Moveble Type <= v3.33 XSS filter evasion vulnerability

Tuesday, 6 February 2007

[Description]
MT (Movable Type) is a Blog software.
MT has a XSS filter to remove scripts from user inputs,
but there are ways to evade the filter using malformed input.

[Affected]
Movable Type <= 3.33

[Exploit]
By the default, Blog readers are allowed to post comments
containing html tags.

Attackers may post malformed comments as below.

1. NULL byte in number entitiy reference.
<A href="javascript[0x00]8;alert();">link</A>

2. Unfinished tag in the tail of comment.
<P><BR style="xss:expression(alert())"

MT's filter fails to sanitize these comments.
Scripts in these comments may run in certain browsers
(maybe in IE ONLY).

[Impact]
- Cookies theft.
- Web pages defacing.

[Solution]
Upgrade MT to the newest version.
Six Apart fixed these problems in v3.34.

[Links]
http://www.sixapart.com/movabletype/beta/distros/MT-3.34-beta-Release-No
tes.html
See #46226.

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teracci2002 (at) yahoo.co (dot) jp