So I just tried installing the patch on one of our stores and it failed. It seems like everything is fine except for one file. app/code/core/Mage/Core/Controller/Varien/Router/Admin.php. I'm not sure where to go from here. Here's the output I got.
|diff --git app/code/core/Mage/Core/Controller/Varien/Router/Admin.php app/code/core/Mage/Core/Controller/Varien/Router/Admin.php
|index 04a9869..2b33a72 100644
Patching file app/code/core/Mage/Core/Controller/Varien/Router/Admin.php using Plan A...
No such line 133 in input file, ignoring
Hunk #1 failed at 131.
1 out of 1 hunks failed while patching app/code/core/Mage/Core/Controller/Varien/Router/Admin.php
Hmm... The next patch looks like a unified diff to me...
The text leading up to this was:
have you applied all previous security patches? Check app/etc/applied.patches.list to see if you've missed one or more.
You should check app/code/core/Mage/Core/Controller/Varien/Router/Admin.php file with the default unmodified file of your Magento version.
You should download the Magento version which you are using and compare for any of the file changes in mentioned file.If there are changes in the file then keep backup of file and replace it with default magento file.
After this reapply the patch.
This is happening because you are missing one of the earlier patches (SUPEE-5994) that created the previously non-existent file:
Applying SUPEE-5994 will need to be done. Create the path app/code/core/Mage/Core/Controller/Varien/Router/ or this patch will fail to apply.