There is a patch from joomlacode issue tracker:

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=29646

But it was not applied to official 2.5.x branch, e.g. see the "evals" in this file:

https://github.com/joomla/joomla-cms/blob/2.5.x/libraries/joomla/updater/adapters/collection.php