Vulnerabilities in PAM and NSS modules using a PostgreSQL databaseDuring the investigation of the problem described in RUS-CERT Advisory 2001-08:01, it became evident that a few PAM and NSS modules which use PostgreSQL as database backend are vulnerable to SQL code injection attacks, too.
All systems using at least one of the following PAM and NSS modules:
libnss-pgsql0.9.0 by Joerg Wendland
nss_postgresql0.6.1 by Alessandro Gardich
pam-pgsql0.9.2 by Joerg Wendland
pam_pgsql0.0.3 by Alessandro Gardich
pam-pgsql0.5.1 by Leon J Breedt
For the PAM authentication modules, the ability to attempt a password-based login on the system is required to exploit the vulnerability. The exact login method (HTTP Authentication, SSH, Telnet) does not matter, as long as PAM is used. For the NSS database modules, an interactive account is usually required to exploit this vulnerability.
The attack can execute arbitrary SQL statements under the database user used for querying the PostgreSQL database. Responses from the database backend can be faked. Exploiting the vulnerability in a PAM module, an attacker might gain unauthorized access. The possibilities of an attacker facing a vulnerable NSS module strongly depend on the system configuration and the offered services.
SQL code insertion attack
The problem has already been described in RUS-CERT Advisory 2001-08:01: An attacker might use specially crafted strings containing embedded SQL statements in order to fake responses from the database backend.
If the attacker can attempt logins using a suitable PAM-based login procedure (which permits spaces and single quotation marks in user names), involving one of the vulnerable PAM modules, or can query one of the NSS based handled by a vulnerable NSS module, he is able to execute arbitrary SQL statements on the database server, under the database account used for the query. In addition, data returned by queries can be manipulated. This can lead to unauthorized access to the system.
We believe that the fact that essentially the same vulnerability is present in many PostgreSQL applications (see also RUS-CERT Advisory 2001-08:01) is related to the lack of a suitable string quoting function in the PostgreSQL client library (and not just to code reuse and overlap among the authors).
Therefore, we propose to include a function into the PostgreSQL client library
libpq which escapes characters treated specially by PostgreSQL, replacing them with safe character sequences.
Joerg Wendland has published fixed versions of his modules.
RUS-CERT contacted the authors of the vulnerable authentication modules on 2001-08-25.
RUS-CERT is the Computer Emergency Response Team located at the Computing Center (RUS) of the University of Stuttgart, Germany.