0xiviel/poc-avro-enum-bypass

PoC: Avro Negative Enum Index Bypass (fastavro + Apache Avro)

Summary

Both fastavro and Apache Avro Python libraries fail to validate negative enum indices. The bounds check if index >= len(symbols) misses negative values. In Python, list[-1] returns the last element, so a crafted .avro file can cause enum fields to return wrong symbol values.

Confirmed on fastavro 1.12.1 and avro 1.12.1 — GUEST role becomes ADMIN.

Root Cause

Apache avro io.py:769:

if index_of_symbol >= len(writers_schema.symbols):
    # Missing: if index_of_symbol < 0

fastavro _read_py.py:280:

writer_schema["symbols"][decoder.read_enum()]  # list[-1] = last element

Demonstrated Bypass

Schema: enum Role { GUEST, USER, ADMIN }
Written: role=GUEST (index 0)
Corrupted: index → -1 (zigzag byte 0x01)
Read: role=ADMIN  ← WRONG!

Same bug in union index: io.py:866-870 and _read_py.py:413.

Impact

• Data integrity violation in ML data pipelines
• Attacker-controlled enum symbol selection
• Affects BOTH major Python Avro libraries

Files

• poc_cython_overflow.py — combined PoC demonstrating enum bypass (section 2)

Reproduction

pip install fastavro avro
python poc_cython_overflow.py

Look for the "Negative Enum Index Bypass" section in output.

Affected Versions

• fastavro <= 1.12.1
• Apache avro (Python) <= 1.12.1