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WordPress 4.2 stored XSS

2015-04-27 08:15
Current versions of WordPress are vulnerable to a stored XSS. An
unauthenticated attacker can inject JavaScript in WordPress comments. The
script is triggered when the comment is viewed.

If triggered by a logged-in administrator, under default settings the
attacker can leverage the vulnerability to execute arbitrary code on the
server via the plugin and theme editors.

Alternatively the attacker could change the administrator’s password,
create new administrator accounts, or do whatever else the currently
logged-in administrator can do on the target system.

If the comment text is long enough, it will be truncated when inserted in
the database. The MySQL TEXT type size limit is 64 kilobytes so the comment
has to be quite long.

The truncation results in malformed HTML generated on the page. The
attacker can supply any attributes in the allowed HTML tags, in the same
way as the previous stored XSS vulnerabilities affecting WordPress.

The vulnerability bears a similarity to the one reported by Cedric Van
Bockhaven in 2014 (patched this week, after 14 months). Instead of using an
invalid UTF-8 character to truncate the comment, this time an excessively
long comment text is used for the same effect.

In these two cases the injected JavaScript apparently can't be triggered in
the administrative Dashboard, so these exploits require getting around
comment moderation e.g. by posting one harmless comment first.

*Proof of Concept*
Enter the following as a comment:

<a title='x onmouseover=alert(unescape(/hello%20world/.source))
AAAAAAAAAAAA [64 kb] ...'></a>

This was tested on WordPress 4.2, 4.1.2, and 4.1.1, MySQL versions 5.1.53
and 5.5.41.

Disable comments (Dashboard, Settings/Discussion, select as restrictive
options as possible). Do not approve any comments.

The vulnerability was discovered by Jouko Pynnönen of Klikki Oy.

An up-to-date version of this document: http://klikki.fi/adv/wordpress2.html

Jouko Pynnönen <jouko () iki fi>
Klikki Oy - http://klikki.fi - @klikkioy

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Source: 48/rpA/5102/erusolcsidlluf/gro.stsilces

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