CVE-2025-12781: Python Software Foundation CPython

base64.b64decode() always accepts "+/" characters, despite setting altchars

CVSS 6.3 MediumEPSS 0.0086 · 58th percentile

Published by the CVE Program (CNA: PSF) at 21 Jan 2026, 19:34 UTC. agentprime has not analysed this yet, so whether it affects any app is not known. This CVE names a vendor and product rather than a package, so the lockfile check doesn’t match it.

Affected products, as the CNA wrote them
Vendor	Product	Versions
Python Software Foundation	CPython	affected before 3.13.10 python affected 3.14.0 to before 3.14.1 python affected 3.15.0a1 to before 3.15.0a2 python Other versions: unaffected

Identifiers

CVE-2025-12781

CNA

PSF

Weakness (CWE)

CWE-704

Not analysed yet

We haven’t published our own analysis of this CVE. If a person reviews it, this page will say what’s affected and what has to be true for it to matter.

Severity and exploitation

Score	Rating	CVSS	Vector	Source
6.3	Medium	4.0	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	CNA (PSF)

CISA KEV Not listed.
EPSS 0.0086, the 58th percentile, on 5 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

This record came from the full history of the CVE Program, published 21 Jan 2026. New records show when each source confirmed them.

Sources

References from the CNA

Facts on this page come from the CVE List as the CNA published them, with CVSS from the sources named above, CISA KEV and FIRST EPSS. Vendors, products and versions are shown as the CNA wrote them. We don’t reproduce the CVE’s description.