CVE-2025-4565: Unbounded recursion in Python Protobuf
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 PROTOCOLBUFFERSPYTHONIMPLEMENTATION=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 decodertest.py and messagetest
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)
Other sources
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. This can result in a Denial of service by crashing the application with a RecursionError. We recommend upgrading to version =>6.31.1 or beyond commit 17838beda2943d08b8a9d4df5b68f5f04f26d901
— Red Hat
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. This can result in a Denial of service by crashing the application with a RecursionError. We recommend upgrading to version =6.31.1 or beyond commit 17838beda2943d08b8a9d4df5b68f5f04f26d901
— IBM
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
pip/protobufto a version that resolves this vulnerability.Fixed in 6.31.1 - Upgrade
Upgrade
pip/protobufto a version that resolves this vulnerability.Fixed in 5.29.5 - Upgrade
Upgrade
pip/protobufto a version that resolves this vulnerability.Fixed in 4.25.8 - Upgrade
Upgrade
protobuf-pythonto a version that resolves this vulnerability.Fixed in 6.31.1Patch 17838beda2943d08b8a9d4df5b68f5f04f26d901
Event History
Frequently Asked Questions
What is the severity of CVE-2025-4565?
CVE-2025-4565 has been classified with a high severity rating due to its potential to cause denial of service through resource exhaustion.
How do I fix CVE-2025-4565?
To fix CVE-2025-4565, upgrade your protobuf package to version 6.31.1 or newer, 5.29.5 or newer, or 4.25.8 or newer.
What software is affected by CVE-2025-4565?
CVE-2025-4565 affects projects using the pure-Python backend of the protobuf library with specific vulnerable versions.
What type of attack does CVE-2025-4565 enable?
CVE-2025-4565 enables denial of service attacks via recursive groups and messages in untrusted Protocol Buffers data.
What should I do if I cannot upgrade my protobuf package due to dependencies and I have CVE-2025-4565?
If you cannot upgrade due to dependencies, consider implementing strict input validation and limiting the size of Protocol Buffers data processed.