatibility: the recommended setting "smtpd_forbid_bare_newline
+  = normalize" requires the standard End-of-DATA sequence
+  <CR><LF>.<CR><LF>, but allows bare newlines from SMTP clients,
+  maintaining more compatibility with existing infrastructure.
+
+- Improved logging for rejected input (it now includes queue ID,
+  helo, mail, and rcpt, if available).
+
+- The setting "smtpd_forbid_bare_newline = reject" requires
+  that input lines end in <CR><LF>, requires the standard End-of-DATA
+  sequence <CR><LF>.<CR><LF>, and rejects a command or message that
+  contains a bare newline. To disconnect the client, specify
+  "smtpd_forbid_bare_newline_reject_code = 521".
+
+- The Postfix SMTP server no longer strips extra <CR> as in
+  <CR><LF>.<CR><CR><LF>, to silence false alarms from test tools
+  that send attack sequences that real mail servers cannot send.
+  Details at https://www.postfix.org/false-smuggling-claims.html
+
+- The old setting "yes" has become an alias for "normalize".
+
+- The old setting "no" has not changed, and allows SMTP smuggling.
+
+The recommended settings are now:
+
+    # Require the standard End-of-DATA sequence <CR><LF>.<CR><LF>.
+    # Otherwise, allow bare <LF> and process it as if the client sent
+    # <CR><LF>.
+    #
+    # This maintains compatibility with many legitimate SMTP client
+    # applications that send a mix of standard and non-standard line
+    # endings, but will fail to receive email from client implementations
+    # that do not terminate DATA content with the standard End-of-DATA
+    # sequence <CR><LF>.<CR><LF>.
+    #
+    # Such clients can be allowlisted with smtpd_forbid_bare_newline_exclusions.
+    # The example below allowlists SMTP clients in trusted networks.
+    #
+    smtpd_forbid_bare_newline = normalize
+    smtpd_forbid_bare_newline_exclusions = $mynetworks
+
+Alternative settings:
+
+    # Reject input lines that contain <LF> and log a "bare <LF> received"
+    # error. Require that input lines end in <CR><LF>, and require the
+    # standard End-of-DATA sequence <CR><LF>.<CR><LF>.
+    #
+    # This will reject email from SMTP clients that send any non-standard
+    # line endings such as web applications, netcat, or load balancer
+    # health checks.
+    #
+    # This will also reject email from services that use BDAT to send
+    # MIME text containing a bare newline (RFC 3030 Section 3 requires
+    # canonical MIME format for text message types, defined in RFC 2045
+    # Sections 2.7 and 2.8).
+    #
+    # Such clients can be allowlisted with smtpd_forbid_bare_newline_exclusions.
+    # The example below allowlists SMTP clients in trusted networks.
+    #
+    smtpd_forbid_bare_newline = reject
+    smtpd_forbid_bare_newline_exclusions = $mynetworks
+    #
+    # Alternatively, in the case of BDAT violations, BDAT can be selectively
+    # disabled with smtpd_discard_ehlo_keyword_address_maps, or globally
+    # disabled with smtpd_discard_ehlo_keywords.
+    #
+    # smtpd_discard_ehlo_keyword_address_maps = cidr:/path/to/file
+    # /path/to/file:
+    #     10.0.0.0/24 chunking, silent-discard
+    # smtpd_discard_ehlo_keywords = chunking, silent-discard
+
 Runtime detection of DNSSEC support
 -----------------------------------

--- a/postfix/html/cleanup.8.html
+++ b/postfix/html/cleanup.8.html
@ -146,6 +146,16 @@ CLEANUP(8)                                                          CLEANUP(8)
              The set of characters that Postfix will remove from message con-
              tent.

+       Available in Postfix version 3.9, 3.8.5, 3.7.10,  3.6.14,  3.5.24,  and
+       later:
+
+       <b><a href="postconf.5.html#cleanup_replace_stray_cr_lf">cleanup_replace_stray_cr_lf</a> (yes)</b>
+              Replace  each  stray  &lt;CR&gt;  or &lt;LF&gt; character in message content
+              with a space character, to prevent outbound SMTP smuggling,  and
+              to make the evaluation of Postfix-added DKIM or other signatures
+              independent from how a remote mail server handles  such  charac-
+              ters.
+
 <b>BEFORE QUEUE MILTER CONTROLS</b>
       As of version 2.3, Postfix supports the Sendmail version 8 Milter (mail
       filter) protocol. When mail is not received via  the  <a href="smtpd.8.html">smtpd(8)</a>  server,
--- a/postfix/html/postconf.5.html
+++ b/postfix/html/postconf.5.html
@ -1442,6 +1442,40 @@ Examples:
 </pre>


+</DD>
+
+<DT><b><a name="cleanup_replace_stray_cr_lf">cleanup_replace_stray_cr_lf</a>
+(default: yes)</b></DT><DD>
+
+<p> Replace each stray &lt;CR&gt; or &lt;LF&gt; character in message
+content with a space character, to prevent outbound SMTP smuggling,
+and to make the evaluation of Postfix-added DKIM or other signatures
+independent from how a remote mail server handles such characters.
+</p>
+
+<p> SMTP does not allow such characters unless they are part of a
+&lt;CR&gt;&lt;LF&gt; sequence, and different mail systems handle
+such stray characters in an implementation-dependent manner. Stray
+&lt;CR&gt; or &lt;LF&gt; characters could be used for outbound
+SMTP smuggling, where an attacker uses a Postfix server to send
+message content with a non-standard End-of-DATA sequence that
+triggers inbound SMTP smuggling at a remote SMTP server.</p>
+
+<p> The replacement happens before all other content management,
+and before Postfix may add a DKIM etc. signature; if the signature
+were created first, the replacement could invalidate the signature.
+</p>
+
+<p> In addition to preventing SMTP smuggling, replacing stray
+&lt;CR&gt; or &lt;LF&gt; characters ensures that the result of
+signature validation by later mail system will not depend on how
+that mail system handles those stray characters in an
+implementation-dependent manner. </p>
+
+<p> This feature is available in Postfix &ge; 3.9, 3.8.5, 3.7.10,
+3.6.14, and 3.5.24. </p>
+
+
 </DD>

 <DT><b><a name="cleanup_service_name">cleanup_service_name</a>
@ -14982,6 +15016,153 @@ This feature is available in Postfix 2.0 and later.
 </p>


