Bug 20616

Created attachment 9514 [details]
patch to avoid DoS in crypt function

There are situations where a user can control a password hash. A user could exploit this and cause a DoS by selecting an insanely high rounds value for a password hash.

The current maximum rounds number is 999999999 (a billion minus 1), which would run several minutes on a modern machine. I don't think there is any valid use case for this.

To avoid DoS I would propose to limit the number of rounds to a sane value. It's a bit arbitrary which value to choose, in the attached patch I limit it to 9999999 (10 million minus 1), which runs 3/5 seconds (depending if sha256/512) on my system. It seems like this is probably above all valid use cases, but doesn't DoS too much. Also the original implementation would cap the rounds to 999999999 for all larger values [1], I changed that to return NULL for too large values (to avoid having identical inputs generating different outputs for different glibc versions). Patch attached, changes it for both sha256/512 in the same way.

(P.S.: musl libc already does essentially the same that I'm proposing here [2])

[1] http://people.redhat.com/drepper/sha-crypt.html
[2] http://git.musl-libc.org/cgit/musl/tree/src/crypt/crypt_sha256.c?id=5505f6afae9acf37ef565c68a07ca3df7b1ae2cb#n168