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Credential Stuffing

Introduction

So, you know how people are absolutely terrible at password hygiene? Yeah, that's where Credential Stuffing comes to play! This attack leverages the sad reality that users recycle the same username/password combinations across multiple services. It's like having one key for your house, car, office, and gym locker - when someone gets that key, they've got access to your entire life!

Credential stuffing takes leaked credentials from one breach and systematically tries them against other services. It's not about guessing passwords like password spraying - we're using REAL credentials that we KNOW worked somewhere. The question is just: where else do they work?

Unlike password spraying where we're playing the odds with common passwords, credential stuffing is more like having a master key collection and trying to see which locks they open. Pretty neat, right?

The Discovery

Credential stuffing isn't exactly "new" - people have been reusing passwords since passwords existed! But it became a massive problem around 2012-2014 when major breaches started happening left and right. Remember the LinkedIn breach in 2012? Yahoo in 2013-2014? Suddenly, attackers had MILLIONS of real username/password combos to work with.

The technique really exploded when these breach databases became easily accessible on underground forums. Tools started popping up specifically designed to automate the process of testing these credentials across hundreds of different services. What used to be a manual, tedious process became an industrial-scale operation.

Is Credential Stuffing still a thing?

Oh, absolutely! In fact, it's gotten WORSE. Think about it - we've had massive breaches almost every year since 2012. Collection #1, Collection #2-5, the Compilation of Many Breaches (COMB), and countless others. We're talking about BILLIONS of compromised credentials floating around.

The scary part? Studies show that 65% of people reuse passwords across multiple accounts. That means if you get someone's Netflix password from a breach, there's a good chance it's also their work email password. Yikes!

Summing it all up:

Credential Stuffing exploits human nature and poor password hygiene by using REAL stolen credentials from data breaches to gain unauthorized access to other services where users have reused the same login information.

Credential Stuffing is devastating because it uses LEGITIMATE CREDENTIALS - no guessing, no brute forcing, just testing real username/password pairs that we KNOW are valid somewhere!

Credential Stuffing Architecture: The Attack Flow

The Data Breach Ecosystem:

Stage 1: The Breach 1. Major service gets compromised (LinkedIn, Yahoo, etc.) 2. Usernames, emails, and passwords are stolen 3. Data gets sold/leaked on underground forums 4. Credentials get compiled into massive databases

Stage 2: The Preparation 1. Attacker downloads breach databases 2. Credentials get cleaned and formatted 3. Duplicates removed, passwords cracked (if hashed) 4. Data organized by email domains, password patterns, etc.

Stage 3: The Attack Flow 1. Target Selection: Choose high-value services (banking, email, cloud) 2. Credential Loading: Load breach database into attack tool 3. Automated Testing: Systematically test credentials across targets 4. Success Identification: Flag successful logins for manual verification 5. Account Takeover: Access accounts, extract data, or establish persistence

The Numbers Game:

Here's why credential stuffing is so effective: - Breach Database: 1 million username/password pairs - Success Rate: Typically 0.1% - 2% (still 1,000 - 20,000 successful logins!) - Time Investment: Mostly automated, minimal manual effort - ROI: Extremely high - one successful login can lead to significant compromise

Credential Stuffing vs Other Attacks

Credential Stuffing vs Password Spraying:

Password Spraying: Few passwords → Many accounts (guessing common passwords) Credential Stuffing: Many credentials → Many services (using known valid pairs)

Credential Stuffing vs Brute Force:

Brute Force: Generate all possible password combinations Credential Stuffing: Use real passwords from actual breaches

Credential Stuffing Requirements and Pre-Requisites

Technical Requirements:

  • Access to credential databases (breach collections)
  • Automated testing tools (proxies, rotation, etc.)
  • Target service enumeration capabilities
  • Large-scale infrastructure (to handle volume)

Credential stuffing makes for an excellent attack vector when you want to test if your target organization's users are reusing passwords from known breaches. No need for sophisticated techniques - just good old data and automation!

Data Sources:

Public Breach Collections: - Collection #1 (773M records) - COMB - Compilation of Many Breaches (3.2B records) - Various individual breaches (LinkedIn, Adobe, Yahoo, etc.) - Leaked database dumps from forums

Credential Quality Indicators: - Fresh breaches: Higher success rates (people haven't changed passwords yet) - Corporate domains: Target-specific email domains for focused attacks - High-value services: Credentials from financial or business services - Password complexity: Complex passwords often reused more frequently

Tools and Capabilities:

Professional/Commercial Tools:

  • Sentry MBA: Most popular credential stuffing platform
  • STORM: Advanced multi-threaded credential testing
  • BlackBullet: Comprehensive credential stuffing suite
  • OpenBullet: Open-source alternative with extensive configs

Custom/Script-Based:

  • Py Credential Stuffing: Python-based custom solutions
  • Burp Suite Extensions: Web application credential testing
  • Custom Selenium Scripts: Browser automation for complex sites
  • API-based tools: Direct API credential testing

Infrastructure Requirements:

  • Proxy Rotation: Avoid IP-based blocking
  • User-Agent Rotation: Mimic legitimate traffic
  • CAPTCHA Solving: Services like 2captcha, Anti-Captcha
  • Distributed Infrastructure: Cloud-based attack platforms

When to Use Credential Stuffing

Ideal Scenarios:

1. Initial Access Operations:

  • You need legitimate credentials to specific high-value targets
  • Perfect for getting into corporate email systems
  • Excellent for cloud service account takeovers
  • Great for establishing persistent access through legitimate accounts

2. Post-Compromise Intelligence Gathering:

  • You've compromised one service, testing if creds work elsewhere
  • Expanding access across user's digital footprint
  • Identifying additional attack vectors through credential reuse
  • Mapping user behavior and service usage patterns

3. Red Team Engagements:

  • Demonstrating real-world attack scenarios to clients
  • Testing organizational password policies and user education
  • Simulating APT-style persistent access techniques
  • Showing impact of third-party breaches on corporate security

4. Threat Intelligence and Research:

  • Understanding credential reuse patterns in your industry
  • Monitoring if your organization's credentials appear in breaches
  • Proactive security assessments before attackers find the data
  • Compliance validation for security awareness programs

Quick Reference Cards:

Q: What makes Credential Stuffing so effective? A: It uses REAL passwords that we know worked somewhere - no guessing required!

Q: What's the typical success rate? A: Usually 0.1% - 2%, but with millions of credentials, that's still thousands of successful logins

Q: What's the difference from password spraying? A: Password spraying guesses common passwords; credential stuffing uses known real username/password pairs from breaches

Q: What's the best defense? A: Multi-factor authentication - even if they have your password, they can't get in without the second factor

Q: Where do attackers get the credential databases? A: Public breach collections, underground forums, dark web markets, and leaked database dumps

Q: What services are most commonly targeted? A: High-value targets like email (O365, Gmail), cloud services (AWS, Azure), banking, and social media

Conclusion

Credential stuffing represents the dark side of our interconnected digital lives. It's a stark reminder that a breach at one service can compromise users across the entire internet. The attack is simple in concept but devastating in practice - why try to guess passwords when you can just use real ones?

For pentesters and red teamers, understanding credential stuffing is crucial not just for offensive operations, but for helping clients understand the real-world impact of poor password hygiene. Sometimes the best way to demonstrate the importance of unique passwords and MFA is to show just how easily reused credentials can be exploited.

Remember: with great power comes great responsibility. Use this knowledge to make the digital world more secure, not less!