SUPEE-10266 is an important security update for Magento to address remote code execution and SQL injection vulnerabilities.
Where to download
Its best downloading the patch from the Magento.com download section - or via MDA - the Magento download tool (this is what this guide will use).
Applying the patch
The application of this patch is relatively straightforward. I'd recommend doing this in staging/development before attempting on live (if you don't have a dev. site follow this guide to create one).
Change directory to your Magento document root and fetch
mda.phar(the Magento downloader tool), if using Enterprise, refer to the documentation here to provide your id/token.
cd /microcloud/data/domains/example/domains/example.com/http wget -O mda.phar --no-check-certificate https://raw.githubusercontent.com/sonassi/magento-download-archive/master/bin/mda.phar php mda.phar
Select either CE/EE patch as appropriate,
1: Ce-patch 3: Ee-patch
Select the auto detected version of Magento,
0: 126.96.36.199 (auto detected)
Select the SUPEE-10266 patch (or press
mto download all missing patches),
17: Missing SUPEE-10266 for CE 188.8.131.52-184.108.40.206 (PATCH_SUPEE-10266_CE_220.127.116.11_v1.sh/SUPEE-10266)
Copy the patch to your Magento document root and apply it,
cp ./downloads/PATCH_SUPEE-10266* . bash PATCH_SUPEE-10266*
Clean your Magento cache using MageRun,
Admin login message
Logging into the admin after an extended period of time will yield,
Invalid Secret Key. Please refresh the page.
This message, whist a nuisance, is believed harmless.
Custom admin themes
Users with custom admin themes will need to ensure the changes made to the following files by the patch are correctly updated. It is recommended to temporarily disable any custom admin theme until updated.