What’s Vulnerable › Vulnerabilities

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

• Severity: medium (CVSS 5.9 v4.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2025-05-15 · updated 2026-07-31
• Weakness: CWE-416

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.

Affected products

• Python: before 3.9.23; from 3.10.0 before 3.10.18; from 3.11.0 before 3.11.13; from 3.12.0 before 3.12.11; from 3.13.0 before 3.13.4; from 3.14.0a1 before 3.14.0b2 — fixed in 3.10.18, 3.11.13, 3.12.11, 3.13.4, 3.14.0b2, 3.9.23

How to fix

Upgrade to a fixed version: Python 3.10.18, 3.11.13, 3.12.11, 3.13.4. Check the vendor advisory for mitigations.

References

• https://github.com/python/cpython/commit/4398b788ffc1f954a2c552da285477d42a571292
• https://github.com/python/cpython/commit/6279eb8c076d89d3739a6edb393e43c7929b429d
• https://github.com/python/cpython/commit/69b4387f78f413e8c47572a85b3478c47eba8142
• https://github.com/python/cpython/commit/73b3040f592436385007918887b7e2132aa8431f
• https://github.com/python/cpython/commit/8d35fd1b34935221aff23a1ab69a429dd156be77
• https://github.com/python/cpython/commit/9f69a58623bd01349a18ba0c7a9cb1dad6a51e8e
• https://github.com/python/cpython/commit/ab9893c40609935e0d40a6d2a7307ea51aec598b
• https://github.com/python/cpython/issues/133767
• https://github.com/python/cpython/pull/129648
• https://mail.python.org/archives/list/security-announce@python.org/thread/L75IPBBTSCYEF56I2M4KIW353BB3AY74/
• http://www.openwall.com/lists/oss-security/2025/05/16/4
• http://www.openwall.com/lists/oss-security/2025/05/19/1

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