+</DD>
+
+<DT><b><a name="smtpd_forbid_bare_newline">smtpd_forbid_bare_newline</a>
+(default: Postfix &lt; 3.9: no)</b></DT><DD>
+
+<p> Reject or restrict input lines from an SMTP client that end in
+&lt;LF&gt; instead of the standard &lt;CR&gt;&lt;LF&gt;. Such line
+endings are commonly allowed with UNIX-based SMTP servers, but they
+violate <a href="http://tools.ietf.org/html/rfc5321">RFC 5321</a>, and allowing such line endings can make a server
+vulnerable to <a href="https://www.postfix.org/smtp-smuggling.html">
+SMTP smuggling</a>.  </p>
+
+<p> Specify one of the following values (case does not matter): </p>
+
+<dl compact>
+
+<dt> <b>normalize</b></dt> <dd> Require the standard
+End-of-DATA sequence &lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt;.
+Otherwise, allow command or message content lines ending in the
+non-standard &lt;LF&gt;, and process them as if the client sent the
+standard &lt;CR&gt;&lt;LF&gt;. <br> <br> This maintains compatibility
+with many legitimate SMTP client applications that send a mix of
+standard and non-standard line endings, but will fail to receive
+email from client implementations that do not terminate DATA content
+with the standard End-of-DATA sequence
+&lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt;. <br> <br> Such clients
+can be excluded with <a href="postconf.5.html#smtpd_forbid_bare_newline_exclusions">smtpd_forbid_bare_newline_exclusions</a>. </dd>
+
+<dt> <b>yes</b> </dt> <dd> Compatibility alias for <b>normalize</b>. </dd>
+
+<dt> <b>reject</b> </dt> <dd> Require the standard End-of-DATA
+sequence &lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt;. Reject a command
+or message content when a line contains bare &lt;LF&gt;, log a "bare
+&lt;LF&gt; received" error, and reply with the SMTP status code in
+$<a href="postconf.5.html#smtpd_forbid_bare_newline_reject_code">smtpd_forbid_bare_newline_reject_code</a>. <br> <br> This will reject
+email from SMTP clients that send any non-standard line endings
+such as web applications, netcat, or load balancer health checks.
+<br> <br> This will also reject email from services that use BDAT
+to send MIME text containing a bare newline (<a href="http://tools.ietf.org/html/rfc3030">RFC 3030</a> Section 3
+requires canonical MIME format for text message types, defined in
+<a href="http://tools.ietf.org/html/rfc2045">RFC 2045</a> Sections 2.7 and 2.8). <br> <br> Such clients can be
+excluded with <a href="postconf.5.html#smtpd_forbid_bare_newline_exclusions">smtpd_forbid_bare_newline_exclusions</a> (or, in the case
+of BDAT violations, BDAT can be selectively disabled with
+<a href="postconf.5.html#smtpd_discard_ehlo_keyword_address_maps">smtpd_discard_ehlo_keyword_address_maps</a>, or globally disabled with
+<a href="postconf.5.html#smtpd_discard_ehlo_keywords">smtpd_discard_ehlo_keywords</a>). </dd>
+
+<dt> <b>no</b> (default)</dt> <dd> Do not require the standard
+End-of-DATA
+sequence &lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt;. Always process
+a bare &lt;LF&gt; as if the client sent &lt;CR&gt;&lt;LF&gt;. This
+option is fully backwards compatible, but is not recommended for
+an Internet-facing SMTP server, because it is vulnerable to <a
+href="https://www.postfix.org/smtp-smuggling.html"> SMTP smuggling</a>.
+</dd>
+
+</dl>
+
+<p> Recommended settings: </p>
+
+<blockquote>
+<pre>
+# Require the standard End-of-DATA sequence &lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt;.
+# Otherwise, allow bare &lt;LF&gt; and process it as if the client sent
+# &lt;CR&gt;&lt;LF&gt;.
+#
+# This maintains compatibility with many legitimate SMTP client
+# applications that send a mix of standard and non-standard line
+# endings, but will fail to receive email from client implementations
+# that do not terminate DATA content with the standard End-of-DATA
+# sequence &lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt;.
+#
+# Such clients can be allowlisted with <a href="postconf.5.html#smtpd_forbid_bare_newline_exclusions">smtpd_forbid_bare_newline_exclusions</a>.
+# The example below allowlists SMTP clients in trusted networks.
+#
+<a href="postconf.5.html#smtpd_forbid_bare_newline">smtpd_forbid_bare_newline</a> = normalize
+<a href="postconf.5.html#smtpd_forbid_bare_newline_exclusions">smtpd_forbid_bare_newline_exclusions</a> = $<a href="postconf.5.html#mynetworks">mynetworks</a>
+</pre>
+</blockquote>
+
+<p> Alternative: </p>
+
+<blockquote>
+<pre>
+# Reject input lines that contain &lt;LF&gt; and log a "bare &lt;LF&gt; received"
+# error. Require that input lines end in &lt;CR&gt;&lt;LF&gt;, and require the
+# standard End-of-DATA sequence &lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt;.
+#
+# This will reject email from SMTP clients that send any non-standard
+# line endings such as web applications, netcat, or load balancer
+# health checks.
+#
+# This will also reject email from services that use BDAT to send
+# MIME text containing a bare newline (<a href="http://tools.ietf.org/html/rfc3030">RFC 3030</a> Section 3 requires
+# canonical MIME format for text message types, defined in <a href="http://tools.ietf.org/html/rfc2045">RFC 2045</a>
+# Sections 2.7 and 2.8).
+#
+# Such clients can be allowlisted with <a href="postconf.5.html#smtpd_forbid_bare_newline_exclusions">smtpd_forbid_bare_newline_exclusions</a>.
+# The example below allowlists SMTP clients in trusted networks.
+#
+<a href="postconf.5.html#smtpd_forbid_bare_newline">smtpd_forbid_bare_newline</a> = reject
+<a href="postconf.5.html#smtpd_forbid_bare_newline_exclusions">smtpd_forbid_bare_newline_exclusions</a> = $<a href="postconf.5.html#mynetworks">mynetworks</a>
+#
+# Alternatively, in the case of BDAT violations, BDAT can be selectively
+# disabled with <a href="postconf.5.html#smtpd_discard_ehlo_keyword_address_maps">smtpd_discard_ehlo_keyword_address_maps</a>, or globally
+# disabled with <a href="postconf.5.html#smtpd_discard_ehlo_keywords">smtpd_discard_ehlo_keywords</a>.
+#
+# <a href="postconf.5.html#smtpd_discard_ehlo_keyword_address_maps">smtpd_discard_ehlo_keyword_address_maps</a> = <a href="cidr_table.5.html">cidr</a>:/path/to/file
+# /path/to/file:
+#     10.0.0.0/24 chunking, silent-discard
+# <a href="postconf.5.html#smtpd_discard_ehlo_keywords">smtpd_discard_ehlo_keywords</a> = chunking, silent-discard
+</pre>
+</blockquote>
+
+<p> This feature with settings <b>yes</b> and <b>no</b> is available
+in Postfix 3.8.4, 3.7.9, 3.6.13, and 3.5.23. Additionally, the
+settings <b>reject</b>, and <b>normalize</b> are available with
+Postfix &ge; 3.9, 3.8.5, 3.7.10, 3.6.14, and 3.5.24. </p>
+
+
+</DD>
+
+<DT><b><a name="smtpd_forbid_bare_newline_exclusions">smtpd_forbid_bare_newline_exclusions</a>
+(default: $<a href="postconf.5.html#mynetworks">mynetworks</a>)</b></DT><DD>
+
+<p> Exclude the specified clients from <a href="postconf.5.html#smtpd_forbid_bare_newline">smtpd_forbid_bare_newline</a>
+enforcement. This setting uses the same syntax and parent-domain
+matching behavior as <a href="postconf.5.html#mynetworks">mynetworks</a>. </p>
+
+<p> This feature is available in Postfix &ge; 3.9, 3.8.4, 3.7.9,
+3.6.13, and 3.5.23. </p>
+
+
+</DD>
+
+<DT><b><a name="smtpd_forbid_bare_newline_reject_code">smtpd_forbid_bare_newline_reject_code</a>
+(default: 550)</b></DT><DD>
+
+<p>
+The numerical Postfix SMTP server response code when rejecting a
+request with "<a href="postconf.5.html#smtpd_forbid_bare_newline">smtpd_forbid_bare_newline</a> = reject".
+Specify a 5XX status code (521 to disconnect).
+</p>
+
+<p> This feature is available in Postfix &ge; 3.9, 3.8.5, 3.7.10,
+3.6.14, and 3.5.24. </p>
+
+
 </DD>

 <DT><b><a name="smtpd_forbidden_commands">smtpd_forbidden_commands</a>
--- a/postfix/html/smtpd.8.html
+++ b/postfix/html/smtpd.8.html
@ -895,6 +895,22 @@ SMTPD(8)                                                              SMTPD(8)
              to  send to this service per time unit, regardless of whether or
              not Postfix actually accepts those commands.

