Broken Access Control (A01:2025)

The #1 OWASP risk for two consecutive cycles. Found in 3.74% of applications with over 1.8 million occurrences and 32,654 CVEs. Access control enforces that users cannot act outside their intended permissions. Failures lead to unauthorized data disclosure, modification, or destruction.

For deep reference on all CWEs and remediation patterns, see references/bac-detail.md.

---

What This Skill Covers

Sub-category	Description	Key CWEs
Missing Authorization	Routes/endpoints with no auth guard	CWE-862, CWE-284
IDOR	Resource ownership not verified	CWE-639, CWE-285
CORS Misconfiguration	Wildcard or overly-permissive origins	CWE-284
SSRF	Outbound requests to internal/private IPs	CWE-918
CSRF	State-changing forms missing token validation	CWE-352
Privilege Escalation	Users accessing higher-privilege roles	CWE-269
JWT/Session Manipulation	Metadata tampering, session fixation	CWE-287

---

Workflow: Auditing a FastAPI or Flask App

Step 1 — Identify All Routes

FastAPI:

# List all routes and their dependencies
for route in app.routes:
    print(route.path, route.methods, route.dependencies)

Flask:

# Print all registered endpoints
for rule in app.url_map.iter_rules():
    print(rule.endpoint, rule.methods, rule.rule)

Flag any route that:

• Handles POST, PUT, PATCH, DELETE without an auth dependency
• Returns user-specific data (profile, orders, files) without ownership check
• Has no rate limiting on sensitive operations

---

Step 2 — Check Authorization Guards

FastAPI — look for missing Depends():

# VULNERABLE — no auth check
@app.get("/users/{user_id}/data")
async def get_user_data(user_id: int):
    return db.query(UserData).filter_by(owner_id=user_id).all()

# SECURE — requires authenticated user
@app.get("/users/{user_id}/data")
async def get_user_data(user_id: int, current_user: User = Depends(get_current_user)):
    if current_user.id != user_id:
        raise HTTPException(status_code=403, detail="Forbidden")
    return db.query(UserData).filter_by(owner_id=user_id).all()

Flask — look for missing @login_required:

# VULNERABLE
@app.route("/dashboard")
def dashboard():
    return render_template("dashboard.html")

# SECURE
@app.route("/dashboard")
@login_required
def dashboard():
    return render_template("dashboard.html")

---

Step 3 — IDOR Check (Resource Ownership)

IDOR is the most common BAC failure. Every query that fetches a user-owned resource must compare resource.owner_id against current_user.id.

# VULNERABLE IDOR — attacker can request any document_id
@app.get("/documents/{doc_id}")
async def get_doc(doc_id: int, user=Depends(get_current_user)):
    return db.get(Document, doc_id)  # No ownership check!

# SECURE
@app.get("/documents/{doc_id}")
async def get_doc(doc_id: int, user=Depends(get_current_user)):
    doc = db.get(Document, doc_id)
    if not doc or doc.owner_id != user.id:
        raise HTTPException(status_code=404)  # 404 preferred over 403 (avoids enumeration)
    return doc

Test vector: Authenticate as User A, record a resource ID, then authenticate as User B and request that same ID. If you get data — it's an IDOR.

---

Step 4 — CORS Misconfiguration

FastAPI:

# VULNERABLE — wildcard origin
app.add_middleware(CORSMiddleware, allow_origins=["*"], allow_credentials=True)

# SECURE — explicit allowlist
app.add_middleware(
    CORSMiddleware,
    allow_origins=["https://app.yourdomain.com"],
    allow_credentials=True,
    allow_methods=["GET", "POST"],
    allow_headers=["Authorization", "Content-Type"],
)

Flask:

# VULNERABLE
CORS(app, origins="*")

# SECURE
CORS(app, origins=["https://app.yourdomain.com"], supports_credentials=True)

⚠️ allow_origins=["*"] combined with allow_credentials=True is doubly dangerous — most browsers block this, but it signals deep misconfiguration.

