A09:2025 — Security Logging & Alerting Failures

OWASP rank: #9 (community-voted for third consecutive cycle)
Renamed in 2025: Added "Alerting" — logging without alerting provides minimal security value.
Stats: 5 CWEs, 723 CVEs, 3.91% average incidence rate
Coverage CWEs: CWE-778, CWE-117, CWE-532, CWE-223, CWE-390

What This Category Covers

Audit Checklist — FastAPI & Flask

Work through each section systematically. For each item, note: PASS, FAIL, or NOT APPLICABLE.

1. Log Injection (CWE-117)

User-controlled input must never flow raw into log statements.

What to look for:

# VULNERABLE — newline injection, ANSI escape injection
logger.info(f"User logged in: {request.form['username']}")
logger.warning("Failed login for: " + username)

# SAFE — sanitize before logging
import re
safe_username = re.sub(r'[\r\n\x1b]', '_', username)
logger.info("Failed login for: %s", safe_username)

Checklist:

• No f-strings or + concatenation with raw request data in log calls
• Newlines (\r, \n) stripped from all user-supplied log values
• ANSI escape codes (\x1b[) stripped (prevent terminal injection)
• Structured logging used (JSON format) — field separation prevents injection by design

2. Sensitive Data in Logs (CWE-532)

Secrets must never appear in log output — even at DEBUG level.

Patterns to flag:

# VULNERABLE
logger.debug(f"Auth token: {token}")
logger.info(f"Processing payment for card: {card_number}")
app.logger.debug(request.json)         # May contain password field
print(f"Password reset for {email}, token={reset_token}")

# SAFE
logger.info("Auth token validated for user_id=%s", user_id)
logger.info("Payment processed for user_id=%s, last4=%s", user_id, card_number[-4:])

What to scan for:

• No logging of: password, passwd, secret, token, api_key, authorization header values
• No logging of: credit card numbers, SSNs, full email+password combos
• request.json / request.form not logged wholesale without field filtering
• No print() statements containing sensitive variables in production code
• Bandit rule B506 (and similar) passes in CI/CD

3. Insufficient Logging (CWE-778)

The following events must produce log entries with full context.

Required log events:

FastAPI implementation pattern:

import logging
import structlog  # recommended for structured logs

log = structlog.get_logger()

# In auth endpoint
@router.post("/login")
async def login(credentials: LoginSchema, request: Request):
    user = await authenticate(credentials)
    if not user:
        log.warning(
            "auth.failed",
            ip=request.client.host,
            username=credentials.username,    # OK — username is not a secret
            user_agent=request.headers.get("user-agent"),
        )
        raise HTTPException(status_code=401, detail="Invalid credentials")
    log.info("auth.success", user_id=user.id, ip=request.client.host)

Flask implementation pattern:

from flask import request
import logging

@app.route("/login", methods=["POST"])
def login():
    user = authenticate(request.form["username"], request.form["password"])
    if not user:
        app.logger.warning(
            "auth.failed ip=%s username=%s ua=%s",
            request.remote_addr,
            request.form.get("username"),
            request.user_agent.string,
        )
        return {"error": "Invalid credentials"}, 401

Checklist:

• All 401/403 responses have a corresponding logger.warning() or logger.error() call
• Log entries include: timestamp (automatic with logging), source IP, user identifier
• Password reset and account modification flows are logged
• Admin panel actions are logged with actor identity
• High-value business transactions produce audit log entries

4. Structured Logging & SIEM Compatibility

Log format must be parseable by observability platforms (ELK, Splunk, Datadog, Sentry).

FastAPI — structlog + JSON:

# main.py
import structlog
import logging

structlog.configure(
    processors=[
        structlog.processors.TimeStamper(fmt="iso"),
        structlog.stdlib.add_log_level,
        structlog.processors.JSONRenderer(),  # machine-parseable
    ],
    wrapper_class=structlog.BoundLogger,
    logger_factory=structlog.PrintLoggerFactory(),
)

Flask — python-json-logger:

from pythonjsonlogger import jsonlogger

handler = logging.StreamHandler()
handler.setFormatter(jsonlogger.JsonFormatter(
    fmt="%(asctime)s %(levelname)s %(name)s %(message)s"
))
app.logger.addHandler(handler)

Checklist:

• Log output is JSON-formatted (not plain text) in production
• Log level is not DEBUG in production (leaks verbose info)
• Logs ship to a centralized platform: Sentry, ELK, Splunk, Datadog, CloudWatch
• Log retention policy exists and meets compliance requirements (e.g., 90 days minimum)

5. Alerting Integration

Logging without alerting = silent failures. Verify active monitoring exists.

Sentry (exception alerting):

# FastAPI
import sentry_sdk
from sentry_sdk.integrations.fastapi import FastApiIntegration
sentry_sdk.init(dsn=settings.SENTRY_DSN, integrations=[FastApiIntegration()])

# Flask
from sentry_sdk.integrations.flask import FlaskIntegration
sentry_sdk.init(dsn=settings.SENTRY_DSN, integrations=[FlaskIntegration()])

Recommended alerting thresholds:

Checklist:

• Sentry or equivalent is initialized and receiving events in production
• Rate-limit violations produce alert events (not just HTTP 429 responses)
• Repeated auth failures trigger alerts (brute force detection)
• Alerting runbooks/playbooks exist for each alert type
• Honeytokens deployed in high-value areas (near-zero false-positive intrusion detection)

Automated Scanning

Run these tools as part of CI/CD:

# Static analysis — catches log injection patterns, sensitive data in logs
bandit -r . -t B106,B107,B110 --severity-level medium

# Search for raw request data in log calls
grep -rn "logger\.\(info\|warning\|error\|debug\)" . \
  | grep -E "request\.(form|json|args|data|headers)\[" \
  | grep -v "\.get(" 

# Search for potential sensitive field logging
grep -rn "logger\|logging\|print" . \
  | grep -iE "password|passwd|secret|token|api_key|card|ssn|cvv"

Key CWEs Reference

Reference Files

• references/remediation-patterns.md — Full before/after code fixes for each CWE
• references/alerting-platforms.md — Integration guides for Sentry, ELK, Splunk, Datadog

Read these when the user needs detailed implementation help beyond the checklist.