+       Available in Postfix 3.9, 3.8.4, 3.7.9, 3.6.13, 3.5.23 and later:
+
+       <b><a href="postconf.5.html#smtpd_forbid_bare_newline">smtpd_forbid_bare_newline</a> (Postfix</b> &lt; <b>3.9: no)</b>
+              Reject or restrict input lines from an SMTP client that  end  in
+              &lt;LF&gt; instead of the standard &lt;CR&gt;&lt;LF&gt;.
+
+       <b><a href="postconf.5.html#smtpd_forbid_bare_newline_exclusions">smtpd_forbid_bare_newline_exclusions</a> ($<a href="postconf.5.html#mynetworks">mynetworks</a>)</b>
+              Exclude  the  specified  clients  from <a href="postconf.5.html#smtpd_forbid_bare_newline">smtpd_forbid_bare_newline</a>
+              enforcement.
+
+       Available in Postfix 3.9, 3.8.5, 3.7.10, 3.6.14, 3.5.24 and later:
+
+       <b><a href="postconf.5.html#smtpd_forbid_bare_newline_reject_code">smtpd_forbid_bare_newline_reject_code</a> (550)</b>
+              The numerical Postfix SMTP server response code when rejecting a
+              request with "<a href="postconf.5.html#smtpd_forbid_bare_newline">smtpd_forbid_bare_newline</a> = reject".
+
 <b>TARPIT CONTROLS</b>
       When a remote SMTP client makes errors, the  Postfix  SMTP  server  can
       insert  delays  before  responding. This can help to slow down run-away
--- a/postfix/man/man5/postconf.5
+++ b/postfix/man/man5/postconf.5
@ -845,6 +845,32 @@ canonical_maps = hash:/etc/postfix/canonical
 .fi
 .ad
 .ft R
+.SH cleanup_replace_stray_cr_lf (default: yes)
+Replace each stray <CR> or <LF> character in message
+content with a space character, to prevent outbound SMTP smuggling,
+and to make the evaluation of Postfix\-added DKIM or other signatures
+independent from how a remote mail server handles such characters.
+.PP
+SMTP does not allow such characters unless they are part of a
+<CR><LF> sequence, and different mail systems handle
+such stray characters in an implementation\-dependent manner. Stray
+<CR> or <LF> characters could be used for outbound
+SMTP smuggling, where an attacker uses a Postfix server to send
+message content with a non\-standard End\-of\-DATA sequence that
+triggers inbound SMTP smuggling at a remote SMTP server.
+.PP
+The replacement happens before all other content management,
+and before Postfix may add a DKIM etc. signature; if the signature
+were created first, the replacement could invalidate the signature.
+.PP
+In addition to preventing SMTP smuggling, replacing stray
+<CR> or <LF> characters ensures that the result of
+signature validation by later mail system will not depend on how
+that mail system handles those stray characters in an
+implementation\-dependent manner.
+.PP
+This feature is available in Postfix >= 3.9, 3.8.5, 3.7.10,
+3.6.14, and 3.5.24.
 .SH cleanup_service_name (default: cleanup)
 The name of the \fBcleanup\fR(8) service. This service rewrites addresses
 into the standard form, and performs \fBcanonical\fR(5) address mapping
@ -10140,6 +10166,154 @@ The smtpd_expansion_filter value is not subject to Postfix configuration
 parameter $name expansion.
 .PP
 This feature is available in Postfix 2.0 and later.
+.SH smtpd_forbid_bare_newline (default: Postfix < 3.9: no)
+Reject or restrict input lines from an SMTP client that end in
+<LF> instead of the standard <CR><LF>. Such line
+endings are commonly allowed with UNIX\-based SMTP servers, but they
+violate RFC 5321, and allowing such line endings can make a server
+vulnerable to
+SMTP smuggling.
+.PP
+Specify one of the following values (case does not matter):
+.IP "\fBnormalize\fR"
+Require the standard
+End\-of\-DATA sequence <CR><LF>.<CR><LF>.
+Otherwise, allow command or message content lines ending in the
+non\-standard <LF>, and process them as if the client sent the
+standard <CR><LF>.
+.br
+.br
+This maintains compatibility
+with many legitimate SMTP client applications that send a mix of
+standard and non\-standard line endings, but will fail to receive
+email from client implementations that do not terminate DATA content
+with the standard End\-of\-DATA sequence
+<CR><LF>.<CR><LF>.
+.br
+.br
+Such clients
+can be excluded with smtpd_forbid_bare_newline_exclusions.
+.br
+.IP "\fByes\fR"
+Compatibility alias for \fBnormalize\fR.
+.br
+.IP "\fBreject\fR"
+Require the standard End\-of\-DATA
+sequence <CR><LF>.<CR><LF>. Reject a command
+or message content when a line contains bare <LF>, log a "bare
+<LF> received" error, and reply with the SMTP status code in
+$smtpd_forbid_bare_newline_reject_code.
+.br
+.br
+This will reject
+email from SMTP clients that send any non\-standard line endings
+such as web applications, netcat, or load balancer health checks.
+.br
+.br
+This will also reject email from services that use BDAT
+to send MIME text containing a bare newline (RFC 3030 Section 3
+requires canonical MIME format for text message types, defined in
+RFC 2045 Sections 2.7 and 2.8).
+.br
+.br
+Such clients can be
+excluded with smtpd_forbid_bare_newline_exclusions (or, in the case
+of BDAT violations, BDAT can be selectively disabled with
+smtpd_discard_ehlo_keyword_address_maps, or globally disabled with
+smtpd_discard_ehlo_keywords).
+.br
+.IP "\fBno\fR (default)"
+Do not require the standard
+End\-of\-DATA
+sequence <CR><LF>.<CR><LF>. Always process
+a bare <LF> as if the client sent <CR><LF>. This
+option is fully backwards compatible, but is not recommended for
+an Internet\-facing SMTP server, because it is vulnerable to  SMTP smuggling.
+.br
+.br
+.PP
+Recommended settings:
+.sp
+.in +4
+.nf
+.na
+.ft C
+# Require the standard End\-of\-DATA sequence <CR><LF>.<CR><LF>.
+# Otherwise, allow bare <LF> and process it as if the client sent
+# <CR><LF>.
+#
+# This maintains compatibility with many legitimate SMTP client
+# applications that send a mix of standard and non\-standard line
