A04:2025 — Cryptographic Failures

OWASP Rank: #4 (dropped from #2 due to improved TLS adoption) Incidence: 3.80% of applications, 1.6M+ occurrences, 32 CWEs

Cryptographic Failures cover the absence, misuse, or weakness of cryptography protecting data at rest and in transit. In Python, there is a distinctive set of pitfalls that differ from Java/.NET.

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What This Skill Covers

Area	Key Risk	Python-Specific Pitfall
Password hashing	Weak/missing algorithm	Using hashlib.md5/sha1 instead of bcrypt/Argon2
Randomness	Predictable values	import random instead of import secrets
Hardcoded keys	Key leakage	SECRET_KEY = "..." literals in source
TLS/SSL	Cert bypass	verify=False in requests/httpx
JWT secrets	Key strength/algorithm	Weak secret, missing algo pinning
Symmetric encryption	Deprecated modes	ECB mode, no authenticated encryption

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Step-by-Step Audit Workflow

1. Static Scan with Bandit

Run Bandit — the primary Python SAST tool for catching crypto issues automatically:

pip install bandit
bandit -r . -t B303,B304,B305,B306,B311,B324,B501,B502,B503,B504,B505,B506 -f json -o bandit_crypto.json

Key Bandit test IDs for cryptographic failures:

Bandit ID	What It Catches	Severity
B303	MD5/SHA1 usage	Medium
B304/B305	Weak cipher modes (ECB, DES)	High
B311	random module in security context	Low
B324	Hashlib use of weak hash	Medium
B501	verify=False in SSL context	High
B502/B503/B504	Insecure SSL/TLS versions	Medium–High
B105/B106	Hardcoded password/secret	Medium

2. Manual Code Pattern Review

Work through these patterns in priority order:

🔴 CRITICAL — Password Hashing

# BAD — MD5/SHA1 for passwords: CWE-328, CWE-759, CWE-916
hashlib.md5(password.encode()).hexdigest()
hashlib.sha1(password.encode()).hexdigest()
hashlib.sha256(password.encode()).hexdigest()  # also bad — no salt, no stretching

# GOOD — use passlib with bcrypt or argon2
from passlib.context import CryptContext
pwd_context = CryptContext(schemes=["argon2"], deprecated="auto")
# OR
pwd_context = CryptContext(schemes=["bcrypt"], bcrypt__rounds=12, deprecated="auto")
hashed = pwd_context.hash(plain_password)

Check for:

• Any hashlib.* call used to hash a password or token
• Missing salt (hash of raw password without hashlib.pbkdf2_hmac)
• bcrypt with rounds < 12 — should be ≥12 to resist brute force

🔴 CRITICAL — Weak Randomness (CWE-330)

# BAD — random is NOT cryptographically secure
import random
token = random.randint(100000, 999999)       # predictable OTP
reset_key = random.choice(string.ascii_letters + string.digits)  # guessable

# GOOD
import secrets
token = secrets.randbelow(900000) + 100000   # secure OTP
reset_key = secrets.token_urlsafe(32)        # secure reset token
session_id = secrets.token_hex(16)           # secure session ID

Check for: Any random.random(), random.randint(), random.choice(), random.shuffle() in code paths involving tokens, OTPs, session IDs, keys, or nonces.

🔴 HIGH — Hardcoded Cryptographic Keys (CWE-321)

# BAD — key in source code
SECRET_KEY = "my-super-secret-key"
JWT_SECRET = "hardcoded_jwt_secret_123"
app.config['SECRET_KEY'] = "flask_secret"

# GOOD — load from environment
import os
SECRET_KEY = os.environ["SECRET_KEY"]        # raises KeyError if missing (intentional)
SECRET_KEY = os.getenv("SECRET_KEY")         # returns None silently (verify it's not None)

Minimum key lengths:

• Flask SECRET_KEY: ≥32 random bytes (use secrets.token_hex(32))
• JWT HMAC secrets: ≥32 characters
• AES keys: 128, 192, or 256 bits (16, 24, or 32 bytes)

🔴 HIGH — TLS/SSL Certificate Bypass (CWE-295)

# BAD — disables certificate validation entirely
requests.get(url, verify=False)
httpx.get(url, verify=False)
ssl._create_unverified_context()
urllib3.disable_warnings()  # companion to verify=False

# GOOD
requests.get(url)               # verify=True is the default
requests.get(url, verify="/path/to/ca-bundle.crt")  # custom CA

🟡 MEDIUM — JWT Configuration (CWE-327)

# BAD — algorithm confusion attacks
jwt.decode(token, key, algorithms=["none"])  # NEVER allow "none"
jwt.decode(token, key)                       # no algorithm pin = vulnerable

