Data Integrity Vulnerability in Python's Base64 Module

CVE-2025-12781

What is CVE-2025-12781?

A data integrity vulnerability exists in Python's base64 module, specifically within the functions b64decode(), standard_b64decode(), and urlsafe_b64decode(). This issue arises when the '+' and '/' characters are indiscriminately accepted, regardless of the set 'altchars' parameter used to define an alternative base64 alphabet. While older base64 RFCs endorse this behavior, updated documentation suggests that characters outside the designated base64 set should either be rejected or cause an error. This discrepancy poses a risk in applications utilizing alternate base64 alphabets, potentially leading to compromised data integrity. The patch issued does not enforce a strict error approach, maintaining existing code functionality while deprecating the current encoding behavior for future revisions. Developers are advised to validate user inputs against the expected base64 alphabet to mitigate risks.

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