Credential Stuffing Attack Logs

AuthenticationAttacks Web Application Credential Stuffing

What This Means

Identify credential stuffing attacks in your authentication logs. Detect automated login attempts using stolen credential pairs from data breaches targeting your web applications.

Example Log

203.0.113.50 - - [08/Mar/2026:14:22:01 +0000] "POST /api/login HTTP/1.1" 401 89 "-" "Mozilla/5.0"
203.0.113.50 - - [08/Mar/2026:14:22:02 +0000] "POST /api/login HTTP/1.1" 401 89 "-" "Mozilla/5.0"
203.0.113.51 - - [08/Mar/2026:14:22:03 +0000] "POST /api/login HTTP/1.1" 200 1205 "-" "Mozilla/5.0"
203.0.113.52 - - [08/Mar/2026:14:22:04 +0000] "POST /api/login HTTP/1.1" 401 89 "-" "Mozilla/5.0"

Indicators of Suspicious Activity

High volume of login POST requests from multiple IPs (distributed botnet)
Low success rate — most attempts fail but a few succeed (valid stolen credentials)
Identical or very similar User-Agent strings across different IPs
Requests arriving at regular intervals suggesting automated tooling
Login attempts using email addresses as usernames (common in breached credential lists)
Geographic distribution of source IPs does not match normal user base

How to Investigate

Aggregate login failures by time window to identify abnormal spikes
Check the success rate — credential stuffing typically shows 0.1-2% success
Identify accounts that had successful logins from suspicious IPs
Review if successful logins led to unusual account activity (profile changes, data export)
Check the attacking IP addresses against proxy/VPN/botnet reputation services
Compare login volume against baseline to quantify the attack scale

Recommended Mitigations

Implement CAPTCHA or progressive challenges after failed login attempts
Deploy bot detection solutions that analyze behavioral patterns
Require MFA for all accounts to make stolen credentials useless
Monitor for and alert on successful logins from unusual geolocations
Implement credential breach detection (HaveIBeenPwned API integration)
Rate limit login endpoints per IP and per account with progressive delays

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Related Log Types

Related Attack Patterns

Frequently Asked Questions

What is credential stuffing? ▼

Credential stuffing uses username/password pairs stolen from data breaches to attempt logins on other services. It exploits password reuse — the same credentials that worked on a breached site may work on yours.

How is credential stuffing different from brute force? ▼

Brute force guesses passwords. Credential stuffing uses known-valid username/password pairs from breaches. The attacker already has real credentials — they just test if users reused them on your site.

What is a typical success rate for credential stuffing? ▼

Industry data shows 0.1-2% of attempts succeed, meaning for every 10,000 credential pairs tried, 10-200 may be valid on your platform due to password reuse.