FKIE_CVE-2025-12781

Vulnerability from fkie_nvd - Published: 2026-01-21 20:16 - Updated: 2026-02-02 17:25
Severity ?
5.3 (Medium) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N
Summary
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.
References
cna@python.orghttps://github.com/python/cpython/commit/13360efd385d1a7d0659beba03787ea3d063ef9bPatch
cna@python.orghttps://github.com/python/cpython/commit/1be80bec7960f5ccd059e75f3dfbd45fca302947Patch
cna@python.orghttps://github.com/python/cpython/commit/9060b4abbe475591b6230b23c2afefeff26fcca5Patch
cna@python.orghttps://github.com/python/cpython/commit/e95e783dff443b68e8179fdb57737025bf02ba76Patch
cna@python.orghttps://github.com/python/cpython/commit/fd17ee026fa9b67f6288cbafe374a3e479fe03a5Patch
cna@python.orghttps://github.com/python/cpython/issues/125346Exploit, Issue Tracking
cna@python.orghttps://github.com/python/cpython/pull/141128Issue Tracking, Patch
cna@python.orghttps://mail.python.org/archives/list/security-announce@python.org/thread/KRI7GC6S27YV5NJ4FPDALS2WI5ENAFJ6/Mailing List, Vendor Advisory
Impacted products
Vendor Product Version
python python *
python python *
python python 3.15.0
To clipboard 

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  "id": "CVE-2025-12781",
  "lastModified": "2026-02-02T17:25:23.933",
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Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
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  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



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