### SummaryAny 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).### SeverityThis is a potential Denial of Service. Parsing nested protobuf data creates unbounded recursions that can be abused by an attacker.### Proof of ConceptFor 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/protobuf/internal/message_test.py#L1436-L1478)### Remediation and MitigationA 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)