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RUS-CERT Advisory 2001-08:01 – Vulnerabilities in several Apache authentication modules

RUS-CERT has discovered that several Apache authentication modules which use SQL databases to store authentication information are vulnerable to a remote SQL code injection attack.

Systems Affected

Any Apache server using database-based authentication with the following modules:

  • AuthPG 1.2b2 by Min S. Kim (also known as mod_auth_pg)
  • mod_auth_mysql 1.9 by Vivek Khera
  • mod_auth_oracle 0.5.1 by Serg Oskin
  • mod_auth_pgsql 0.9.5 and 0.9.6 by Guiseppe Tanzilli and Matthias Eckermann
  • mod_auth_pgsql_sys 0.9.4 (by the same authors, modifications by Victor Glushchenko)

It is possible that other authentication modules not listed above are affected.

Systems Not Affected

RUS-CERT has examined the following authentication modules and verified that an Apache server using these modules is not vulnerable to the problem described in this document:

  • mod_auth_mysql 2.20 by Zeev Suraski
  • mod_auth_ora7 1.0 by Ben Reser
  • mod_auth_ora8 1.0 by Ben Reser

Attack vector

HTTP requests sent to the Apache server using the vulnerable authentication modules.

Impact

In the case of the PostgreSQL modules, an attack can execute arbitrary SQL statements or cause the database query for the password to return arbitrary data. As a result, unauthorized access to the web server is possible.

With the Oracle module, the attacker can call stored procedures and cause the database query for the password to return arbitrary data. The impact with MySQL is currently unclear, but with the advent of stored procedures, harmful side effects might become possible as well.

Vulnerability Type

SQL code insertion attack

Description

During the authentication process, the password hash has to be looked up in the database, so a SQL SELECT statement has to be built. In the vulnerable modules, this is done using code equivalent to the following pseudocode:

Query := Sprintf ("SELECT %s FROM %s WHERE %s = '%s'",
                  Password_Column, User_Table, User_Column,
                  User);

Later on, the retrieved password hash is compared with the one supplied by the user trying to authenticate.

However, the value of User has been received over the network. Suppose an attacker choses the string (note the single quotation mark at the beginning):

'; SELECT 'wA8aGH92dPQnIDD

Now the resulting string contains two SQL statements:

SELECT password_column FROM user_table WHERE user_column = '';
SELECT 'wA8aGH92dPQnIDD'

PostgreSQL’s libpq client library will transmit both statements to the PostgreSQL server. The server will execute both statements and return the result of the second to the client. This way, an attacker can make it appear to the authentication code that the database contains the proper hash for the password it just has provided.

Other forms of attacks are possible by issuing INSERT or DELETE statements in essentially the same manner, of course.

In the MySQL and Oracle cases, the impact of the vulnerability is different. Oracle does not seem to allow multiple SQL statements per query, but using a UNION clause to add additional data seems to be possible, so the attack given above can be duplicated. In addition, stored procedures can be called, with a potential for harmful side effects. We were unable to obtain a definite answer if the vulnerability is exploitable if a MySQL database is used, since MySQL neither supports UNION clauses nor stored procedures.

Proposed Solution

PostgreSQL

We believe that the fact that the essentially the same vulnerability is present in many PostgreSQL applications (see also RUS-CERT Advisory 2001-09:01) 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.

Some of the fixed versions below already implement this suggestion.

Update: libpq now provides a suitable function for quoting too.

MySQL and Oracle

Both the MySQL and Oracle client libraries provide a suitable function for quoting strings in SQL queries. The authentication modules which are not vulnerable (see above) use them, so we propose to use these modules, or the fixed versions below.

Patched Versions

Several authors have already reacted and released new versions:

Serg Oskin has announced a fixed version as well.

Contact Status

RUS-CERT contacted the authors of the vulnerable authentication modules on 2001-08-23.

About RUS-CERT

RUS-CERT is the Computer Emergency Response Team located at the Computing Center (RUS) of the University of Stuttgart, Germany.

Version History

2001-08-29: Published.
2001-09-04: Patch for PostgreSQL’s libpq updated.
2001-09-04: mod_auth_pgsql 0.9.6 is still vulnerable.