CVE-2025-12781

What it is

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.

At a glance

Exploitability

Can be exploited over the network without logging in. Limited direct impact.

Exploit odds

0.9% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2026-01-21 20:16 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-06-17 08:32 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 0.9% in the next 30 days (EPSS).
• Severity: 6.3 MEDIUM (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2026-01-21 20:16 UTC

Last modified

2026-06-17 08:32 UTC

Exploit probability

0.9% · EPSS, next 30 days

Weakness

CWE-704

Record status

Analyzed

Affected

• Python < 3.13.10
• Python < 3.14.1
• Python 3.15.0

Attack profile CVSS 4.0 · 6.3

Attack vector

Network

Complexity

High

Attack requirements

Present

Privileges needed

None

User interaction

None

Confidentiality

None

Integrity

Low

Availability

None

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2025-12781.md and follow it. Check whether this project is affected by CVE-2025-12781. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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