fastapi-code-generator: Import-time RCE via response status-code key -> unescaped @app.get(..., responses={...}) dict

Summary

fastapi-code-generator (koxudaxi) emits an operation's extra responses into the route decorator's responses={...} dict, using each response's status-code key as a single-quoted string literal without escaping the '. A ' in a status-code key closes the dict-key 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. The status-code key is a pure data field. Verified on fastapi-code-generator 0.8.0 / Python 3 / FastAPI.

Details

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

The responses={...} dict is emitted only when the operation has a second (non-primary) response, so the malicious key is any additional response status. The generator post-processes 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. The response content-type key is NOT a sink (normalized to 'model'). info.* are repr-escaped; operationId -> function_name fails closed (DoS-resistant).

Not a duplicate: same product as the path -> @app.get('…') and summary -> handler-docstring findings, but a different field (response status-code key) and position (the responses={...} decorator dict key). Distinct from OpenAPI Generator's python-fastapi. Models come from datamodel-code-generator (repr-escaped).

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 (multi-response operation) 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 each response status-code key for the Python string literal (escape ', , newlines via repr) before emitting the responses={...} dict, and validate that status-code keys are numeric / a known pattern. maintainer-report.txt make_spec.py reproduce.sh