GHSA-p2mm-64f6-gmpp

Summary

fastapi-code-generator (koxudaxi) emits the OpenAPI path into the generated main.py route decorator as a single-quoted string literal (@app.get('/u/<path>/x')) without escaping the '. A ' in the path closes the decorator string and, via concatenation, injects a live expression that the decorator argument evaluates when main.py is imported (the ASGI server imports it at startup) -- code execution, no request needed. The path is a pure data field. Verified on fastapi-code-generator 0.8.0 / Python 3 / FastAPI.

Details

@app.get('/u/'+(__import__("os").system('…') or "")+'/x', response_model=None)
def get_thing() -> None:
 pass

The generator post-processes output with black/isort, so a syntax-breaking payload aborts generation (DoS); a valid-Python concat ('+expr+', kept valid with (… or "")) is written and runs at import. info.description (description='…'), tags, and parameter defaults are '-escaped; the path in @app.get('…') is the unescaped sink. Models come from datamodel-code-generator (repr-escaped, no RCE).

Not a duplicate: distinct product from OpenAPI Generator's python-fastapi backend (different codebase/template/decorator); and distinct from the operation-summary docstring sink in the same product.

PoC

reproduce.sh (+ make_spec.py) attached: generates the app and merely imports main.py; the injected expression runs at decorator evaluation and writes a marker. Verified on 0.8.0.

Impact

Code execution at import/startup in any application/CI that generates a FastAPI app with fastapi-code-generator from an attacker-controlled or attacker-influenced spec and imports/serves it. Estimated Critical, e.g. CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N.

Suggested fix

Escape the path for the Python string literal (escape ', , newlines via repr) before emitting @app.get('…'), and validate path characters.

maintainer-report.txt make_spec.py reproduce.sh