Bug 2431736 (CVE-2025-12781)

Summary: CVE-2025-12781 cpython: base64.b64decode() always accepts "+/" characters, despite setting altchars
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: bbrownin, dfreiber, drow, gotiwari, jburrell, jgrulich, jhorak, jkoehler, ljawale, lphiri, luizcosta, mvyas, nweather, rbobbitt, sdawley, teagle, tpopela, vkumar
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the base64 module in the Python standard library. The b64decode, standard_b64decode and urlsafe_b64decode functions will always accept the '+' and '/' characters even when an alternative base64 alphabet is specified via the altchars parameter that excludes them. This input validation bypass allows malformed or unexpected data to pass through decoding filters, potentially causing logical errors or data integrity issues in applications relying on strict character sets.
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Description OSIDB Bzimport 2026-01-21 20:01:19 UTC 
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.