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vulnerability note CVE-2011-2483

crypt_blowfish: hash collision

Synthesis of the vulnerability

When the user has a password containing 8 bit characters, the Blowfish hashing algorithm of crypt() generates an invalid hash, which is potentially faster to find with a brute force.
Vulnerable systems: Debian, Mandriva Linux, NLD, OES, openSUSE, PostgreSQL, RHEL, SUSE Linux Enterprise Desktop, SLES, Unix (platform) ~ not comprehensive.
Severity of this threat: 2/4.
Consequences of a hack: user access/rights.
Pirate's origin: intranet client.
Creation date: 19/08/2011.
Références of this weakness: CVE-2011-2483, DSA-2340-1, MDVSA-2011:161, MDVSA-2011:178, MDVSA-2011:179, MDVSA-2011:180, openSUSE-SU-2011:0921-1, openSUSE-SU-2011:0921-2, openSUSE-SU-2011:0970-1, openSUSE-SU-2011:0972-1, openSUSE-SU-2012:0480-1, openSUSE-SU-2013:1670-1, openSUSE-SU-2013:1676-1, RHSA-2011:1377-01, RHSA-2011:1378-01, SUSE-SA:2011:035, SUSE-SU-2011:0922-1, SUSE-SU-2011:0923-1, SUSE-SU-2011:0927-1, SUSE-SU-2011:0971-1, SUSE-SU-2011:0974-1, SUSE-SU-2011:0991-1, SUSE-SU-2011:1081-1, SUSE-SU-2011:1081-2, VIGILANCE-VUL-10934.

Description of the vulnerability

The crypt() function hashes the password of a user. When a user is added, the hash is stored in the /etc/shadow file. When the user authenticates, the hash is compared to the hash from /etc/shadow.

The crypt() function supports several hash algorithms:
 - DES
 - MD5 (prefix $1$)
 - Blowfish (prefix $2a$), which is implemented in the crypt_blowfish library

However, crypt_blowfish uses signed C characters (-128 to 127), instead of unsigned characters (0 to 255). The generated hash is thus invalid if the password contains 8 bit characters.

This error has no impact of user authentication, because the invalid hash was stored in the /etc/shadow file, and the invalid hash of the entered password is the same.

However, the generated hash is subject to collisions: several passwords can have the same hash. A brute force attack thus requires to test less passwords before finding user's password.

When the user has a password containing 8 bit characters, the Blowfish hashing algorithm of crypt() therefore generates an invalid hash, which is potentially faster to find with a brute force.
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