---

Step 5 — SSRF (Now part of A01:2025)

Check any code that makes outbound HTTP requests with user-controlled URLs.

import ipaddress, re

PRIVATE_RANGES = [
    ipaddress.ip_network("10.0.0.0/8"),
    ipaddress.ip_network("172.16.0.0/12"),
    ipaddress.ip_network("192.168.0.0/16"),
    ipaddress.ip_network("127.0.0.0/8"),
    ipaddress.ip_network("169.254.0.0/16"),  # AWS metadata
]

def is_safe_url(url: str) -> bool:
    from urllib.parse import urlparse
    import socket
    try:
        host = urlparse(url).hostname
        ip = ipaddress.ip_address(socket.gethostbyname(host))
        return not any(ip in net for net in PRIVATE_RANGES)
    except Exception:
        return False

# In your endpoint:
@app.post("/fetch")
async def fetch_url(url: str, user=Depends(get_current_user)):
    if not is_safe_url(url):
        raise HTTPException(status_code=400, detail="URL not allowed")
    return await httpx.get(url)

Test vectors: http://169.254.169.254/latest/meta-data/ (AWS metadata), http://localhost:6379 (Redis), http://10.0.0.1/admin

---

Step 6 — CSRF (Flask-specific risk)

FastAPI APIs using JWT/Bearer tokens are generally CSRF-safe. Flask apps using session cookies need explicit protection.

# Flask — verify Flask-WTF is installed and CSRF is enabled
from flask_wtf.csrf import CSRFProtect
csrf = CSRFProtect(app)

# All state-changing forms must include {{ form.hidden_tag() }} or {{ csrf_token() }}

Check that WTF_CSRF_ENABLED = True in config (it is by default, but verify it hasn't been disabled).

---

Step 7 — JWT & Session Security

# Verify JWT expiry is set (short-lived: 15 min–1 hr)
jwt.encode({"sub": user_id, "exp": datetime.utcnow() + timedelta(minutes=30)}, SECRET_KEY)

# Flask — verify session security flags
app.config["SESSION_COOKIE_SECURE"] = True    # HTTPS only
app.config["SESSION_COOKIE_HTTPONLY"] = True  # No JS access
app.config["SESSION_COOKIE_SAMESITE"] = "Lax" # CSRF mitigation
app.config["PERMANENT_SESSION_LIFETIME"] = timedelta(hours=2)

---

Quick Audit Checklist

Run through this for any Python web app:

• Every non-public route has an auth dependency / decorator
• All user-owned resource queries verify resource.owner_id == current_user.id
• CORS origins are an explicit allowlist, not *
• Outbound HTTP calls validate URL is not in private IP ranges
• Flask apps have Flask-WTF CSRF protection on all state-changing forms
• JWTs have short exp claims; sessions have SECURE, HTTPONLY, SAMESITE flags
• Session IDs are invalidated on logout
• Access control failures are logged and alerted on

---

Automated Scanning

Tool	What it catches	Command
Bandit	SSRF patterns, hardcoded secrets, dangerous functions	bandit -r . -t B105,B106,B107,B501,B601
OWASP ZAP	IDOR, missing auth at runtime, CORS	zap-baseline.py -t http://localhost:8000
truffleHog	Leaked tokens/keys in git history	trufflehog git file://. --only-verified

---

Key CWEs Reference

CWE	Name	Typical Pattern
CWE-284	Improper Access Control	Route accessible without any auth
CWE-285	Improper Authorization	Auth present but role/ownership not checked
CWE-352	CSRF	State-changing endpoint accepts cookie auth, no CSRF token
CWE-639	Auth Bypass via User-Controlled Key	IDOR — GET /orders/{id} with no ownership check
CWE-862	Missing Authorization	No @login_required or Depends(get_current_user)
CWE-918	SSRF	requests.get(user_input_url) without validation
CWE-200	Sensitive Info Exposure	Unauthorized user receives another's PII/data

---

See Also

• references/bac-detail.md — Expanded CWE descriptions, more attack examples, compliance mapping
• OWASP A01:2025 official page: https://owasp.org/Top10/A01_2025-Broken_Access_Control/