Doc: short description of CVE-2016-9963

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+CVE ID:     CVE-2016-9963
+Date:       2016-12-15
+Credits:    Bjoern Jacke <bjoern@j3e.de>
+Version(s): 4.69 -> 4.87
+Issue:      If several conditions are met, Exim leaks private information
+            to a remote attacker.
+
+Conditions
+==========
+
+If *all* of the following conditions are met
+
+    Build options
+    -------------
+
+    * Exim is built with DKIM enabled (default for newer versions)
+      exim -bV | grep 'Support.*DKIM'
+
+    Runtime options
+    ---------------
+
+    * Exim uses DKIM signing (transport options dkim_private_key,
+      dkim_domain, and other)
+
+    * The dkim_private_key option names a file containing the key.
+
+      exim -bP transports | grep 'dkim_private_key = .'
+
+    * Exim uses PRDR (transport option hosts_try_prdr) (default
+      since 4.86)
+
+      exim -bP transports | grep 'hosts_try_prdr = .'
+
+      *OR*
+
+      Exim uses the LMTP protocol variant for SMTP transport.
+
+      exim -bP transports | grep 'protocol = lmtp'
+
+    Operation
+    ---------
+
+    * Exim transports a multi-recipient message
+
+    * The destination host supports PRDR
+      OR
+      the message transport uses LMTP
+
+    * One or more recipients are rejected after the DATA phase
+
+Impact
+======
+
+Exim leaks the private DKIM signing key to the log files.  Additionally,
+if the build option EXPERIMENTAL_DSN_INFO=yes is used, the key material
+is included in the bounce message.
+
+Fix
+===
+
+Install a fixed Exim version:
+
+    4.88        (available soon)
+    4.87.1      (available soon)
+
+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 PRDR in your outgoing transport(s): set hosts_try_prdr to an
+empty string.
+
+AND do not use the LMTP protocol variant of the SMTP driver.
+
+Indication
+==========
+
+You can check if you where affected already. The mainlog entries look like this:
+
+2016-12-17 09:44:33 10HmaX-0005vi-00 ** baduser@test.ex R=client T=send_to_server H=ip4.ip4.ip4.ip4 [ip4.ip4.ip4.ip4]: PRDR error after -----BEGIN RSA PRIVATE KEY-----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\nfKQax6vhiMq+0qIiEwLA1wavyLVKZ7Mfag+/4NTcDUVC\n-----END RSA PRIVATE KEY-----\n: 550 PRDR R=<baduser@test.ex> refusal
+