CVE-2025-4516: Use-after-free in "unicode_escape" decoder with error handler

Vulnerability summary

CVE-2025-4516 is a medium (CVSS 5.9) Use After Free vulnerability in python software foundation cpython. EPSS estimates a 0.22% chance of exploitation in the next 30 days, higher than 13.3% of scored vulnerabilities.

Timeline

1. 15-05-2025
   PublishedVulnerability first disclosed.
2. 31-07-2026
   Last modifiedVulnerability information updated.

Description

There is an issue in CPython when using `bytes.decode("unicode_escape", error="ignore|replace")`. If you are not using the "unicode_escape" encoding or an error handler your usage is not affected. To work-around this issue you may stop using the error= handler and instead wrap the bytes.decode() call in a try-except catching the DecodeError.

• CWE-416Use After FreeThe product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.