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AUSCERT External Security Bulletin Redistribution
ESB-2001.390 -- RUS-CERT Advisory 2001-08:01
Vulnerabilities in several Apache authentication modules
11 September 2001
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AusCERT Security Bulletin Summary
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Product: Apache using authentication 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 by Guiseppe Tanzilli
and Matthias Eckermann
* mod_auth_pgsql_sys 0.9.4
(modified by Victor Glushchenko)
Impact: Execute Arbitrary Code/Commands
Access Privileged Data
Access Required: Remote
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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 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 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 that a function which escapes characters treated
specially by the PostgreSQL by replacing them with safe character
sequences is included in the PostgreSQL client library. We provide a
mostly untested sample implementation:
* Escaping Strings in PostgreSQL Queries
(http://cert.uni-stuttgart.de/doc/postgresql/escape/)
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:
* AuthPG 1.3 by Min S. Kim (http://authpg.sourceforge.net/)
* mod_auth_mysql 1.10 by Vivek Khera (ftp://ftp.kcilink.com/pub/)
* mod_auth_pgsql 0.9.6 by Guiseppe Tanzilli
(http://www.giuseppetanzilli.it/mod_auth_pgsql/dist/)
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 (http://cert.uni-stuttgart.de/) is the Computer Emergency
Response Team located at the Computing Center (RUS) of the
University of Stuttgart, Germany.
URI For This Advisory
http://cert.uni-stuttgart.de/advisories/apache_auth.php
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