CVE-2025-12781

Jan. 22, 2026, 9:15 p.m.

6.3
Medium

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.

Product(s) Impacted

Vendor Product Versions
Python
  • Base64
  • *

Weaknesses

Common security weaknesses mapped to this vulnerability.

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

*CPE(s)

Affected systems and software identified for this CVE.

Type Vendor Product Version Update Edition Language Software Edition Target Software Target Hardware Other Information
a python base64 / / / / / / / /

References

https://github.com/python/cpython/commit/13360efd385d1a7d0659beba03787ea3d063ef9b
https://github.com/python/cpython/commit/1be80bec7960f5ccd059e75f3dfbd45fca302947
https://github.com/python/cpython/commit/9060b4abbe475591b6230b23c2afefeff26fcca5
https://github.com/python/cpython/commit/e95e783dff443b68e8179fdb57737025bf02ba76
https://github.com/python/cpython/commit/fd17ee026fa9b67f6288cbafe374a3e479fe03a5
https://github.com/python/cpython/issues/125346
https://github.com/python/cpython/pull/141128
https://mail.python.org/archives/list/[email protected]/thread/KRI7GC6S27YV5NJ4FPDALS2WI5ENAFJ6/

Tags

[email protected]
CWE-704
2026-01-21
CVE-2025-12781

CVSS Score

6.3 / 10

CVSS Data - 4.0

  • Attack Vector: NETWORK
  • Attack Complexity: HIGH
  • Attack Requirements: PRESENT
  • Privileges Required: NONE
  • User Interaction: NONE
  • Scope:
  • Confidentiality Impact: NONE
  • Integrity Impact: LOW
  • Availability Impact: NONE
  • Exploit Maturity: NOT_DEFINED

    • CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

    View Vector String

Timeline

Published: Jan. 21, 2026, 8:16 p.m.
Last Modified: Jan. 22, 2026, 9:15 p.m.

Status : Received

CVE has been recently published to the CVE List and has been received by the NVD.

More info

Source

[email protected]

*Disclaimer: Some vulnerabilities do not have an associated CPE. To enhance the data, we use AI to infer CPEs based on CVE details. This is an automated process and might not always be accurate.