Python Incorrect Type Conversion or Cast Vulnerability - CVE-2025-12781

When passing data to the b64decode() standard_b64decode() and urlsafe_b64decode() functions in the quotbase64quot module the characters quot/quot will always be accepted regardless of the value of quotaltcharsquot parameter typically used to establish an quotalternative base64 alphabetquot such as the URL safe alphabet. This behavior matches what is recommended in earlier base64 RFCs but newer RFCs now recommend either dropping characters outside the specified base64 alphabet or raising an error. The old behavior has the possibility of causing data integrity issues. This behavior can only be insecure if your application uses an alternate base64 alphabet (without quot/quot). If your application does not use the quotaltcharsquot parameter or the urlsafe_b64decode() function then your application does not use an alternative base64 alphabet. The attached patches DOES NOT make the base64-decode behavior raise an error as this would be a change in behavior and break existing programs. Instead the patch deprecates the behavior which will be replaced with the newly recommended behavior in a future version of Python.Users are recommended to mitigate by verifying user-controlled inputs match the base64 alphabet they are expecting or verify that their application would not be affected if the b64decode() functions accepted quotquot or quot/quot outside of altchars.