CVE ID: CVE-2016-9963 Date: 2016-12-15 Credits: Bjoern Jacke 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 4.87.1 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.) If you think that you MIGHT be affected, we HIGHLY recommend to create a new set of DKIM keys and fade out the previous DKIM key soon to make sure that a possibly leaked DKIM key can not be misused in the future. 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= refusal Even if there is no evidence in the existing log files, that a DKIM key leakage happened this might have happened in the past, log files might have been deleted already but a key leak could have ended up via mail bounce in a user mail box [Heiko Schlittermann ]