+# endings, but will fail to receive email from client implementations
+# that do not terminate DATA content with the standard End\-of\-DATA
+# sequence <CR><LF>.<CR><LF>.
+#
+# Such clients can be allowlisted with smtpd_forbid_bare_newline_exclusions.
+# The example below allowlists SMTP clients in trusted networks.
+#
+smtpd_forbid_bare_newline = normalize
+smtpd_forbid_bare_newline_exclusions = $mynetworks
+.fi
+.ad
+.ft R
+.in -4
+.PP
+Alternative:
+.sp
+.in +4
+.nf
+.na
+.ft C
+# Reject input lines that contain <LF> and log a "bare <LF> received"
+# error. Require that input lines end in <CR><LF>, and require the
+# standard End\-of\-DATA sequence <CR><LF>.<CR><LF>.
+#
+# This will reject email from SMTP clients that send any non\-standard
+# line endings such as web applications, netcat, or load balancer
+# health checks.
+#
+# This will also reject email from services that use BDAT to send
+# MIME text containing a bare newline (RFC 3030 Section 3 requires
+# canonical MIME format for text message types, defined in RFC 2045
+# Sections 2.7 and 2.8).
+#
+# Such clients can be allowlisted with smtpd_forbid_bare_newline_exclusions.
+# The example below allowlists SMTP clients in trusted networks.
+#
+smtpd_forbid_bare_newline = reject
+smtpd_forbid_bare_newline_exclusions = $mynetworks
+#
+# Alternatively, in the case of BDAT violations, BDAT can be selectively
+# disabled with smtpd_discard_ehlo_keyword_address_maps, or globally
+# disabled with smtpd_discard_ehlo_keywords.
+#
+# smtpd_discard_ehlo_keyword_address_maps = cidr:/path/to/file
+# /path/to/file:
+#     10.0.0.0/24 chunking, silent\-discard
+# smtpd_discard_ehlo_keywords = chunking, silent\-discard
+.fi
+.ad
+.ft R
+.in -4
+.PP
+This feature with settings \fByes\fR and \fBno\fR is available
+in Postfix 3.8.4, 3.7.9, 3.6.13, and 3.5.23. Additionally, the
+settings \fBreject\fR, and \fBnormalize\fR are available with
+Postfix >= 3.9, 3.8.5, 3.7.10, 3.6.14, and 3.5.24.
+.SH smtpd_forbid_bare_newline_exclusions (default: $mynetworks)
+Exclude the specified clients from smtpd_forbid_bare_newline
+enforcement. This setting uses the same syntax and parent\-domain
+matching behavior as mynetworks.
+.PP
+This feature is available in Postfix >= 3.9, 3.8.4, 3.7.9,
+3.6.13, and 3.5.23.
+.SH smtpd_forbid_bare_newline_reject_code (default: 550)
+The numerical Postfix SMTP server response code when rejecting a
+request with "smtpd_forbid_bare_newline = reject".
+Specify a 5XX status code (521 to disconnect).
+.PP
+This feature is available in Postfix >= 3.9, 3.8.5, 3.7.10,
+3.6.14, and 3.5.24.
 .SH smtpd_forbidden_commands (default: CONNECT, GET, POST)
 List of commands that cause the Postfix SMTP server to immediately
 terminate the session with a 221 code. This can be used to disconnect
--- a/postfix/man/man8/cleanup.8
+++ b/postfix/man/man8/cleanup.8
@ -146,6 +146,14 @@ content.
 .IP "\fBmessage_strip_characters (empty)\fR"
 The set of characters that Postfix will remove from message
 content.
+.PP
+Available in Postfix version 3.9, 3.8.5, 3.7.10, 3.6.14,
+3.5.24, and later:
+.IP "\fBcleanup_replace_stray_cr_lf (yes)\fR"
+Replace each stray <CR> or <LF> character in message
+content with a space character, to prevent outbound SMTP smuggling,
+and to make the evaluation of Postfix\-added DKIM or other signatures
+independent from how a remote mail server handles such characters.
 .SH "BEFORE QUEUE MILTER CONTROLS"
 .na
 .nf
--- a/postfix/man/man8/smtpd.8
+++ b/postfix/man/man8/smtpd.8
@ -791,6 +791,20 @@ Available in Postfix version 3.1 and later:
 The maximal number of AUTH commands that any client is allowed to
 send to this service per time unit, regardless of whether or not
 Postfix actually accepts those commands.
+.PP
+Available in Postfix 3.9, 3.8.4, 3.7.9, 3.6.13, 3.5.23 and later:
+.IP "\fBsmtpd_forbid_bare_newline (Postfix < 3.9: no)\fR"
+Reject or restrict input lines from an SMTP client that end in
+<LF> instead of the standard <CR><LF>.
+.IP "\fBsmtpd_forbid_bare_newline_exclusions ($mynetworks)\fR"
+Exclude the specified clients from smtpd_forbid_bare_newline
+enforcement.
+.PP
+Available in Postfix 3.9, 3.8.5, 3.7.10, 3.6.14, 3.5.24 and
+later:
+.IP "\fBsmtpd_forbid_bare_newline_reject_code (550)\fR"
+The numerical Postfix SMTP server response code when rejecting a
+request with "smtpd_forbid_bare_newline = reject".
 .SH "TARPIT CONTROLS"
 .na
 .nf
--- a/postfix/mantools/postlink
+++ b/postfix/mantools/postlink
@ -548,6 +548,10 @@ while (<>) {
    s;\bsmtpd_etrn_restrictions\b;<a href="postconf.5.html#smtpd_etrn_restrictions">$&</a>;g;
    s;\bsmtpd_expansion_filter\b;<a href="postconf.5.html#smtpd_expansion_filter">$&</a>;g;
    s;\bsmtpd_for[-</bB>]*\n*[ <bB>]*bidden_commands\b;<a href="postconf.5.html#smtpd_forbidden_commands">$&</a>;g;
+    s;\bsmtpd_for[-</bB>]*\n*[ <bB>]*bid_bare_new[-</bB>]*\n*[ <bB>]*line\b;<a href="postconf.5.html#smtpd_forbid_bare_newline">$&</a>;g;
+    s;\bsmtpd_for[-</bB>]*\n*[ <bB>]*bid_bare_new[-</bB>]*\n*[ <bB>]*line_reject_code\b;<a href="postconf.5.html#smtpd_forbid_bare_newline_reject_code">$&</a>;g;
+    s;\bsmtpd_for[-</bB>]*\n*[ <bB>]*bid_bare_new[-</bB>]*\n*[ <bB>]*line_exclusions\b;<a href="postconf.5.html#smtpd_forbid_bare_newline_exclusions">$&</a>;g;
+    s;\bcleanup_replace_stray_cr_lf\b;<a href="postconf.5.html#cleanup_replace_stray_cr_lf">$&</a>;g;
    s;\bsmtpd_hard_error_limit\b;<a href="postconf.5.html#smtpd_hard_error_limit">$&</a>;g;
    s;\bsmtpd_helo_required\b;<a href="postconf.5.html#smtpd_helo_required">$&</a>;g;
    s;\bsmtpd_helo_restrictions\b;<a href="postconf.5.html#smtpd_helo_restrictions">$&</a>;g;
--- a/postfix/proto/postconf.proto
+++ b/postfix/proto/postconf.proto
@ -17681,3 +17681,169 @@ value to disable the feature. </p>

