oss-security - Re: CVE request: crypt_blowfish 8-bit character mishandling

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Message-ID: <20110621201838.GA8278@openwall.com>
Date: Wed, 22 Jun 2011 00:18:38 +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 Tue, Jun 21, 2011 at 04:34:41PM +0200, Ludwig Nussel wrote:
> I wonder whether it would make sense to patch pam_unix (resp 
> pam_unix2 in our case) to detect the problem and activate the 
> workaround automatically. pam_unix has the clear text password so 
> knows when it contains 8bit characters. It also has the shadow entry 
> which tells when the password was set. If that date is before the 
> update was installed the 2x method could be tried if 2a failed and a 
> warning could be logged to syslog.

This is tricky.  When implementing things like that, 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).

My response above is generic, not focused on your specific proposed
approach.  Overall, I think we'll need to give this more thought.

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.

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?

Meanwhile, here's my announcement of crypt_blowfish 1.1 and the Owl
glibc security update:

http://www.openwall.com/lists/announce/2011/06/21/1

It includes my latest summary of the bug's impact.

Thanks,

Alexander

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