Date: Thu, 23 Jun 2011 18:00:25 +0400 From: Solar Designer <solar@...nwall.com> To: oss-security@...ts.openwall.com Cc: Michael Matz <matz@...e.de>, Thorsten Kukuk <kukuk@...e.de>, Andreas Jaeger <aj@...e.de> Subject: Re: CVE request: crypt_blowfish 8-bit character mishandling On Wed, Jun 22, 2011 at 02:02:53PM +0200, Ludwig Nussel wrote: > Solar Designer wrote: > > ... we need to consider > >timing leaks (do we care if an observer of ssh traffic is able to > >tell whether the password contained 8-bit chars or not? perhaps we do) > >and leaks via the hash encodings themselves (if only some are changed to > >a certain type, this may leak some info about the corresponding > >passwords, thereby speeding up offline attacks on the hashes). > > For SUSE Linux we're not that paranoid I guess :-) You should be. One of the lessons we should learn from this bug is that we were not paranoid enough. Clearly, I was not. (Yet, of course, we should limit our paranoia to a reasonable level - e.g., don't introduce paranoid checks so complicated that they could likely contain security bugs themselves. This is something I need to consider when I work on those mandatory self-tests, introduced in crypt_blowfish 1.1 and likely to be revised somewhat in future revisions.) > The extra time > would only hit accounts that are not converted yet. I suppose there > are not too many anyways and there will be less over time. Yes, but you'd expose them in a worse manner than they currently are. And the extra password hashing call would be usable by a DoS attacker, thereby halving the maximum iteration count for new password hashes that an admin can configure. > >One idea is to allocate yet another prefix, which will mean the same > >thing as 2a, but "certified" as passing a certain specific test suite > >(which will include 8-bit chars). So we'll have: > > > >2a - unknown correctness (may be correct, may be buggy) > >2x - sign extension bug > >2y - definitely correct > > > >Newly set/changed passwords will be getting the new prefix. I am leaning towards this approach. It is flexible and it may be implemented safely, without added risks on top of what we already have. (Of course, we'll need to be careful with the implementation. We must not reveal anything about the actual passwords through the prefix used. The prefixes should depend solely on system settings and on when a given password was set, but not on the password itself. The "when" is stored in shadow anyway, so we're not revealing anything new.) > >Then we'll be able to do things such as optionally have a PAM module > >deny logins with 8-bit char passwords to accounts that have 2a or/and > >2x hashes. (Rationale for the admin: passwords weaker than expected.) > >With another option, we'll be able to have 2a treated as 2x. (Rationale > >for the admin: minimum inconvenience to the users.) Perhaps there can > >be other reasonable settings as well. > > > >What do you think? > > I'm not sure we can expect admins to put that much thought into the > issue and expect them to configure things. You don't need to "expect" them to do it, but giving advanced admins the option could be nice. The drawback is code complexity, though (extra options, extra if's). > I think for the system > logins we can get away with patching pam_unix2 to have a fallback to > 2x See above re: timing and halved maximum iteration count. > and log a message for the admin that tells him to run "passwd -e" > on the account. This is even worse: you'd be storing some info about the password itself in plaintext! Yes, that's just one bit, yet it may allow for more focused attacks. If an intruder retrieves not only the shadow file, but also the log file, they'd be able to attack those hashes faster. Similarly, if another user has read access to the log file, but not to the shadow file, they'd know which accounts (not) to target with 8-bit chars in an online password guessing attack. So please don't do that. If you need to make things mostly transparent for admins and users, here's my suggestion: Add an option to treat 2a in existing hashes as 2x. Maybe this should even be the default. For new hashes, produced by crypt_blowfish 1.1+ with the mandatory self-test in it, use a new prefix - 2y. When checking, treat it as 2a (and I may make it official and transparent in 1.1.1+). How does this sound to you? Additionally, for the paranoid, when the option to treat 2a as 2x is disabled, disallow logins with passwords containing 0xff chars (possible attack). Maybe only for 2a hashes, but not for 2y. In order not to leak this fact via timings, perform the hashing anyway. (I'll consider making this built-in in a new version of crypt_blowfish, which should let us be more careful with timings.) The 0xff char rejection idea was also proposed by iabervon in the thread at LWN: http://lwn.net/Articles/448699/ (subscription required). iabervon also wrote: "Alternatively, the function could replace any char 255 with a char 127, based on the fact that no common encoding uses both of these (so it's not interesting to confuse them), and get the same effect without prohibiting any newly-entered passwords." but I think 2y takes care of that same goal better. Alexander
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