 <p> This feature was backported from Postfix 3.6 to Postfix versions
 3.5.9, 3.4.19, 3.3.16. 3.2.21. </p>
+
+%PARAM smtpd_forbid_bare_newline Postfix &lt; 3.9: no
+
+<p> Reject or restrict input lines from an SMTP client that end in
+&lt;LF&gt; instead of the standard &lt;CR&gt;&lt;LF&gt;. Such line
+endings are commonly allowed with UNIX-based SMTP servers, but they
+violate RFC 5321, and allowing such line endings can make a server
+vulnerable to <a href="https://www.postfix.org/smtp-smuggling.html">
+SMTP smuggling</a>.  </p>
+
+<p> Specify one of the following values (case does not matter): </p>
+
+<dl compact>
+
+<dt> <b>normalize</b></dt> <dd> Require the standard
+End-of-DATA sequence &lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt;.
+Otherwise, allow command or message content lines ending in the
+non-standard &lt;LF&gt;, and process them as if the client sent the
+standard &lt;CR&gt;&lt;LF&gt;. <br> <br> This maintains compatibility
+with many legitimate SMTP client applications that send a mix of
+standard and non-standard line endings, but will fail to receive
+email from client implementations that do not terminate DATA content
+with the standard End-of-DATA sequence
+&lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt;. <br> <br> Such clients
+can be excluded with smtpd_forbid_bare_newline_exclusions. </dd>
+
+<dt> <b>yes</b> </dt> <dd> Compatibility alias for <b>normalize</b>. </dd>
+
+<dt> <b>reject</b> </dt> <dd> Require the standard End-of-DATA
+sequence &lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt;. Reject a command
+or message content when a line contains bare &lt;LF&gt;, log a "bare
+&lt;LF&gt; received" error, and reply with the SMTP status code in
+$smtpd_forbid_bare_newline_reject_code. <br> <br> This will reject
+email from SMTP clients that send any non-standard line endings
+such as web applications, netcat, or load balancer health checks.
+<br> <br> This will also reject email from services that use BDAT
+to send MIME text containing a bare newline (RFC 3030 Section 3
+requires canonical MIME format for text message types, defined in
+RFC 2045 Sections 2.7 and 2.8). <br> <br> Such clients can be
+excluded with smtpd_forbid_bare_newline_exclusions (or, in the case
+of BDAT violations, BDAT can be selectively disabled with
+smtpd_discard_ehlo_keyword_address_maps, or globally disabled with
+smtpd_discard_ehlo_keywords). </dd>
+
+<dt> <b>no</b> (default)</dt> <dd> Do not require the standard
+End-of-DATA
+sequence &lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt;. Always process
+a bare &lt;LF&gt; as if the client sent &lt;CR&gt;&lt;LF&gt;. This
+option is fully backwards compatible, but is not recommended for
+an Internet-facing SMTP server, because it is vulnerable to <a
+href="https://www.postfix.org/smtp-smuggling.html"> SMTP smuggling</a>.
+</dd>
+
+</dl>
+
+<p> Recommended settings: </p>
+
+<blockquote>
+<pre>
+# Require the standard End-of-DATA sequence &lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt;.
+# Otherwise, allow bare &lt;LF&gt; and process it as if the client sent
+# &lt;CR&gt;&lt;LF&gt;.
+#
+# This maintains compatibility with many legitimate SMTP client
+# applications that send a mix of standard and non-standard line
+# endings, but will fail to receive email from client implementations
+# that do not terminate DATA content with the standard End-of-DATA
+# sequence &lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt;.
+#
+# Such clients can be allowlisted with smtpd_forbid_bare_newline_exclusions.
+# The example below allowlists SMTP clients in trusted networks.
+#
+smtpd_forbid_bare_newline = normalize
+smtpd_forbid_bare_newline_exclusions = $mynetworks
+</pre>
+</blockquote>
+
+<p> Alternative: </p>
+
+<blockquote>
+<pre>
+# Reject input lines that contain &lt;LF&gt; and log a "bare &lt;LF&gt; received"
+# error. Require that input lines end in &lt;CR&gt;&lt;LF&gt;, and require the
+# standard End-of-DATA sequence &lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt;.
+#
+# This will reject email from SMTP clients that send any non-standard
+# line endings such as web applications, netcat, or load balancer
+# health checks.
+#
+# This will also reject email from services that use BDAT to send
+# MIME text containing a bare newline (RFC 3030 Section 3 requires
+# canonical MIME format for text message types, defined in RFC 2045
+# Sections 2.7 and 2.8).
+#
+# Such clients can be allowlisted with smtpd_forbid_bare_newline_exclusions.
+# The example below allowlists SMTP clients in trusted networks.
+#
+smtpd_forbid_bare_newline = reject
+smtpd_forbid_bare_newline_exclusions = $mynetworks
+#
+# Alternatively, in the case of BDAT violations, BDAT can be selectively
+# disabled with smtpd_discard_ehlo_keyword_address_maps, or globally
+# disabled with smtpd_discard_ehlo_keywords.
+#
+# smtpd_discard_ehlo_keyword_address_maps = cidr:/path/to/file
+# /path/to/file:
+#     10.0.0.0/24 chunking, silent-discard
+# smtpd_discard_ehlo_keywords = chunking, silent-discard
+</pre>
+</blockquote>
+
+<p> This feature with settings <b>yes</b> and <b>no</b> is available
+in Postfix 3.8.4, 3.7.9, 3.6.13, and 3.5.23. Additionally, the
+settings <b>reject</b>, and <b>normalize</b> are available with
+Postfix &ge; 3.9, 3.8.5, 3.7.10, 3.6.14, and 3.5.24. </p>
+
+%PARAM smtpd_forbid_bare_newline_exclusions $mynetworks
+
+<p> Exclude the specified clients from smtpd_forbid_bare_newline
+enforcement. This setting uses the same syntax and parent-domain
+matching behavior as mynetworks. </p>
+
+<p> This feature is available in Postfix &ge; 3.9, 3.8.4, 3.7.9,
+3.6.13, and 3.5.23. </p>
+
+%PARAM smtpd_forbid_bare_newline_reject_code 550
+
+<p>
+The numerical Postfix SMTP server response code when rejecting a
+request with "smtpd_forbid_bare_newline = reject".
+Specify a 5XX status code (521 to disconnect).
+</p>
+
+<p> This feature is available in Postfix &ge; 3.9, 3.8.5, 3.7.10,
+3.6.14, and 3.5.24. </p>
+
+%PARAM cleanup_replace_stray_cr_lf yes
+
+<p> Replace each stray &lt;CR&gt; or &lt;LF&gt; character in message
+content with a space character, to prevent outbound SMTP smuggling,
+and to make the evaluation of Postfix-added DKIM or other signatures
+independent from how a remote mail server handles such characters.
+</p>
+
+<p> SMTP does not allow such characters unless they are part of a
+&lt;CR&gt;&lt;LF&gt; sequence, and different mail systems handle
+such stray characters in an implementation-dependent manner. Stray
+&lt;CR&gt; or &lt;LF&gt; characters could be used for outbound
+SMTP smuggling, where an attacker uses a Postfix server to send
+message content with a non-standard End-of-DATA sequence that
+triggers inbound SMTP smuggling at a remote SMTP server.</p>
+
+<p> The replacement happens before all other content management,
+and before Postfix may add a DKIM etc. signature; if the signature
+were created first, the replacement could invalidate the signature.
+</p>
+
+<p> In addition to preventing SMTP smuggling, replacing stray
+&lt;CR&gt; or &lt;LF&gt; characters ensures that the result of
+signature validation by later mail system will not depend on how
+that mail system handles those stray characters in an
+implementation-dependent manner. </p>
+
+<p> This feature is available in Postfix &ge; 3.9, 3.8.5, 3.7.10,
+3.6.14, and 3.5.24. </p>
+
--- a/postfix/src/cleanup/cleanup.c
+++ b/postfix/src/cleanup/cleanup.c
@ -128,6 +128,14 @@
 /* .IP "\fBmessage_strip_characters (empty)\fR"
 /*	The set of characters that Postfix will remove from message
 /*	content.
+/* .PP
+/*	Available in Postfix version 3.9, 3.8.5, 3.7.10, 3.6.14,
+/*	3.5.24, and later:
+/* .IP "\fBcleanup_replace_stray_cr_lf (yes)\fR"
+/*	Replace each stray <CR> or <LF> character in message
+/*	content with a space character, to prevent outbound SMTP smuggling,
+/*	and to make the evaluation of Postfix-added DKIM or other signatures
+/*	independent from how a remote mail server handles such characters.
 /* BEFORE QUEUE MILTER CONTROLS
 /* .ad
 /* .fi
--- a/postfix/src/cleanup/cleanup_init.c
+++ b/postfix/src/cleanup/cleanup_init.c
@ -173,6 +173,7 @@ int     var_auto_8bit_enc_hdr;		/* auto-detect 8bit encoding header */
 int     var_always_add_hdrs;		/* always add missing headers */
 int     var_virt_addrlen_limit;		/* stop exponential growth */
 char   *var_hfrom_format;		/* header_from_format */
+int     var_cleanup_mask_stray_cr_lf;	/* replace stray CR or LF with space */

