OWASP A10:2025 — Mishandling of Exceptional Conditions

Brand new category for 2025, replacing SSRF (absorbed into A01). Contains 24 CWEs with 769,581 recorded occurrences. Addresses programs that fail to prevent, detect, and respond to unusual situations — leading to crashes, unexpected behaviour, information disclosure, and exploitable states.

Core Concepts

What It Covers

Prevention Principles

1. Catch errors where they occur — at the function that can handle them, not three layers up.
2. Fail closed — when auth/authz logic throws, deny access; never pass through.
3. Roll back completely — partial DB transactions must be rolled back, never committed.
4. Separate user messages from technical details — log the stack trace server-side; return a safe, generic message to the client.
5. Global exception handler as safety net — every app needs one, but it is not a substitute for local handling.
6. Rate-limit and quota — prevent exceptional conditions (e.g., resource exhaustion) from being triggered intentionally.

FastAPI Testing Checklist

1. Global Exception Handler

# REQUIRED — catches anything not handled locally
@app.exception_handler(Exception)
async def global_handler(request: Request, exc: Exception):
    logger.error("Unhandled exception", exc_info=exc)
    return JSONResponse(status_code=500, content={"detail": "Internal server error"})

Flag if absent. The default FastAPI 500 response may include traceback in development mode.

2. Fail-Closed Auth Middleware (CWE-636)

# BAD — exception causes pass-through (fail open)
@app.middleware("http")
async def auth_middleware(request: Request, call_next):
    try:
        verify_token(request)
    except Exception:
        pass                       # ← VULNERABILITY
    return await call_next(request)

# GOOD — exception causes denial (fail closed)
@app.middleware("http")
async def auth_middleware(request: Request, call_next):
    try:
        verify_token(request)
    except Exception as e:
        logger.warning("Auth check failed", exc_info=e)
        return JSONResponse(status_code=401, content={"detail": "Unauthorized"})
    return await call_next(request)

3. Custom HTTP Error Responses (CWE-756 / CWE-209)

# Register handlers for all common error codes
@app.exception_handler(404)
async def not_found(request, exc):
    return JSONResponse(status_code=404, content={"detail": "Not found"})

@app.exception_handler(422)
async def validation_error(request, exc):
    return JSONResponse(status_code=422, content={"detail": "Invalid input"})

Flag any handler that returns `str(exc)`, `repr(exc)`, or `traceback.format_exc()` in the response body.

4. Resource Cleanup in Exception Paths

# BAD — connection leaked if query raises
conn = db.connect()
result = conn.execute(query)   # raises → conn never closed
conn.close()

# GOOD — context manager guarantees cleanup
with db.connect() as conn:
    result = conn.execute(query)

Scan for: db.connect(), open(file), socket.socket() not inside a with block or try/finally.

5. SQLAlchemy Transaction Rollback

# BAD — partial write committed on error
session.add(obj)
risky_operation()        # raises → session still commits elsewhere
session.commit()

# GOOD — rollback on exception
try:
    session.add(obj)
    risky_operation()
    session.commit()
except Exception:
    session.rollback()
    raise

Flag: any session.commit() not paired with session.rollback() in an except branch.

Flask Testing Checklist

1. Global Exception Handler

@app.errorhandler(Exception)
def global_error(e):
    app.logger.error("Unhandled exception", exc_info=e)
    return {"error": "Internal server error"}, 500

Also register specific handlers:

@app.errorhandler(404)
def not_found(e):
    return {"error": "Not found"}, 404

@app.errorhandler(500)
def server_error(e):
    return {"error": "Server error"}, 500

2. Flask Debug Mode — Doubles as A02 + A10 Issue

Debug mode (debug=True or FLASK_DEBUG=1) enables the Werkzeug interactive debugger, which:

• Exposes full tracebacks to the browser.
• Allows arbitrary code execution via the debugger console.

Flag immediately if found in production config or environment variables.

3. request.args.get() Silent None Returns

# BAD — returns None silently; downstream code may crash
user_id = request.args.get("user_id")
user = db.query(User).filter_by(id=user_id).first()  # id=None → wrong query

# GOOD — explicit validation
user_id = request.args.get("user_id")
if user_id is None:
    abort(400, description="user_id is required")

FastAPI's Pydantic models prevent this automatically; Flask requires manual guards.

4. Fail-Closed before_request Hooks

# BAD — exception in auth check bypasses protection
@app.before_request
def check_auth():
    try:
        verify_session()
    except:
        pass   # ← VULNERABILITY

# GOOD
@app.before_request
def check_auth():
    try:
        verify_session()
    except Exception as e:
        app.logger.warning("Session verify failed", exc_info=e)
        abort(401)

Automated Scanning Approach

Static Analysis (Bandit)

bandit -r . -t B110,B112   # try-except-pass and try-except-continue
bandit -r . --severity-level medium

Key Bandit test IDs for this category:

• B110 — try: ... except: pass (swallowed exception)
• B112 — try: ... except: continue (swallowed in loop)

Pattern Search (grep / ripgrep)

# Fail-open patterns
rg "except.*:\s*(pass|continue|return True)" --type py

# Stack traces leaking to response
rg "traceback\.format_exc\(\)|str\(exc\)|repr\(exc\)" --type py

# Missing context managers
rg "open\(|\.connect\(\)" --type py   # review each hit for 'with'

# Missing rollback
rg "session\.commit\(\)" --type py    # check each for paired rollback

Runtime / DAST

• Trigger deliberate 404, 500, and validation errors (422 on FastAPI).
• Inspect HTTP responses for presence of: file paths, stack frames, SQL query strings, exception class names.
• Probe middleware by sending a malformed Authorization header — verify the response is 401, not 200.

Key CWEs (A10:2025)

Remediation Priority

1. Critical: Fail-open auth middleware / before_request hooks (CWE-636) — direct authentication bypass.
2. Critical: Debug mode enabled in production (Werkzeug debugger = RCE).
3. High: Stack traces / DB details in HTTP responses (CWE-209) — aids attacker reconnaissance.
4. High: Missing global exception handler — unhandled paths crash app and may reveal details.
5. Medium: Missing transaction rollback — data integrity corruption.
6. Medium: Resource leaks in exception paths — connection pool exhaustion under attack.
7. Low: Silent None returns from request.args.get() — logic bugs, not directly exploitable.