CVE-2025-12781

MEDIUM EPSS 39.8%
Published Jan 21, 20265mo ago · Modified Jun 17, 20261w ago
6.3 CVSS 4.0
Medium
Find Similar
Published Jan 21, 2026 5mo ago
Last Modified Jun 17, 2026 1w ago

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.

CVSS Details

Base Score
6.3
Exploitability
—
Impact
—
Vector string
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
Attack Vector Network
Attack Complexity High
Privileges Required None
User Interaction None
Scope X

Threat Intelligence

EPSS Exploit Probability
39.8% percentile
Exploit & Patch Status
Public Exploit Known
Patch Available

Weaknesses 1

CWE-704

Affected Products 3

VendorProductVersionRange
pythonpython* <3.13.10
pythonpython*≥3.14.0  –  <3.14.1
pythonpython3.15.0any

References 8

  • github.com https://github.com/python/cpython/commit/13360efd385d1a7d0659beba03787ea3d063ef9b
    Patch
  • github.com https://github.com/python/cpython/commit/1be80bec7960f5ccd059e75f3dfbd45fca302947
    Patch
  • github.com https://github.com/python/cpython/commit/9060b4abbe475591b6230b23c2afefeff26fcca5
    Patch
  • github.com https://github.com/python/cpython/commit/e95e783dff443b68e8179fdb57737025bf02ba76
    Patch
  • github.com https://github.com/python/cpython/commit/fd17ee026fa9b67f6288cbafe374a3e479fe03a5
    Patch
  • github.com https://github.com/python/cpython/issues/125346
    ExploitIssue Tracking
  • github.com https://github.com/python/cpython/pull/141128
    Issue TrackingPatch
  • mail.python.org https://mail.python.org/archives/list/security-announce@python.org/thread/KRI7GC6S27YV5NJ4FPDALS2WI5ENAFJ6/
    Mailing ListVendor Advisory

Remediation

  • github.com https://github.com/python/cpython/commit/13360efd385d1a7d0659beba03787ea3d063ef9b
    Patch
  • github.com https://github.com/python/cpython/commit/1be80bec7960f5ccd059e75f3dfbd45fca302947
    Patch
  • github.com https://github.com/python/cpython/commit/9060b4abbe475591b6230b23c2afefeff26fcca5
    Patch
  • github.com https://github.com/python/cpython/commit/e95e783dff443b68e8179fdb57737025bf02ba76
    Patch
  • github.com https://github.com/python/cpython/commit/fd17ee026fa9b67f6288cbafe374a3e479fe03a5
    Patch
  • github.com https://github.com/python/cpython/pull/141128
    Issue TrackingPatch