dovecot -- Buffer overflow reading extension header

Aki Tuomi reports:

Vulnerability Details:
When reading FTS or POP3-UIDL header from dovecot index, the input
buffer size is not bound, and data is copied to target structure causing
stack overflow.

Risk:
This can be used for local root privilege escalation or executing
arbitrary code in dovecot process context. This requires ability to
directly modify dovecot indexes.
Steps to reproduce:
Produce dovecot.index.log entry that creates an FTS header which has
more than 12 bytes of data.
Trigger dovecot indexer-worker or run doveadm index.
Dovecot will crash.

Mitigations:
Since 2.3.0 dovecot has been compiled with stack smash protection, ASLR,
read-only GOT tables and other techniques that make exploiting this bug
much harder.