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.
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
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.
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.