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Date: Fri, 4 May 2012 19:31:51 +0400
From: Solar Designer <solar@...nwall.com>
To: oss-security@...ts.openwall.com
Subject: Debian/Ubuntu php_crypt_revamped.patch

Hi,

Boaz Rymland reported to me what he thought was a phpass bug, where any
password would be valid when authenticated against a NULL or empty
password hash.  (Indeed, the password hash shouldn't normally be NULL or
empty, but it is better for authentication code to be fail-close rather
than fail-open.)  At first, I was not able to reproduce the problem, but
after exchanging a few e-mails we were able to narrow it down to PHP
crypt() call returning an empty string when called with NULL or an empty
string for the salt argument on Boaz' Ubuntu 11.04 system with php5
5.3.5-1ubuntu7.7.  I was still not able to reproduce that on other
systems (with other versions of PHP).

Today, I downloaded and built clean PHP 5.3.5 on an Owl system.
I still could not trigger the problem.  Then I applied
debian/patches/php_crypt_revamped.patch from Debian's
php5_5.3.5-1.diff.gz - and the problem finally appeared.

Original:

php@...:~ $ ~/php-5.3.5/bin/php -r 'echo crypt("pass", null), "\n";'
$1$l5Nwx5hu$NhostJ7i8jP1B.4C4zaiM78.

With Debian patch:

php@...:~ $ ~/php-5.3.5-debian/bin/php -r 'echo crypt("pass", null), "\n";'

php@...:~ $ 

(empty string was printed).

It turns out that the patch first appeared in Debian's 5.3.2-1 in
response to almost a non-issue (different behavior across PHP versions
for an invalid salt string) and general feeling that PHP should be using
system-provided crypto instead of its bundled code when possible
(questionable to me: each approach has its pros and cons):

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=572601

The handling of NULL/empty salt strings was corrected in 5.3.6-1, as
well as in 5.3.3-7+squeeze4 (stable-security):

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=581170

Apparently, that fix never made it into Ubuntu 11.04 updates - so I
guess this should happen now.

Overall, the patch looks problematic to me.  Here's another problem with
it, still reproducible on 5.3.10-1ubuntu3 (Ubuntu 12.04):

user@...ntu:~$ php -r 'echo crypt("pass", "_J9..Salt"), "\n";'
_J9..Saltr2Hq6I3ZH0s
user@...ntu:~$ php -r 'echo crypt("pass", "_J9..Saltr2Hq6I3ZH0s"), "\n";'
_J0LlWX63dRZg

That non-security bug is with the "salt_len == 9" check added with the
patch.  So phpass' authentication against CRYPT_EXT_DES hashes, which it
tries to support, would be failing on Debian/Ubuntu systems.  I guess I
need to introduce a workaround for it now, complicating the code. :-(

I think it may be best to drop this patch from further versions of
Debian/Ubuntu - and not reintroduce it even in response to the likely
"bug" reports from Debian users complaining about the behavior change
from previous versions of Debian.

I agree that the code in upstream PHP may need improvement, but that
patch does not improve it, and the deviation from upstream is bad.
Altering the behavior of PHP on specific distros beyond what may
normally happen due to PHP's ./configure is undesirable and should only
be done for very good reasons.

Sorry for the rant.

Alexander

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