7003907 2001-08-29 19:24 +0200 /156 rader/ Florian Weimer <Florian.Weimer@RUS.Uni-Stuttgart.DE>
Sänt av: joel@lysator.liu.se
Importerad: 2001-08-29 23:06 av Brevbäraren
Extern mottagare: bugtraq@securityfocus.com
Mottagare: Bugtraq (import) <18993>
Ärende: RUS-CERT Advisory 2001-08:01
------------------------------------------------------------
From: Florian Weimer <Florian.Weimer@RUS.Uni-Stuttgart.DE>
To: bugtraq@securityfocus.com
Message-ID: <tgu1yqzsm6.fsf@mercury.rus.uni-stuttgart.de>
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
--
Florian Weimer Florian.Weimer@RUS.Uni-Stuttgart.DE
University of Stuttgart http://cert.uni-stuttgart.de/
RUS-CERT +49-711-685-5973/fax +49-711-685-5898
(7003907) /Florian Weimer <Florian.Weimer@RUS.Uni-Stuttgart.DE>/(Ombruten)