"Dovecot is an open source IMAP and POP3 server for Linux/UNIX-like systems, written with security primarily in mind. Although it's written in C, it uses several coding techniques to avoid most of the common pitfalls."
Dovecot IMAP/POP3 Server suffers from a buffer overflow vulnerability when mmap_disable=yes setting is used.
* Dovecot IMAP/POP3 Server versions 1.0test53 to 1.0.rc14.
* Dovecot IMAP/POP3 Server versions 0.99.x (they don't have mmap_disable setting).
When mmap_disable=yes setting is used (not default), dovecot.index.cache file is read to memory using "file cache" code. It contains a "mapped pages" bitmask buffer. In some conditions when updating the buffer it allocates one byte too little.
Exploitability: It's going to be pretty difficult to cause anything else than a crash, but not impossible. Only logged in IMAP/POP3 users can exploit this.
In theory you might be able to exploit this for other users as well by sending them a lot of specially crafted emails, but this requires knowing what dovecot.index.cache file contains. Normally its contents can't be predicted, although perhaps with POP3 users it gets empty often enough that the exploit could be tried. Then again, the exploit requires having at least 4MB cache file, which won't happen with POP3 users before the mailbox has about 170k mails (if I counted right).
With IMAP the cache file is used more, so it's easier to fill the 4MB with for example a lot of To-headers.
Use INDEX=MEMORY so the cache files aren't used at all.