Code insertion in Blogger comments

[Antone Roundy <antone@xxxxxxxxxxxxxx>]

Having notified Blogger of this twice over the course of a number of 
months, and not seeing them take any action (beyond saying that they'll 
look at it) or warn their users, I think it's time to warn people.  
Under the following conditions, Blogger weblogs are vulnerable to 
executable code insertion by third parties:

* Comments must be enabled.
* The server must support server-side processing, such as PHP, ASP, 
SSI, etc. (I'm pretty sure Blogspot-hosted blogs are NOT vulnerable).
* The Archive Filename (in the Settings/Archiving tab) must have an 
extension which triggers server-side processing, such as .php, .asp, 
.shtml, etc.  Depending on one's server configuration, files with 
extensions like .html and .htm may also be server-side-processed--no 
particular extension is necessarily safe.
* It may be necessary to have individual post pages enabled (also in 
the Settings/Archiving tab)--I haven't checked where the comments go 
with that setting off.

Under these circumstances, an attacker may inject executable code into 
the archive page by posting a comment to the weblog because, while 
Blogger automatically strips most HTML from comments, they do not strip 
processing instructions.  Blogger should be stripping out EVERYTHING 
between a "<" and the next ">" unless it is one of the allowed HTML 
tags, or should be stripping all unapproved HTML and converting any 
remaining "<" characters that aren't part of approved HTML to &lt;.

Antone Roundy
antone@xxxxxxxxxxxxxx

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