CVE-2025-12781: base64.b64decode() always accepts "+/" characters, despite setting altchars

Vulnerability summary

CVE-2025-12781 is a medium (CVSS 6.3) Incorrect Type Conversion or Cast vulnerability in python software foundation cpython. EPSS estimates a 0.86% chance of exploitation in the next 30 days, higher than 58.5% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 21-01-2026
   PublishedVulnerability first disclosed.
2. 22-01-2026
   Last modifiedVulnerability information updated.

Description

When passing data to the b64decode(), standard_b64decode(), and urlsafe_b64decode() functions in the "base64" module the characters "+/" will always be accepted, regardless of the value of "altchars" parameter, typically used to establish an "alternative base64 alphabet" 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 "+/"). If your application does not use the "altchars" 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 "+" or "/" outside of altchars.

• CWE-704Incorrect Type Conversion or CastThe product does not correctly convert an object, resource, or structure from one type to a different type.