CVE-2025-12781 base64.b64decode() always accepts "+/" characters, despite setting altchars
Python's base64 decoder accepts standard characters even when an alternative alphabet is specified, potentially causing data integrity issues in applications relying on strict validation.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-12781?
CVE-2025-12781 is a medium-severity vulnerability in Python Python, published on 21 Jan 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-704
Which versions are affected by CVE-2025-12781?
3 open source packages ship an affected version, including libpython, python and python-min. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
libpython Bitnami | All versions | 3.15.0 |
python Bitnami | All versions | 3.15.0 |
python-min Bitnami | All versions | 3.15.0 |
Products include Python Python.
How do I fix CVE-2025-12781?
Upgrade libpython to 3.15.0 or later.
Is CVE-2025-12781 being exploited?
There is no known exploitation of CVE-2025-12781 in the wild and no public exploit code.
Other vulnerabilities in Python Python
Sources
Also known as AZL-109683, AZL-75225, AZL-75231, AZL-84287, BELL-CVE-2025-12781, BIT-libpython-2025-12781 and 15 more.