In Exim 4.90 and earlier, there is a buffer overflow in an utility function, if some pre-conditions are met. Using a handcrafted message, remote code execution seems to be possible.
More information to follow.
Created exim tracking bugs for this issue:
Affects: epel-all [bug 1543269]
Affects: fedora-all [bug 1543270]
The issue here is a buffer overflow in the b64decode() function, that seems to be limited to just one byte. It's triggered when Base64 encoded input to decode does not have proper length (multiple of 4). Attacker controlled byte is written past of the buffer.
This issue affects the versions of Exim as shipped in Red Hat Enterprise Linux 5. Red Hat Enterprise Linux 5 is already in the Extended Life Phase of its life cycle and Exim is not on the list of components supported via Red Hat Enterprise Linux 5 Extended Life-cycle Support (ELS) add-on, therefore there's currently no plan to address this issue in Red Hat Enterprise Linux 5. For more information about Red Hat Enterprise Linux 5 life cycle and ELS add-on scope of support, see:
The Exim mail server is not shipped in Red Hat Enterprise Linux 6 and 7.