acFreeProxy Cross-Site Scripting Vulnerability/Possible DoS

["Matthew Murphy" <mattmurphy () kc rr com>]

Product Information

acFreeProxy (aka "acfp") is an HTTP/1.x proxy for Microsoft Windows
environments.  It offers caching, and several other features, and has a
plug-in format designed for extensibility.  A flaw in the product may allow
attackers to execute content across domains.

Description

The proxy server may generate an error message if given a host that it
cannot reach, or some other exceptional condition.  The error page generated
during this process does not have any input validation, and is vulnerable to
cross-site scripting.  This allows an attacker to inject code as *any site*
the victim can visit, because this problem is in the proxy, and not a
specific site.

Impact

This vulnerability is significantly more dangerous than any site-specific
flaw, as it can be exploited to read content from any domain, instead of the
limited scope of a typical cross-site scripting flaw, where the site that is
flawed is the only site that can be impacted.

Exploit

http://www.hotmail.com:41997/%3CSCRIPT%3Ealert%28document%3EURL%29%3C/SCRIPT
%3E/

If a vulnerable proxy is being run, script execution begins.

I've also found bizarre crash behavior within acfp.  When it accesses
www.hotmail.com it crashes for some reason that I have yet to isolate.  I
believe that this may have something to do with empty entities in responses.
Any ideas?