CVE-2025-12781 in CPython
Summary
by MITRE • 01/21/2026
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.
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Analysis
by VulDB Data Team • 04/24/2026
The vulnerability identified as CVE-2025-12781 pertains to a behavioral inconsistency in Python's base64 module that affects the b64decode(), standard_b64decode(), and urlsafe_b64decode() functions. This issue stems from the module's handling of the altchars parameter which is designed to specify an alternative base64 alphabet typically used for URL-safe encoding. The current implementation maintains backward compatibility by accepting the characters "+" and "/" regardless of the altchars parameter value, a behavior that originated from earlier base64 RFC specifications but conflicts with modern RFC recommendations.
The technical flaw manifests when applications expect strict adherence to a specified base64 alphabet while the functions continue to accept characters outside that alphabet. This creates potential data integrity issues because the functions do not validate that input data strictly conforms to the expected character set defined by altchars. According to CWE-20: Improper Input Validation, this represents a weakness where input validation is insufficient, potentially allowing malformed data to be processed. The vulnerability becomes particularly concerning when applications rely on alternative base64 alphabets without "+" and "/" characters, as the functions may silently accept invalid input that would otherwise be rejected by strict RFC-compliant implementations.
Operationally, this vulnerability impacts applications that process user-controlled data through base64 decoding functions, particularly those implementing security-sensitive operations such as authentication tokens, encrypted data handling, or API request processing. The risk is elevated when applications assume that the altchars parameter enforces strict character validation, leading to potential security bypasses or data corruption scenarios. Attackers could potentially exploit this behavior by crafting base64-encoded data that includes "+" or "/" characters in contexts where these characters should be rejected, potentially causing unexpected behavior in downstream processing. This aligns with ATT&CK technique T1070.004: Indicator Removal on Host, where improper validation might allow malicious input to bypass security controls.
The Python development team has addressed this issue by deprecating the current behavior rather than implementing a breaking change that would affect existing applications. This approach follows the principle of maintaining backward compatibility while signaling future changes through deprecation warnings. The patch does not immediately raise errors upon encountering invalid characters, which would constitute a behavioral change that could break existing codebases. Instead, the deprecation serves as a warning to developers that their applications may need to be updated to handle the stricter validation that will be implemented in future Python versions. Organizations should implement mitigation strategies by validating user-controlled inputs against expected base64 character sets or by ensuring their applications would not be negatively affected by the relaxed validation behavior, particularly when dealing with security-critical applications that depend on strict base64 decoding rules.