Authentication mechanisms are a critical component of web application security. However, weak credential management and insufficient protections against login attempts can expose systems to targeted credential attacks.

In this write-up, I demonstrate how valid login credentials can be discovered through a targeted credential attack using two different approaches:

• Burp Suite Intruder (Cluster Bomb attack)
• ffuf (Command-line fuzzing tool)

This lab highlights how attackers can efficiently test combinations of usernames and passwords when login attempts are limited by rate-limiting or account lockout mechanisms.

This article is part of my Web Security Series, where I document practical web security labs and explain common authentication and authorization vulnerabilities.

Understanding Targeted Credential Attacks

A targeted credential attack (sometimes referred to as credential stuffing or constrained brute-force) attempts combinations of usernames and passwords against an authentication endpoint when valid credentials are unknown.

Unlike traditional brute-force attacks that test thousands of combinations, targeted attacks are designed to operate within strict attempt limits.

When an application enforces protections such as:

• Login rate limiting
• Account lockouts
• Attempt thresholds

attackers may use a clustered approach, testing a small curated list of usernames against a small curated list of passwords.

This approach maximizes the probability of discovering valid credentials while remaining within the allowed attempt budget.

When This Attack Technique Is Used

This attack method is typically used during the exploitation phase of a penetration test when:

• No valid credentials are available.
• The application limits the number of login attempts (for example, 5 attempts).
• A small list of high-probability usernames and passwords is available.
• Attempts must be carefully managed to avoid triggering account lockouts or security alerts.

Lab Objective

The objective of this lab is to:

Discover valid login credentials by testing combinations of usernames and passwords within a strict attempt limit using both Burp Suite Intruder and the ffuf fuzzing tool.

Lab Environment

This challenge lab provides a login page where credentials must be discovered through controlled attack techniques.

The lab allows only five login attempts, requiring a carefully planned credential testing strategy.

Method 1 — Credential Discovery Using Burp Suite Intruder

Burp Suite provides a powerful feature called Intruder, which allows testers to automate parameter manipulation and test multiple input combinations.

In this lab, the Cluster Bomb attack type is used to test combinations of usernames and passwords.

Step 1 — Generate a Sample Login Request

First, submit any credentials on the login page to generate a request.

Example:

username = jeremy
password = letmein

With Burp Proxy enabled, navigate to:

Proxy → HTTP History

Locate the captured POST request to the authentication endpoint.

Right-click the request and select:

Send to Intruder

Step 2 — Configure Intruder Positions

Inside Burp Intruder, mark the username and password parameters as payload positions.

Navigate to:

Intruder → Positions

Add markers to both parameters and select the attack type:

Cluster Bomb

The cluster bomb attack will test all combinations of the selected username and password lists.

Step 3 — Load Username Wordlist

A username list can be obtained from the SecLists collection.

Example command to locate username lists:

find /usr/share/seclists -name '*user*'

For this lab, the following list is used:

/usr/share/seclists/Usernames/top-usernames-shortlist.txt

Load this list into the username payload slot in Burp Intruder.

Step 4 — Prepare a Small Password List

Because the application limits login attempts, only a small set of high-probability passwords should be used.

Using SecLists:

head /usr/share/seclists/Passwords/xato-net-10-million-passwords-10.txt

Example selected passwords:

123456
password
password123
letmein

Load these passwords into the password payload slot.

Step 5 — Launch the Cluster Bomb Attack

Start the attack by clicking:

Start Attack

Burp Intruder will automatically test all username and password combinations.

Step 6 — Analyze the Results

After the attack completes, analyze the responses by examining:

• Response status codes
• Response length
• Content differences

In this lab, the successful credential pair was discovered as:

username = admin
password = letmein

Entering these credentials into the login page confirms successful authentication.

Method 2 — Credential Discovery Using ffuf

While Burp Intruder provides a graphical interface for attacks, similar techniques can also be performed using command-line tools such as ffuf.

ffuf is a fast web fuzzing tool commonly used for automated testing of parameters and endpoints.

Step 1 — Capture the Authentication Request

As before, generate a login request and capture it using Burp Proxy.

Navigate to:

Proxy → HTTP History

Copy the raw request and save it to a file:

request2.txt

Step 2 — Modify the Request with Fuzz Markers

Open the request file in a text editor:

mousepad request2.txt

Replace the username and password values with fuzz markers:

username=FUZZUSER&password=FUZZPASS

Step 3 — Create a Small Password File

Create a local password file named pass.txt containing common passwords:

123456
password
password123
letmein

Step 4 — Run ffuf in Cluster Bomb Mode

Execute the following command:

ffuf -request request2.txt -request-proto http -mode clusterbomb \
-w /usr/share/seclists/Usernames/top-usernames-shortlist.txt:FUZZUSER \
-w pass.txt:FUZZPASS

Explanation:

• -request loads the raw HTTP request
• -mode clusterbomb tests all combinations
• -w <file>:<marker> maps wordlists to parameters

Step 5 — Identify Valid Credentials

During execution, ffuf highlights responses that differ from the baseline.

By analyzing these responses, the correct credentials were again identified as:

admin : letmein

Why This Vulnerability Occurs

This vulnerability occurs because the application lacks sufficient protection mechanisms such as:

• Strong password policies
• Account lockout enforcement
• Login attempt rate limiting
• Multi-factor authentication

Without these controls, attackers can systematically test credential combinations until a valid pair is discovered.

Security Impact

Weak authentication protections may lead to:

• Unauthorized account access
• Privilege escalation
• Exposure of sensitive user data
• System compromise

Credential attacks remain one of the most common techniques used in real-world breaches.

How to Prevent Credential Attacks

Developers should implement several protective mechanisms, including:

• Account lockouts after multiple failed login attempts
• Rate limiting on authentication endpoints
• CAPTCHA or bot detection systems
• Multi-factor authentication
• Monitoring suspicious login behavior

These controls significantly reduce the effectiveness of automated credential attacks.

Conclusion

This lab demonstrated how targeted credential attacks can be performed using both Burp Suite Intruder and ffuf.

By carefully selecting a small set of usernames and passwords and applying a cluster bomb attack strategy, it was possible to discover valid login credentials within a strict attempt limit.

Understanding these attack techniques helps security professionals identify weaknesses in authentication systems and design more secure login mechanisms.

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