 const CONFIG_INT_TABLE cleanup_int_table[] = {
    VAR_HOPCOUNT_LIMIT, DEF_HOPCOUNT_LIMIT, &var_hopcount_limit, 1, 0,
@ -189,6 +190,7 @@ const CONFIG_BOOL_TABLE cleanup_bool_table[] = {
    VAR_VERP_BOUNCE_OFF, DEF_VERP_BOUNCE_OFF, &var_verp_bounce_off,
    VAR_AUTO_8BIT_ENC_HDR, DEF_AUTO_8BIT_ENC_HDR, &var_auto_8bit_enc_hdr,
    VAR_ALWAYS_ADD_HDRS, DEF_ALWAYS_ADD_HDRS, &var_always_add_hdrs,
+    VAR_CLEANUP_MASK_STRAY_CR_LF, DEF_CLEANUP_MASK_STRAY_CR_LF, &var_cleanup_mask_stray_cr_lf,
    0,
 };

--- a/postfix/src/cleanup/cleanup_message.c
+++ b/postfix/src/cleanup/cleanup_message.c
@ -926,6 +926,23 @@ static void cleanup_message_headerbody(CLEANUP_STATE *state, int type,
    const char *cp;
    char   *dst;

+    /*
+     * Replace each stray CR or LF with one space. These are not allowed in
+     * SMTP, and can be used to enable outbound (remote) SMTP smuggling.
+     * Replacing these early ensures that our later DKIM etc. signature will
+     * not be invalidated. Besides preventing SMTP smuggling, replacing stray
+     * <CR> or <LF> ensures that the result of signature validation by a
+     * later mail system will not depend on how that mail system handles
+     * those stray characters in an implementation-dependent manner.
+     * 
+     * The input length is not changed, therefore it is safe to overwrite the
+     * input.
+     */
+    if (var_cleanup_mask_stray_cr_lf)
+	for (dst = (char *) buf; dst < buf + len; dst++)
+	    if (*dst == '\r' || *dst == '\n')
+		*dst = ' ';
+
    /*
     * Reject unwanted characters.
     * 
--- a/postfix/src/global/cleanup_strerror.c
+++ b/postfix/src/global/cleanup_strerror.c
@ -67,6 +67,7 @@ static const CLEANUP_STAT_DETAIL cleanup_stat_map[] = {
    CLEANUP_STAT_SIZE, 552, "5.3.4", "message file too big",
    CLEANUP_STAT_CONT, 550, "5.7.1", "message content rejected",
    CLEANUP_STAT_WRITE, 451, "4.3.0", "queue file write error",
+    CLEANUP_STAT_BARE_LF, 521, "5.5.2", "bare <LF> received",
 };

 static CLEANUP_STAT_DETAIL cleanup_stat_success = {
--- a/postfix/src/global/cleanup_user.h
+++ b/postfix/src/global/cleanup_user.h
@ -63,6 +63,12 @@
 #define CLEANUP_STAT_PROXY	(1<<7)	/* Proxy reject */
 #define CLEANUP_STAT_DEFER	(1<<8)	/* Temporary reject */

+ /*
+  * Non-cleanup errors that live in the same bitmask space, to centralize
+  * error handling.
+  */
+#define CLEANUP_STAT_BARE_LF   (1<<16)	/* Bare <LF> received */
+
 /*
  * These are set when we can't bounce even if we were asked to.
  */
--- a/postfix/src/global/mail_params.h
+++ b/postfix/src/global/mail_params.h
@ -4126,7 +4126,23 @@ extern char *var_smtp_dns_re_filter;
 extern char *var_smtpd_dns_re_filter;

 /*
-  * Share TLS sessions through tlproxy(8).
+  * Backwards compatibility.
+  */
+#define VAR_SMTPD_FORBID_BARE_LF	"smtpd_forbid_bare_newline"
+#define DEF_SMTPD_FORBID_BARE_LF	"no"
+
+#define VAR_SMTPD_FORBID_BARE_LF_EXCL	"smtpd_forbid_bare_newline_exclusions"
+#define DEF_SMTPD_FORBID_BARE_LF_EXCL	"$" VAR_MYNETWORKS
+
+#define VAR_SMTPD_FORBID_BARE_LF_CODE	"smtpd_forbid_bare_newline_reject_code"
+#define DEF_SMTPD_FORBID_BARE_LF_CODE	550
+
+#define VAR_CLEANUP_MASK_STRAY_CR_LF	"cleanup_replace_stray_cr_lf"
+#define DEF_CLEANUP_MASK_STRAY_CR_LF	1
+extern int var_cleanup_mask_stray_cr_lf;
+
+ /*
+  * Share TLS sessions through tlsproxy(8).
  */
 #define VAR_SMTP_TLS_CONN_REUSE		"smtp_tls_connection_reuse"
 #define DEF_SMTP_TLS_CONN_REUSE		0
--- a/postfix/src/global/smtp_stream.c
+++ b/postfix/src/global/smtp_stream.c
@ -50,6 +50,9 @@
 /*	VSTREAM *stream;
 /*	char	*format;
 /*	va_list	ap;
+/*
+/*	int	smtp_detect_bare_lf;
+/*	int	smtp_got_bare_lf;
 /* AUXILIARY API
 /*	int	smtp_get_noexcept(vp, stream, maxlen, flags)
 /*	VSTRING	*vp;
@ -127,6 +130,11 @@
 /*	without timeouts and without making long jumps. Instead,
 /*	query the stream status with vstream_feof() etc.
 /*
+/*	This function assigns smtp_got_bare_lf = smtp_detect_bare_lf,
+/*	if smtp_detect_bare_lf is non-zero and the last read line
+/*	was terminated with a bare newline. Otherwise, this function
+/*	sets smtp_got_bare_lf to zero.
+/*
 /*	smtp_timeout_setup() is a backwards-compatibility interface
 /*	for programs that don't require per-record deadline support.
 /* DIAGNOSTICS
@ -201,6 +209,9 @@

 #include "smtp_stream.h"

+int     smtp_detect_bare_lf;
+int     smtp_got_bare_lf;
+
 /* smtp_timeout_reset - reset per-stream error flags, restart deadline timer */

 static void smtp_timeout_reset(VSTREAM *stream)
@ -362,6 +373,8 @@ int     smtp_get_noexcept(VSTRING *vp, VSTREAM *stream, ssize_t bound, int flags
    int     last_char;
    int     next_char;

+    smtp_got_bare_lf = 0;
+
    /*
     * It's painful to do I/O with records that may span multiple buffers.
     * Allow for partial long lines (we will read the remainder later) and
@ -404,8 +417,15 @@ int     smtp_get_noexcept(VSTRING *vp, VSTREAM *stream, ssize_t bound, int flags
 	 */
    case '\n':
 	vstring_truncate(vp, VSTRING_LEN(vp) - 1);
-	while (VSTRING_LEN(vp) > 0 && vstring_end(vp)[-1] == '\r')
-	    vstring_truncate(vp, VSTRING_LEN(vp) - 1);
+	if (smtp_detect_bare_lf) {
+	    if (VSTRING_LEN(vp) == 0 || vstring_end(vp)[-1] != '\r')
+		smtp_got_bare_lf = smtp_detect_bare_lf;
+	    else
+		vstring_truncate(vp, VSTRING_LEN(vp) - 1);
+	} else {
+	    while (VSTRING_LEN(vp) > 0 && vstring_end(vp)[-1] == '\r')
+		vstring_truncate(vp, VSTRING_LEN(vp) - 1);
+	}
 	VSTRING_TERMINATE(vp);
 	/* FALLTRHOUGH */

--- a/postfix/src/global/smtp_stream.h
+++ b/postfix/src/global/smtp_stream.h
@ -43,6 +43,8 @@ extern void smtp_fputs(const char *, ssize_t len, VSTREAM *);
 extern void smtp_fwrite(const char *, ssize_t len, VSTREAM *);
 extern void smtp_fread_buf(VSTRING *, ssize_t len, VSTREAM *);
 extern void smtp_fputc(int, VSTREAM *);
+extern int smtp_detect_bare_lf;
+extern int smtp_got_bare_lf;

 extern void smtp_vprintf(VSTREAM *, const char *, va_list);

