CVE-2025-12781

Publication date 23 January 2026

Last updated 23 January 2026

Ubuntu priority

Medium

Why this priority?

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.

Status

Package Ubuntu Release Status
pypy3 25.10 questing
Needs evaluation
24.04 LTS noble
Needs evaluation
22.04 LTS jammy
Needs evaluation
20.04 LTS focal
Needs evaluation
python2.7 25.10 questing Not in release
24.04 LTS noble Not in release
22.04 LTS jammy
Needs evaluation
20.04 LTS focal
Needs evaluation
18.04 LTS bionic
Needs evaluation
16.04 LTS xenial
Needs evaluation
14.04 LTS trusty
Needs evaluation
python3.4 25.10 questing Not in release
24.04 LTS noble Not in release
22.04 LTS jammy Not in release
14.04 LTS trusty
Needs evaluation
python3.5 25.10 questing Not in release
24.04 LTS noble Not in release
22.04 LTS jammy Not in release
16.04 LTS xenial
Needs evaluation
14.04 LTS trusty
Needs evaluation
python3.6 25.10 questing Not in release
24.04 LTS noble Not in release
22.04 LTS jammy Not in release
18.04 LTS bionic
Needs evaluation
python3.7 25.10 questing Not in release
24.04 LTS noble Not in release
22.04 LTS jammy Not in release
18.04 LTS bionic
Needs evaluation
python3.8 25.10 questing Not in release
24.04 LTS noble Not in release
22.04 LTS jammy Not in release
20.04 LTS focal
Needs evaluation
18.04 LTS bionic
Needs evaluation
python3.9 25.10 questing Not in release
24.04 LTS noble Not in release
22.04 LTS jammy Not in release
20.04 LTS focal
Needs evaluation
python3.10 25.10 questing Not in release
24.04 LTS noble Not in release
22.04 LTS jammy
Needs evaluation
python3.11 25.10 questing Not in release
24.04 LTS noble Not in release
22.04 LTS jammy
Needs evaluation
python3.12 25.10 questing Not in release
24.04 LTS noble
Needs evaluation
22.04 LTS jammy Not in release
python3.13 25.10 questing
Needs evaluation
24.04 LTS noble Not in release
22.04 LTS jammy Not in release
python3.14 25.10 questing
Needs evaluation
24.04 LTS noble Not in release
22.04 LTS jammy Not in release

References

Other references