8.2

CVSS Score

4.0

-

CVSS Score

Basic Information

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CVE-2025-4565

CVE-2025-4565: 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)

(GitHub Advisory)

Miggo Vulnerability Database

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CVE-2025-4565

CVE-2025-4565:

8.2

CVSS Score

4.0

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
protobuf	pip	< 4.25.8	4.25.8
protobuf	pip	>= 5.26.0rc1, < 5.29.5	5.29.5
protobuf	pip	>= 6.30.0rc1, < 6.31.1	6.31.1

Technical Details

Vulnerability Intelligence
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Root Cause Analysis

The vulnerability (CVE-2025-4565, GHSA-8qvm-5x2c-j2w7) is a Denial of Service in the pure-Python implementation of Google's Protocol Buffers library. It arises from uncontrolled recursion when parsing protobuf messages containing an arbitrary number of recursive groups, recursive messages, or a series of SGROUP tags.

The root cause is the lack of recursion depth checking in the decoder functions responsible for parsing message fields. Specifically, the functions DecodeRepeatedField, DecodeField, and _DecodeUnknownField in python/google/protobuf/internal/decoder.py would make calls that could lead to further recursion (e.g., calling _InternalParse on a sub-message or _DecodeUnknownFieldSet for nested groups) without first checking the current recursion depth.

An attacker could craft a malicious protobuf message with excessively deep nesting. When parsed by the vulnerable pure-Python decoder, these functions would recurse deeply, eventually exceeding Python's default recursion limit, leading to a RecursionError (surfaced as a DecodeError by the library) and causing a Denial of Service for the application processing the malicious data.

The commit 17838beda2943d08b8a9d4df5b68f5f04f26d901 mitigates this vulnerability by introducing a recursion counter (current_depth) and a configurable limit (_recursion_limit) within these identified decoder functions. Before making a call that could deepen the recursion, the current depth is incremented and checked against the limit. If the limit is exceeded, a _DecodeError is raised, preventing the stack overflow.

The identified functions are precisely where these checks were missing and subsequently added, making them the locations containing the vulnerability. During exploitation, these functions, along with Message._InternalParse (which is called by DecodeField and DecodeRepeatedField), would appear in the stack trace as the recursion unfolds.

Vulnerable functions

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WAF Rule

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Reasoning

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