W3 Total Cache Implementation Vulnerability

Just in time for Christmas, it was announced on the full disclosure list a security (configuration/implementation) bug on W3 Total cache (W3TC), one of the most popular WordPress plugins.

The issue is connected to the way W3TC stores the database cache (in a public accessible directory). It can be used to retrieve password hashes and other database information.

By default the plugin will store the caches inside /wp-content/w3tc/dbcache/ and if you have directory listing enabled, anyone can browse to yoursite.com/wp-content/w3tc/dbcache/ and download them. The second issue is that even if you don’t have directory listing enabled, it is still possible to guess those directories/files in order to extract the database cache queries and results.

There is more information about this issue on the full disclosure list: http://seclists.org/fulldisclosure/2012/Dec/242:

Confirmed vulnerability

We tested and were able to exploit this vulnerability on our test sites. Many shared hosts have directory listing enabled by default, making the problem even worse.

On the sites without directory listing, we were able to download the whole db cache directory remotely and then search for specific strings (user_pass) and get the password hashes:

a:6:{s:10:"last_error”;s:0:””;s:10:”last_query”;s:41:”SELECT * FROM wp_users WHERE ID = ’15′”;s:11:”last_result”;a:1:{i:0;O:8:”stdClass”:10:{s:2:”ID”;s:2:”15″;s:10:”user_login”;s:21:”Guest Blogger”;s:9:”user_pass”;s:34:”$P$BPtuFcIxFXXXX3MJbBBN4dxJ1″;s:13:”user_nicename”;..

Patching

It seems the easiest way to protect your sites is by disabling database cache or creating an .htaccess file inside the wp-content/w3tc directory denying direct access there:

deny from all

If you are not using Apache, you will need a similar configuration entry to prevent direct access to the w3tc folder.

If you have any questions, let us know.

UPDATE (30 December 2012): W3 Total Cache has been patched (0.9.2.5). Head over to the WordPress Plugin Directory to download or update right from your WordPress dashboard.

Here is the official response to the vulnerability from Frederick Townes, the plugin author:

For those of you that use W3 Total Cache to make your sites more performant, thank you. Security issues are always of paramount interest, no matter the scope.

The root of the possible vulnerability lies in the intersection of two configuration settings, one at the Web Server level and the other at the W3 Total Cache database caching level. You may be vulnerable if the following are true: your server is configured to allow directory listing with enabled public access on W3TC’s database caching directories and also use database caching via the disk caching method. These settings would allow a hacker to break the md5 hashing used for the then publicly accessible cached database objects. The manner, extent and timing of the vulnerability’s report leave much to be desired; nonetheless, the versions have now been patched on wordpress.org. Thanks to those that offered remediation advice. I’m sorry for the delay in turning this around, none of the proposed solutions were satisfactory.

The hotfix (tested with WordPress version 3.5) will help those who are just now upgrading to 0.9.2.4 or are otherwise getting started with W3 Total Cache. Specifically, the hash logic is improved via wp_hash(), significantly stronger than the previous md5 hashing at the compromise of a bit of speed. I’ve also made sure that a web server’s lack of security around directory listings and the standard file structure of W3TC’s hashing logic are no longer of consequence for those attempting to download them from your server.

For those who are using database caching to disk already, please be sure to disable directory indexing and deny web access to the “wp-content/w3tc/dbcache/” directory in your web configuration, then empty the database cache for good measure. Or, simply deactivate W3 Total Cache, uninstall it, and re-install it via wordpress.org to have the hotfix applied upon re-activation. Again, empty the database cache for good measure. Your settings will not be lost during this process. If all of this is gibberish to you, then simply disable database caching to disk until the next release or use another method if available. Once again, empty the database cache using the button of the same name available on the database caching settings tab.

If you’re reading this and have seen a post about the issue that does not have this response on it, please do post this for me. Thanks in advance. Happy Holidays.