protobuf-python has a potential Denial of Service issue
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 ecosystem@trailofbits.com
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
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
A mitigation 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)
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | protobuf | < 4.25.8, >= 5.26.0rc1, < 5.29.5, >= 6.30.0rc1, < 6.31.1 | 4.25.8, 5.29.5, 6.31.1 |
References
- https://github.com/protocolbuffers/protobuf/security/advisories/GHSA-735f-pc8j-v9w8
- https://github.com/protocolbuffers/protobuf/security/advisories/GHSA-8qvm-5x2c-j2w7
- https://nvd.nist.gov/vuln/detail/CVE-2025-4565
- https://github.com/protocolbuffers/protobuf/commit/17838beda2943d08b8a9d4df5b68f5f04f26d901
- https://github.com/protocolbuffers/protobuf
- https://github.com/protocolbuffers/protobuf/blob/main/python/google/protobuf/internal/decoder_test.py#L87-L98
- https://github.com/protocolbuffers/protobuf/blob/main/python/google/protobuf/internal/message_test.py#L1436-L1478
- https://github.com/protocolbuffers/protobuf/tree/main/python#implementation-backends
- https://pypi.org/project/protobuf
- https://github.com/advisories/GHSA-8qvm-5x2c-j2w7
- CVSS vector
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N- Weakness (CWE)
CWE-674- Platform
- Python (PyPI)
- Package
protobuf- Disclosed
- 16.06.2025
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-1806 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-8qvm-5x2c-j2w7 - CC BY 4.0
Vulnerability history: protobuf
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 5
- First → latest
- 09.2017 01.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 5
Disclosures per year
- 2017: 1
- 2018: 0
- 2019: 0
- 2020: 0
- 2021: 0
- 2022: 2
- 2023: 0
- 2024: 0
- 2025: 1
- 2026: 1
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