Docs: CVE note

Cherry-picked from: 5a8fc0799314

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+CVE ID:     CVE-2023-51766
+Date:       2016-12-15
+Credits:    https://sec-consult.com/blog/detail/smtp-smuggling-spoofing-e-mails-worldwide/
+Version(s): all up to 4.97 inclusive
+Issue:      Given a buggy relay, Exim can be induced to accept a second message embedded
+            as part of the body of a first message
+
+Conditions
+==========
+
+If *all* the following conditions are met
+
+    Runtime options
+    ---------------
+
+    * Exim offers PIPELINING on incoming connections
+
+    * Exim offers CHUNKING on incoming connections
+
+    Operation
+    ---------
+
+    * DATA (as opposed to BDAT) is used for a message reception
+
+    * The relay host sends to the Exim MTA message data including
+      one of "LF . LF" or "CR LF . LF" or "LF . CR LF".
+
+    * Exim interprets the sequence as signalling the end of data for
+      the SMTP DATA command, and hence a first message.
+
+    * Exim interprets further input which the relay had as message body
+      data, as SMTP commands and data. This could include a MAIL, RCPT,
+      BDAT (etc) sequence, resulting in a further message acceptance.
+
+Impact
+======
+
+One or more messages can be accepted by Exim that have not been
+properly validated by the buggy relay.
+
+Fix
+===
+
+Install a fixed Exim version:
+
+    4.98 (once available)
+    4.97.1
+
+If you can't install one of the above versions, ask your package
+maintainer for a version containing the backported fix. On request and
+depending on our resources we will support you in backporting the fix.
+(Please note, that Exim project officially doesn't support versions
+prior the current stable version.)
+
+
+Workaround
+==========
+
+  Disable CHUNKING advertisement for incoming connections.
+
+  An attempt to "smuggle" a DATA command will trip a syncronisation
+  check.
+
+*or*
+
+  Disable PIPELINING advertisement for incoming connections.
+
+  The "smuggled" MAIL FROM command will then trip a syncronisation
+  check.