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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.
We provide a mostly untested sample implementation:
Some of the fixed versions below already implement this suggestion.
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.
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