Strange domains in mod_pagespeed cache folder

Question

About a year ago I have installed mod_pagespeed on my VPS server, set it up and left it running. Recently I was exploring files on my server, went to pagespeed cache folder and discovered some strange folders.

All folders usually named this way ,2Fwww.mydomain.com or ,2F111.111.111.111 for IP addresses. I was surprised to see some domains that does not belong to me, like:

24x7-allrequestsallowed.com
allrequestsallowed.com
m.odnoklassniki.ru
www.fbi.gov
www.securitylab.ru

It looks like something dodgy is going on, was my server compromised, is there any reasonable explanation?

Answer 1

That does look peculiar. Everything in the cache folder should be files that mod_pagespeed tried to rewrite. There are two ways that I know of that this can happen:

1) You reference some third-party resource (say an image from another domain, or google analytics script) and you have explicitly enabled rewriting of that domain with ModPagespeedDomain www.example.com or ModPagespeedDomain *.

2) If your server accepts HTTP requests with invalid Host headers. Try (for example) wget --header="Host: www.fbi.gov" www.yourdomain.com/foo/bar.html. If your server accepts requests like that it may be providing mod_pagespeed with an incorrect base domain, and then subresources would be fetched from the same domain (so if www.yourdomain.com/foo/bar.html references some.jpeg, and your server accepts invalid host headers, we could fetch www.fbi.gov/foo/some.jpeg as the resource). There was a recent security release that makes sure all of these subrequests are done against localhost (not arbitrary third-party websites). Please see: https://developers.google.com/speed/docs/mod_pagespeed/CVE-2012-4001

You might want to look through these folders and see what specific resources are in there. I think that the biggest concern you should have is that someone might be trying to perform an XSS attack on your users or maybe a DDoS attack against another website (like www.fbi.gov), using your server as one vector. I do not think that these folders are indicative that your server itself is compromised.

If you would like to discuss this more, https://groups.google.com/forum/?fromgroups#!forum/mod-pagespeed-discuss is a good list to join and email.