Below is a centralised list of useful Magento security patch links involving the most recent patches for Magento Community/Open Source 1.x versions.
All credit goes to those who took time out to answer questions, write tools and provide information to help users patch their Magento store.
Community information:
https://magento.stackexchange.com/questions/215292/security-patch-supee-10570-possible-issues
Community information:
Community information:
https://magento.stackexchange.com/questions/203567/security-patch-supee-10415-possible-issues
Community information:
https://magento.stackexchange.com/questions/193279/security-patch-supee-10266-possible-issues
Community information:
https://magento.stackexchange.com/questions/176871/security-patch-supee-9767-possible-issues
Magento has released a v2 of this security patch to address many of the issues from the initial patch. If you applied V1, you should revert and then apply V2.
Tool that checks if your custom checkout templates require a form key:
https://github.com/experius/Magento-1-Experius-Patch-Helper#magento
Tool that adds a form key to your custom checkout templates.
Linux: https://gist.github.com/schmengler/c42acc607901a887ef86b4daa7a0445b
OSX: https://gist.github.com/aadmathijssen/fa5f30b7e5a59a57fff3f78d1404e3a1
Community information:
PayPal post:
https://www.paypal-knowledge.com/infocenter/index?page=content&id=FAQ1916
Community information:
Information regarding mail sending settings in the Magento admin:
https://magento.com/security/news/new-zend-framework-1-security-vulnerability
Community information:
https://magento.stackexchange.com/questions/140550/security-patch-supee-8788-possible-problems
Community information:
https://magento.stackexchange.com/questions/98220/security-patch-supee-7405-possible-problems
Community information:
https://magento.stackexchange.com/questions/98220/security-patch-supee-7405-possible-problems
Tool to check Magento files for appsec affected code:
https://github.com/Schrank/magento-appsec-file-check
Community information:
https://magento.stackexchange.com/questions/76600/magento-security-patch-supee-6482-what-is-patched
Community information:
https://magento.stackexchange.com/questions/73481/supee-6285-patch-what-has-been-changed