ModSecurity: more security by obscurity

Yesterday, Philippe Baumgart showed me that my obscurity setup is not yet perfect. In fact, he could very easily enter an URL that didn’t exist and caused the webserver behind my proxy to respond with a 404. In this 404 the name and the version of the webserver were exposed.

After some testing i found that adding the following to my config worked very well.

# enable output scanning in Mod Security.
SecFilterScanOutput On

# hide outgoing 404 by webserver behind proxy
SecFilterSelective OUTPUT_STATUS 404 deny,status:404

This catches outgoing 404 errors, and replaces them by the 404 from the proxy. For some reason, this still didn’t exaclty look like the 404 from the proxy itself, because it contained a message that an additional 404 was encountered. I solved this by changing the ErrorDocument in de Apache config:

ErrorDocument 404 “<html><head><title>404 Not Found</title></head><body><h1>Not Found</h1><p>The requested URL was not found on this server.</p></body></html>”

After this, there was no longer any difference between 404′s produced by the proxy and by the webserver behind it.

Next, Phil showed me that i also leaked my version number of PHP. By using WordPress hiding the fact that I use PHP is impossible and pointless, but hiding the exact version still looks like a good idea. The version was leaked in the header from a server response: X-Powered-By: PHP/4.4.1build1. Solving this requires the mod_header module again:

# unset X-Powered-By to prevent leaking the PHP version
Header unset X-Powered-By

This hides this header. Thanks to Phil for doing some pen-testing :-)

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