Authentication Failures — OWASP A07:2025

Rank: #7 (stable) | CWEs covered: 36 | Occurrences: 1.1 million+ New 2025 threat: Hybrid password attacks / password spray — attackers increment leaked credentials (e.g., Password1! → Password2!).

What This Skill Covers

Authentication Failures occur when an application's identity verification mechanisms are absent, weak, or bypassable. Key attack patterns:

• Credential stuffing / brute force — automated login attempts using leaked creds
• Weak/default passwords — accounts created with guessable or known-breached passwords
• Insecure credential recovery — password reset flows that leak information or are bypassable
• Missing or bypassed MFA — no second factor, or MFA that can be skipped
• Exposed session identifiers — tokens in URLs, logs, or referrer headers
• Session fixation — attacker sets a known session ID before authentication
• Improper session invalidation — sessions that survive logout or token revocation

Testing Checklist for FastAPI & Flask

Work through each section systematically. Mark ✓ pass, ✗ fail, N/A where not applicable.

1. JWT Implementation

# ✓ GOOD — short-lived tokens, claim validation
token = jwt.encode(
    {"sub": user_id, "exp": datetime.utcnow() + timedelta(minutes=15),
     "iss": "myapp", "aud": "myapp-client"},
    settings.SECRET_KEY, algorithm="HS256"
)
jwt.decode(token, settings.SECRET_KEY, algorithms=["HS256"],
           audience="myapp-client", issuer="myapp")

# ✗ BAD — no expiry, no claim validation, accepts any algorithm
jwt.decode(token, settings.SECRET_KEY, algorithms=["HS256", "RS256", "none"])

Checks:

• [ ] Access tokens expire in 15–30 minutes
• [ ] Refresh tokens rotate on use and are invalidated on logout
• [ ] aud (audience) and iss (issuer) claims are set AND verified on decode
• [ ] Algorithm is pinned to an explicit list — never include "none"
• [ ] JWT secret loaded from os.environ, minimum 32 characters
• [ ] Algorithm confusion attack prevented: RS256 keys never accepted as HS256 secrets

2. Rate Limiting on Auth Endpoints

# FastAPI — slowapi
from slowapi import Limiter
from slowapi.util import get_remote_address
limiter = Limiter(key_func=get_remote_address)

@app.post("/login")
@limiter.limit("5/minute")          # ✓ per-IP rate limit
async def login(request: Request, credentials: LoginSchema):
    ...

# Flask — Flask-Limiter
from flask_limiter import Limiter
limiter = Limiter(app, key_func=get_remote_address, default_limits=["200/day"])

@app.route("/login", methods=["POST"])
@limiter.limit("5 per minute")      # ✓
def login():
    ...

Checks:

• [ ] Login endpoint rate-limited (≤5–10 attempts/minute per IP)
• [ ] Per-account lockout after repeated failures (not just per-IP)
• [ ] Registration and password-reset endpoints also rate-limited
• [ ] Rate limit responses return 429 Too Many Requests, not 401

3. Password Policy & Breached-Password Checking

# ✓ GOOD — check against breached databases
import hashlib, httpx

def is_pwned(password: str) -> bool:
    sha1 = hashlib.sha1(password.encode()).hexdigest().upper()
    prefix, suffix = sha1[:5], sha1[5:]
    resp = httpx.get(f"https://api.pwnedpasswords.com/range/{prefix}")
    return suffix in resp.text

# ✓ GOOD — enforce minimum complexity
def validate_password(password: str):
    if len(password) < 12:
        raise ValueError("Password must be at least 12 characters")
    if is_pwned(password):
        raise ValueError("Password found in known data breach")

Checks:

• [ ] Minimum length enforced (NIST 800-63b recommends ≥8; prefer ≥12)
• [ ] Check against known-breached passwords (HaveIBeenPwned API or local list)
• [ ] Do NOT force periodic rotation (NIST 800-63b deprecates this)
• [ ] Password screened against top-10,000 worst passwords list
• [ ] No maximum password length below 64 characters

