Summary

Any project that uses Protobuf pure-Python backend to parse untrusted Protocol Buffers data containing an arbitrary number of recursive groups, recursive messages or a series of SGROUP tags can be corrupted by exceeding the Python recursion limit.

Reporter: Alexis Challande, Trail of Bits Ecosystem Security Team
[email protected]

Affected versions: This issue only affects the pure-Python implementation of protobuf-python backend. This is the implementation when PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION=python environment variable is set or the default when protobuf is used from Bazel or pure-Python PyPi wheels. CPython PyPi wheels do not use pure-Python by default.

This is a Python variant of a previous issue affecting protobuf-java.

Severity

CVE-2025-4565 High CVSS4.0 Score 8.2 (NOTE: there may be a delay in publication)
This is a potential Denial of Service. Parsing nested protobuf data creates unbounded recursions that can be abused by an attacker.

Proof of Concept

For reproduction details, please refer to the unit tests decoder_test.py and message_test

Remediation and Mitigation

We have been working diligently to address this issue and have released a mitigation that is available now. Please update to the latest available versions of the following packages:

• protobuf-python(4.25.8, 5.29.5, 6.31.1)