# BAD — weak secret loaded without validation
JWT_SECRET = os.getenv("JWT_SECRET", "default")  # fallback default is dangerous

# GOOD
JWT_SECRET = os.environ["JWT_SECRET"]        # fail hard if missing
assert len(JWT_SECRET) >= 32, "JWT secret too short"
jwt.decode(token, JWT_SECRET, algorithms=["HS256"])  # explicit allowlist

Also check:

• Token expiry (exp claim) — access tokens should be 15–30 min
• aud and iss claims are validated on decode

🟡 MEDIUM — Deprecated Symmetric Encryption (CWE-327)

# BAD — ECB mode leaks patterns in data
from Crypto.Cipher import AES
cipher = AES.new(key, AES.MODE_ECB)

# BAD — no authentication = vulnerable to tampering
cipher = AES.new(key, AES.MODE_CBC, iv)

# GOOD — authenticated encryption
from cryptography.hazmat.primitives.ciphers.aead import AESGCM
aesgcm = AESGCM(key)
ciphertext = aesgcm.encrypt(nonce, plaintext, associated_data)
# ChaCha20-Poly1305 is also acceptable

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FastAPI-Specific Checks

# 1. Verify OAuth2/JWT middleware algorithm pinning
from fastapi.security import OAuth2PasswordBearer
from jose import JWTError, jwt

ALGORITHM = "HS256"  # must be explicit, not a variable that could be overridden
# Check: is ALGORITHM loaded from config where it could be tampered with?

# 2. Verify password hashing in user creation/login endpoints
@router.post("/register")
async def register(user: UserCreate, db: Session = Depends(get_db)):
    hashed = pwd_context.hash(user.password)  # must NOT store plain or MD5

# 3. Token generation for password resets / email verification
def generate_reset_token() -> str:
    return secrets.token_urlsafe(32)   # GOOD
    # NOT: str(uuid.uuid4())  — UUID v4 uses os.urandom, acceptable but secrets is preferred
    # NOT: random-based anything

Flask-Specific Checks

# 1. Session cookie signing uses SECRET_KEY — must be strong
app.secret_key = os.environ["SECRET_KEY"]  # GOOD
app.secret_key = "dev"                     # BAD — common in tutorials, dangerous in prod

# 2. Flask-Login password verification
from flask_bcrypt import Bcrypt
bcrypt = Bcrypt(app)
bcrypt.generate_password_hash(password, rounds=12)  # rounds≥12

# 3. HTTPS enforcement via HSTS
from flask_talisman import Talisman
Talisman(app, force_https=True)  # adds HSTS header

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Key CWEs Reference

CWE	Name	Common Manifestation
CWE-327	Use of Broken/Risky Crypto Algorithm	MD5, SHA1, ECB, DES, RC4
CWE-328	Reversible One-Way Hash	MD5/SHA1 for passwords (reversible via rainbow tables)
CWE-330	Insufficiently Random Values	random module for tokens/keys
CWE-321	Hard-coded Cryptographic Key	Secrets in source code
CWE-759	One-Way Hash Without a Salt	hashlib.sha256(password) — no salt
CWE-916	Password Hash With Insufficient Effort	bcrypt rounds<12, PBKDF2 iterations<600k
CWE-295	Improper Certificate Validation	verify=False in HTTP clients

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Remediation Checklists

Minimum Viable Crypto Stack (Python 2025)

Password hashing:   passlib[argon2] or passlib[bcrypt] (rounds≥12)
Token generation:   secrets module exclusively
Symmetric crypto:   cryptography library, AES-GCM or ChaCha20-Poly1305
JWT:                python-jose or PyJWT, algorithm pinned, secret≥32 chars
TLS:                verify=True always; pin CA bundle where needed
Config/secrets:     os.environ (fail-hard) or a secrets manager (Vault, AWS SSM)
HSTS:               flask-talisman (Flask) or custom middleware (FastAPI)

Quick Regex Patterns for Code Search

# Find weak hashing
grep -rn "hashlib\.md5\|hashlib\.sha1\|hashlib\.sha224" --include="*.py"

# Find insecure random
grep -rn "import random\b" --include="*.py"
grep -rn "random\.randint\|random\.choice\|random\.random()" --include="*.py"

# Find hardcoded secrets
grep -rn "SECRET_KEY\s*=\s*['\"]" --include="*.py"
grep -rn "verify=False" --include="*.py"

# Find ECB mode
grep -rn "MODE_ECB\|AES\.MODE_ECB" --include="*.py"

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See Also

• references/owasp-a04-full.md — full OWASP A04:2025 spec and extended CWE list
• For supply chain crypto issues (pinned dependency hashes), see the software-supply-chain skill
• For JWT-related auth logic beyond crypto (expiry, claims), see the authentication-failures skill