--- a/postfix/src/smtpd/smtpd.c
+++ b/postfix/src/smtpd/smtpd.c
@ -745,6 +745,20 @@
 /*	The maximal number of AUTH commands that any client is allowed to
 /*	send to this service per time unit, regardless of whether or not
 /*	Postfix actually accepts those commands.
+/* .PP
+/*	Available in Postfix 3.9, 3.8.4, 3.7.9, 3.6.13, 3.5.23 and later:
+/* .IP "\fBsmtpd_forbid_bare_newline (Postfix < 3.9: no)\fR"
+/*	Reject or restrict input lines from an SMTP client that end in
+/*	<LF> instead of the standard <CR><LF>.
+/* .IP "\fBsmtpd_forbid_bare_newline_exclusions ($mynetworks)\fR"
+/*	Exclude the specified clients from smtpd_forbid_bare_newline
+/*	enforcement.
+/* .PP
+/*	Available in Postfix 3.9, 3.8.5, 3.7.10, 3.6.14, 3.5.24 and
+/*	later:
+/* .IP "\fBsmtpd_forbid_bare_newline_reject_code (550)\fR"
+/*	The numerical Postfix SMTP server response code when rejecting a
+/*	request with "smtpd_forbid_bare_newline = reject".
 /* TARPIT CONTROLS
 /* .ad
 /* .fi
@ -1447,6 +1461,12 @@ char   *var_tlsproxy_service;
 char   *var_smtpd_uproxy_proto;
 int     var_smtpd_uproxy_tmout;

+char   *var_smtpd_forbid_bare_lf;
+char   *var_smtpd_forbid_bare_lf_excl;
+int     var_smtpd_forbid_bare_lf_code;
+static int bare_lf_mask;
+static NAMADR_LIST *bare_lf_excl;
+
 /*
  * Silly little macros.
  */
@ -1539,6 +1559,40 @@ static int ask_client_cert;
  */
 static DICT *smtpd_cmd_filter;

+ /*
+  * Bare LF and End-of-DATA controls (bare CR is handled elsewhere).
+  * 
+  * At the smtp_get*() line reader level, setting any of these flags in the
+  * smtp_detect_bare_lf variable enables the detection of bare newlines. The
+  * line reader will set the same flags in the smtp_got_bare_lf variable
+  * after it detects a bare newline, otherwise it clears smtp_got_bare_lf.
+  * 
+  * At the SMTP command level, the flags in smtp_got_bare_lf control whether
+  * commands ending in a bare newline are rejected.
+  * 
+  * At the DATA and BDAT content level, the flags in smtp_got_bare_lf control
+  * whether the standard End-of-DATA sequence CRLF.CRLF is required, and
+  * whether lines ending in bare newlines are rejected.
+  * 
+  * Postfix implements "delayed reject" after detecting a bare newline in BDAT
+  * or DATA content. The SMTP server delays a REJECT response until the
+  * command is finished, instead of replying and hanging up immediately. The
+  * End-of-DATA detection is secured with BARE_LF_FLAG_WANT_STD_EOD.
+  */
+#define BARE_LF_FLAG_WANT_STD_EOD	(1<<0)	/* Require CRLF.CRLF */
+#define BARE_LF_FLAG_REPLY_REJECT	(1<<1)	/* Reject bare newline */
+
+#define IS_BARE_LF_WANT_STD_EOD(m)	((m) & BARE_LF_FLAG_WANT_STD_EOD)
+#define IS_BARE_LF_REPLY_REJECT(m)	((m) & BARE_LF_FLAG_REPLY_REJECT)
+
+static const NAME_CODE bare_lf_mask_table[] = {
+    "normalize", BARE_LF_FLAG_WANT_STD_EOD,	/* Default */
+    "yes", BARE_LF_FLAG_WANT_STD_EOD,	/* Migration aid */
+    "reject", BARE_LF_FLAG_WANT_STD_EOD | BARE_LF_FLAG_REPLY_REJECT,
+    "no", 0,
+    0, -1,				/* error */
+};
+
 #ifdef USE_SASL_AUTH

 /*
@ -3479,6 +3533,7 @@ static void receive_data_message(SMTPD_STATE *state,
    int     curr_rec_type;
    int     prev_rec_type;
    int     first = 1;
+    int     prev_got_bare_lf = 0;

    /*
     * Copy the message content. If the cleanup process has a problem, keep
@ -3492,12 +3547,15 @@ static void receive_data_message(SMTPD_STATE *state,
     * XXX Deal with UNIX-style From_ lines at the start of message content
     * because sendmail permits it.
     */
-    for (prev_rec_type = 0; /* void */ ; prev_rec_type = curr_rec_type) {
+    for (prev_rec_type = 0; /* void */ ; prev_rec_type = curr_rec_type,
+	 prev_got_bare_lf = smtp_got_bare_lf) {
 	if (smtp_get(state->buffer, state->client, var_line_limit,
 		     SMTP_GET_FLAG_NONE) == '\n')
 	    curr_rec_type = REC_TYPE_NORM;
 	else
 	    curr_rec_type = REC_TYPE_CONT;
+	if (IS_BARE_LF_REPLY_REJECT(smtp_got_bare_lf))
+	    state->err |= CLEANUP_STAT_BARE_LF;
 	start = vstring_str(state->buffer);
 	len = VSTRING_LEN(state->buffer);
 	if (first) {
@ -3510,9 +3568,14 @@ static void receive_data_message(SMTPD_STATE *state,
 	    if (len > 0 && IS_SPACE_TAB(start[0]))
 		out_record(out_stream, REC_TYPE_NORM, "", 0);
 	}
-	if (prev_rec_type != REC_TYPE_CONT && *start == '.'
-	    && (proxy == 0 ? (++start, --len) == 0 : len == 1))
-	    break;
+	if (prev_rec_type != REC_TYPE_CONT && *start == '.') {
+	    if (len == 1 && IS_BARE_LF_WANT_STD_EOD(smtp_detect_bare_lf)
+		&& (smtp_got_bare_lf || prev_got_bare_lf))
+		/* Do not store or send to proxy filter. */
+		continue;
+	    if (proxy == 0 ? (++start, --len) == 0 : len == 1)
+		break;
+	}
 	if (state->err == CLEANUP_STAT_OK) {
 	    if (var_message_limit > 0 && var_message_limit - state->act_size < len + 2) {
 		state->err = CLEANUP_STAT_SIZE;
@ -3664,6 +3727,11 @@ static int common_post_message_handling(SMTPD_STATE *state)
 	else
 	    smtpd_chat_reply(state,
 			     "250 2.0.0 Ok: queued as %s", state->queue_id);
+    } else if ((state->err & CLEANUP_STAT_BARE_LF) != 0) {
+	state->error_mask |= MAIL_ERROR_PROTOCOL;
+	log_whatsup(state, "reject", "bare <LF> received");
+	smtpd_chat_reply(state, "%d 5.5.2 %s Error: bare <LF> received",
+			 var_smtpd_forbid_bare_lf_code, var_myhostname);
    } else if (why && IS_SMTP_REJECT(STR(why))) {
 	state->error_mask |= MAIL_ERROR_POLICY;
 	smtpd_chat_reply(state, "%s", STR(why));
@ -3981,6 +4049,8 @@ static int bdat_cmd(SMTPD_STATE *state, int argc, SMTPD_TOKEN *argv)
 		/* Skip the out_record() and VSTRING_RESET() calls below. */
 		break;
 	    }
+	    if (IS_BARE_LF_REPLY_REJECT(smtp_got_bare_lf))
+		state->err |= CLEANUP_STAT_BARE_LF;
 	    start = vstring_str(state->bdat_get_buffer);
 	    len = VSTRING_LEN(state->bdat_get_buffer);
 	    if (state->err == CLEANUP_STAT_OK) {
@ -4633,6 +4703,9 @@ static int xclient_cmd(SMTPD_STATE *state, int argc, SMTPD_TOKEN *argv)
     */
    xclient_allowed =
 	namadr_list_match(xclient_hosts, state->name, state->addr);
+    smtp_detect_bare_lf = (SMTPD_STAND_ALONE((state)) == 0 && bare_lf_mask
+	    && !namadr_list_match(bare_lf_excl, state->name, state->addr)) ?
+	bare_lf_mask : 0;
    /* NOT: tls_reset() */
    if (got_helo == 0)
 	helo_reset(state);
@ -5614,6 +5687,13 @@ static void smtpd_proto(SMTPD_STATE *state)
 	    }
 	    watchdog_pat();
 	    smtpd_chat_query(state);
+	    if (IS_BARE_LF_REPLY_REJECT(smtp_got_bare_lf)) {
+		log_whatsup(state, "reject", "bare <LF> received");
+		state->error_mask |= MAIL_ERROR_PROTOCOL;
+		smtpd_chat_reply(state, "%d 5.5.2 %s Error: bare <LF> received",
+			     var_smtpd_forbid_bare_lf_code, var_myhostname);
+		break;
+	    }
 	    /* Safety: protect internal interfaces against malformed UTF-8. */
 	    if (var_smtputf8_enable && valid_utf8_string(STR(state->buffer),
 						 LEN(state->buffer)) == 0) {
@ -5956,6 +6036,13 @@ static void smtpd_service(VSTREAM *stream, char *service, char **argv)
    xforward_allowed = SMTPD_STAND_ALONE((&state)) == 0 &&
 	namadr_list_match(xforward_hosts, state.name, state.addr);

