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 <.
Antone Roundy
antone@geckotribe.com
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