The DKIM implementation in Exim 4.7x before 4.76 permits matching for DKIM identities to apply to lookup items, instead of only strings, which allows remote attackers to execute arbitrary code or access a filesystem via a crafted identity.
https://lists.exim.org/lurker/message/20110512.102909.8136175a.en.html
https://lists.exim.org/lurker/message/20110509.091632.daed0206.en.html
http://www.ubuntu.com/usn/USN-1135-1