It is possible to read the contents of an extension's XML file and find the version number of the installed extension. This could allow people to exploit a known security flaws for a specific version of an extension.
All 1.5.x installs prior to and including 1.5.14 are affected.
Turn on Apache mod_rewrite and configure your .htaccess file to filter out XML files. In the htaccess.txt file shipped with version 1.5.15, lines 35-39 contain example code that will deny access to XML files. You can incorporate this code (or similar code) into your .htaccess file. Be sure to test that it does not cause problems on your site.
Reported by WHK and Gergő Erdősi
The JSST at the Joomla! Security Center.