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Advisory 02/2007: WordPress Trackback Charset Decoding SQL Injection Vulnerability
Jan 05 2007 03:14PM
Stefan Esser (sesser hardened-php net)
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-= Security Advisory =-
Advisory: WordPress Trackback Charset Decoding SQL Injection Vulnerability
Release Date: 2007/01/05
Last Modified: 2007/01/05
Author: Stefan Esser [sesser (at) hardened-php (dot) net [email concealed]]
Application: WordPress <= 2.0.5
Severity: The support of trackbacks in different charsets can
be used to bypass WordPress's SQL injection protection.
This might result in a compromise of the admin
account and the execution of arbitrary PHP code
on the server
Vendor Status: Vendor has released WordPress 2.0.6 which fixes this issue
Quote from http://www.wordpress.org
"WordPress was born out of a desire for an elegant, well-
architectured personal publishing system built on PHP and MySQL
and licensed under the GPL. It is the official successor of
b2/cafelog. WordPress is fresh software, but its roots and
development go back to 2001. It is a mature and stable product.
We hope by focusing on user experience and web standards we can
create a tool different from anything else out there."
While testing WordPress it was discovered that WordPress supports
trackbacks in different charsets when PHP's mbstring extension is
installed. This feature can be abused to bypass WordPress's SQL
parameter escaping which leads to an SQL injection vulnerability
that can result in a compromise of the admin account and end in
a server compromise.
WordPress supports decoding trackbacks with different charsets
when PHP's mbstring extension is activated. Because the decoding
happens after the database escaping is performed choosing the
right charset for the input data allows bypassing the protection
against SQL injection.
For the demonstration exploit that was shared with the WordPress
developers the UTF-7 charset was chosen, because it is the
easiest to work with. Other multibyte charsets that for example
allow multibyte sequences ending in '\' can also be used.
The exploit first checks if the mbstring extension is loaded by
sending only an UTF-7 encoded '0' in the title field. Because
only a decoded '0' will pass the empty() check this will trigger
two different error situations depending on mbstring availability.
The exploit continues with injecting bogus SQL commands into the
SQL query which will result in WordPress giving out detailed
information about the failed query. This error message is used
to determine the configure database table prefix, which is needed
for the correct exploitation.
In the next step a harmless looking bogus comment is injected
into the comment table with a timestamp in the future. This is
done to stop WordPress from sending notification emails to the
admin during the password hash retrieval.
The rest of the exploit is a simple UNION SELECT injection that
determines password hashes by issuing a request for every bit of
the hash (128 bits) and checking which error message is returned.
With the reconstructed password hash it is possible to create a
WordPress login cookie that can be used to become admin.
As admin it is possible to edit (overwrite) all files within the
blog directory that are writeable. This can be used to inject
arbitrary PHP code in most installations. There exists a protection
that denies loading the wp-config.php file into the file editor,
but it can be tricked by directly sending a POST request that
only saves the files. The wp-config.php file is usually left
writeable after the installation has finished.
Proof of Concept:
The Hardened-PHP Project is not going to release a proof of concept
exploit for this vulnerability.
29. December 2006 - Notified security (at) wordpress (dot) org [email concealed]
05. January 2007 - WordPress 2.0.6 release
05. January 2007 - Public Disclosure
We strongly recommend to upgrade to WordPress 2.0.6 which also
fixes several other security vulnerabilities not covered by this
pub 1024D/0A864AA1 2004-04-17 Hardened-PHP Signature Key
Key fingerprint = 066F A6D0 E57E 9936 9082 7E52 4439 14CC 0A86 4AA1
Copyright 2007 Stefan Esser. All rights reserved.
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