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CVE-2025-4565 — Unbounded recursion in Python Protobuf

Affected: protocolbuffers Python-Protobuf

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

GHSA-8qvm-5x2c-j2w7 — protobuf-python has a potential Denial of Service issue

Affected: PyPI: 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`](https://protobuf.dev/programming-guides/encoding/#groups) tags** can be corrupted by exceeding the Python recursion limit.

Reporter: Alexis Challande, Trail of Bits Ecosystem Security Team [ecosystem@trailofbits.com](mailto:ecosystem@trailofbits.com)

Affected versions: This issue only affects the [pure-Python implementation](https://github.com/protocolbuffers/protobuf/tree/main/python#implementation-backends) 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](https://github.com/protocolbuffers/protobuf/security/advisories/GHSA-735f-pc8j-v9w8).

### 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](https://github.com/protocolbuffers/protobuf/blob/main/python/google/protobuf/internal/decoder_test.py#L87-L98) and [message_test](https://github.com/protocolbuffers/protobuf/blob/main/python/google/protobu…

PYSEC-2026-1806 — protobuf-python has a potential Denial of Service issue

Affected: PyPI: protobuf

Reporter: Alexis Challande, Trail of Bits Ecosystem Security Team [ecosystem@trailofbits.com](mailto:ecosystem@trailofbits.com)

This is a Python variant of a [previous issue affecting protobuf-java](https://github.com/protocolbuffers/protobuf/security/advisories/GHSA-735f-pc8j-v9w8).

### Severity This is a potential Denial of Service. Parsing nested protobuf data creates unbounded recursions that can be abused by an attacker.