avro-schema-recursion-poc

PoC: Uncontrolled Recursion in Avro Schema Parsing → Process Crash (SIGSEGV)

Affected: avro (Apache Avro Python package) 1.12.1 Vulnerability class: CWE-674 (Uncontrolled Recursion)

What this repo contains

malicious_schema.avsc — a valid-syntax Avro schema file, 8,756 bytes, consisting of 350 nested "array" type wrappers around a base "long" type. Every individual level is syntactically valid Avro; only the nesting depth is abusive.

How to reproduce

pip install avro
python3 reproduce.py

On an affected system, this crashes the Python process with SIGSEGV (exit code 139 / "Segmentation fault") — not a caught Python exception — when the schema is parsed on a thread with a reduced stack size (256KB, the standard default greenlet stack size used by gevent/eventlet, common in async Kafka/AMQP consumer deployments written in Python).

On a default (full-size) thread stack, the same file only produces a catchable RecursionError — the crash specifically requires the reduced-stack execution context, which is realistic but not universal. See the full report for details.

Why this matters

Any Python service that (a) accepts externally-supplied Avro schemas (e.g. a Kafka consumer that reads schema definitions from message metadata or a schema registry) and (b) runs on a gevent/eventlet-style worker, can have its entire worker process crashed by a single ~9KB malicious schema — not just the one message being processed.

Full write-up, environment details, threshold measurements, and suggested remediation are in the accompanying disclosure report.