+    /*
+     * Reject or normalize bare LF, with compatibility exclusions.
+     */
+    smtp_detect_bare_lf = (SMTPD_STAND_ALONE((&state)) == 0 && bare_lf_mask
+	      && !namadr_list_match(bare_lf_excl, state.name, state.addr)) ?
+	bare_lf_mask : 0;
+
    /*
     * See if we need to turn on verbose logging for this client.
     */
@ -6018,6 +6105,14 @@ static void pre_jail_init(char *unused_name, char **unused_argv)
    hogger_list = namadr_list_init(VAR_SMTPD_HOGGERS, MATCH_FLAG_RETURN
 				   | match_parent_style(VAR_SMTPD_HOGGERS),
 				   var_smtpd_hoggers);
+    bare_lf_excl = namadr_list_init(VAR_SMTPD_FORBID_BARE_LF_EXCL,
+				    MATCH_FLAG_RETURN
+				    | match_parent_style(VAR_MYNETWORKS),
+				    var_smtpd_forbid_bare_lf_excl);
+    if ((bare_lf_mask = name_code(bare_lf_mask_table, NAME_CODE_FLAG_NONE,
+				  var_smtpd_forbid_bare_lf)) < 0)
+	msg_fatal("bad parameter value: '%s = %s'",
+		  VAR_SMTPD_FORBID_BARE_LF, var_smtpd_forbid_bare_lf);

    /*
     * Open maps before dropping privileges so we can read passwords etc.
@ -6314,6 +6409,7 @@ int     main(int argc, char **argv)
 	VAR_VIRT_MAILBOX_CODE, DEF_VIRT_MAILBOX_CODE, &var_virt_mailbox_code, 0, 0,
 	VAR_RELAY_RCPT_CODE, DEF_RELAY_RCPT_CODE, &var_relay_rcpt_code, 0, 0,
 	VAR_PLAINTEXT_CODE, DEF_PLAINTEXT_CODE, &var_plaintext_code, 0, 0,
+	VAR_SMTPD_FORBID_BARE_LF_CODE, DEF_SMTPD_FORBID_BARE_LF_CODE, &var_smtpd_forbid_bare_lf_code, 500, 599,
 	VAR_SMTPD_CRATE_LIMIT, DEF_SMTPD_CRATE_LIMIT, &var_smtpd_crate_limit, 0, 0,
 	VAR_SMTPD_CCONN_LIMIT, DEF_SMTPD_CCONN_LIMIT, &var_smtpd_cconn_limit, 0, 0,
 	VAR_SMTPD_CMAIL_LIMIT, DEF_SMTPD_CMAIL_LIMIT, &var_smtpd_cmail_limit, 0, 0,
@ -6485,6 +6581,8 @@ int     main(int argc, char **argv)
 	VAR_SMTPD_POLICY_CONTEXT, DEF_SMTPD_POLICY_CONTEXT, &var_smtpd_policy_context, 0, 0,
 	VAR_SMTPD_DNS_RE_FILTER, DEF_SMTPD_DNS_RE_FILTER, &var_smtpd_dns_re_filter, 0, 0,
 	VAR_SMTPD_REJ_FTR_MAPS, DEF_SMTPD_REJ_FTR_MAPS, &var_smtpd_rej_ftr_maps, 0, 0,
+	VAR_SMTPD_FORBID_BARE_LF_EXCL, DEF_SMTPD_FORBID_BARE_LF_EXCL, &var_smtpd_forbid_bare_lf_excl, 0, 0,
+	VAR_SMTPD_FORBID_BARE_LF, DEF_SMTPD_FORBID_BARE_LF, &var_smtpd_forbid_bare_lf, 1, 0,
 	0,
    };
    static const CONFIG_RAW_TABLE raw_table[] = {
--- a/postfix/src/smtpd/smtpd_check.c
+++ b/postfix/src/smtpd/smtpd_check.c
@ -48,6 +48,11 @@
 /*
 /*	char	*smtpd_check_queue(state)
 /*	SMTPD_STATE *state;
+/* AUXILIARY FUNCTIONS
+/*	void	log_whatsup(state, action, text)
+/*	SMTPD_STATE *state;
+/*	const char *action;
+/*	const char *text;
 /* DESCRIPTION
 /*	This module implements additional checks on SMTP client requests.
 /*	A client request is validated in the context of the session state.
@ -146,6 +151,11 @@
 /*	The recipient address given with the RCPT TO or VRFY command.
 /* .IP size
 /*	The message size given with the MAIL FROM command (zero if unknown).
+/* .PP
+/*	log_whatsup() logs "<queueid>: <action>: <protocol state>
+/*	from: <client-name[client-addr]>: <text>" plus the protocol
+/*	(SMTP or ESMTP), and if available, EHLO, MAIL FROM, or RCPT
+/*	TO.
 /* BUGS
 /*	Policies like these should not be hard-coded in C, but should
 /*	be user-programmable instead.
@ -946,8 +956,8 @@ void    smtpd_check_init(void)

 /* log_whatsup - log as much context as we have */

-static void log_whatsup(SMTPD_STATE *state, const char *whatsup,
-			        const char *text)
+void    log_whatsup(SMTPD_STATE *state, const char *whatsup,
+		            const char *text)
 {
    VSTRING *buf = vstring_alloc(100);

--- a/postfix/src/smtpd/smtpd_check.h
+++ b/postfix/src/smtpd/smtpd_check.h
@ -25,6 +25,7 @@ extern char *smtpd_check_etrn(SMTPD_STATE *, char *);
 extern char *smtpd_check_data(SMTPD_STATE *);
 extern char *smtpd_check_eod(SMTPD_STATE *);
 extern char *smtpd_check_policy(SMTPD_STATE *, char *);
+extern void log_whatsup(SMTPD_STATE *, const char *, const char *);

 /* LICENSE
 /* .ad