Version: 1.0test53 .. 1.0.rc14 (ie. all 1.0alpha, 1.0beta and 1.0rc versions so far).

0.99.x versions are safe (they don't even have mmap_disable setting).

Problem: When mmap_disable=yes setting is used, 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: I think it's going to be pretty difficult to cause anything else than a crash, but I wouldn't say 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.

Workaround: Use INDEX=MEMORY so the cache files aren't used at all.

Fix: 1.0.rc15 fixes this. You can also use this patch: http://dovecot.org/patches/1.0/file-cache-buffer-overflow-fix.diff