4. Session Management (Flask)

# ✓ GOOD — Flask-Login with secure session config
app.config.update(
    SESSION_COOKIE_SECURE=True,       # HTTPS only
    SESSION_COOKIE_HTTPONLY=True,     # no JS access
    SESSION_COOKIE_SAMESITE='Lax',    # CSRF mitigation
    PERMANENT_SESSION_LIFETIME=timedelta(hours=1),
)

@app.route("/logout")
@login_required
def logout():
    logout_user()           # ✓ invalidates session
    session.clear()         # ✓ belt-and-suspenders
    return redirect("/login")

Checks:

• [ ] Flask-Login or Flask-Security-Too in use (no hand-rolled session management)
• [ ] Session invalidated on logout (both server-side and cookie cleared)
• [ ] Session ID regenerated after successful login (prevents fixation)
• [ ] SESSION_COOKIE_SECURE, SESSION_COOKIE_HTTPONLY, SESSION_COOKIE_SAMESITE all set
• [ ] Session lifetime is bounded; idle sessions expire

5. Error Message Enumeration

# ✗ BAD — reveals which field was wrong
if not user:
    return {"error": "Username not found"}
if not verify_password(password, user.hashed_password):
    return {"error": "Incorrect password"}

# ✓ GOOD — generic message regardless of failure reason
if not user or not verify_password(password, user.hashed_password):
    return JSONResponse({"error": "Invalid credentials"}, status_code=401)

Checks:

• [ ] Auth failures always return identical message: "Invalid credentials" (or equivalent)
• [ ] HTTP status codes consistent: don't return 404 for missing users vs 401 for bad password
• [ ] Password reset flow doesn't confirm whether email is registered
• [ ] Response timing is constant (use hmac.compare_digest and always hash even for unknown users)

6. Default Credentials & Hard-coded Secrets

# ✗ BAD — Bandit will flag these
ADMIN_PASSWORD = "admin"
TEST_USER = {"username": "test", "password": "test123"}
JWT_SECRET = "supersecretkey"

# ✓ GOOD
JWT_SECRET = os.environ["JWT_SECRET"]  # loaded from environment / secrets manager

Checks:

• [ ] Run Bandit: bandit -r . -t B105,B106,B107 (hardcoded password checks)
• [ ] No admin/admin, test/test, root/root patterns in config, seeds, or fixtures
• [ ] All secrets loaded from environment variables or a secrets manager (Vault, AWS SSM)
• [ ] .env files excluded from version control via .gitignore

7. MFA Enforcement

Checks:

• [ ] MFA offered for all user accounts; enforced for admin/privileged roles
• [ ] MFA cannot be bypassed by manipulating URL parameters or session state
• [ ] TOTP codes are single-use and expire after 30 seconds (standard TOTP)
• [ ] Recovery codes are single-use and stored as hashed values

Key CWEs Reference

Automated Scanning Commands

# Static analysis — catches CWE-798, weak crypto, debug mode
bandit -r . -ll -ii

# Specifically target hardcoded credentials
bandit -r . -t B105,B106,B107,B108

# Dependency vulnerabilities (may expose auth libraries with known CVEs)
pip-audit
safety check

# Check JWT library versions for known vulns
pip show python-jose PyJWT authlib

Remediation Priority Order

1. Critical: JWT algorithm confusion / "none" accepted → immediate fix
2. Critical: No rate limiting on login → add slowapi/Flask-Limiter immediately
3. High: Hard-coded secrets in source → move to environment variables
4. High: Sessions not invalidated on logout → fix session management
5. Medium: Username enumeration via error messages → unify error responses
6. Medium: No breached-password check → integrate HaveIBeenPwned API
7. Low: Missing MFA option → plan rollout for privileged accounts first

References

• OWASP A07:2025 full spec: https://owasp.org/Top10/A07_2021-Identification_and_Authentication_Failures/
• NIST SP 800-63b (Digital Identity Guidelines): https://pages.nist.gov/800-63-3/sp800-63b.html
• HaveIBeenPwned API: https://haveibeenpwned.com/API/v3