22 Feb, 2024

Username Enumeration through Response Analysis


By comparing the  responses the system gives to different username inputs, one can determine which usernames are valid within a system through a process known as “username enumeration through response analysis.” When a system responds differently to authentication attempts or data requests, it can give away whether or not a submitted username exists. This is known as a security vulnerability. By taking advantage of these variations, attackers can generate a list of legitimate usernames that they can use for additional attacks like phishing, targeted social engineering, or brute-force password cracking.

By verifying which usernames are legitimate targets, username enumeration carries the risk of minimizing the attacker’s effort to compromise accounts. In addition, it violates user privacy and may result in further exploitation of system vulnerabilities and unauthorized access.

When an attacker can spot behavioral shifts on the website to determine whether a given username is true, this is known as username enumeration.

Username enumeration usually happens on registration forms when you enter a username that is already taken, or on the login page when you enter a valid username but an incorrect password. Because the attacker can quickly generate a shortlist of valid usernames, this significantly reduces the time and effort required to brute-force a login.

When trying to brute-force a login page, you should be especially aware of any variations in

Status codes: Because the majority of guesses made during a brute-force attack will be incorrect, the returned HTTP status code will almost certainly be the same for the great majority of guesses. A distinct status code in response to a guess suggests that the username was accurate. Although it is recommended that websites always return the same status code, regardless of the result, this recommendation is not always adhered to.

Error messages: Depending on whether the username and password are wrong together or if the password is wrong alone, the error message that is returned may vary at times. While using identical, generic messages in both scenarios is best practice for websites, occasional typos do happen. Even in situations where the character is not visible on the rendered page, the two messages can be distinguished by just one character being out of place.

Response times: If most of the requests were handled with a similar response time, any that deviate from this suggest that something different was happening behind the scenes. This is another indication that the guessed username might be correct. For example, a website might only check whether the password is correct if the username is valid. This extra step might cause a slight increase in the response time. This may be subtle, but an attacker can make this delay more obvious by entering an excessively long password that the website takes noticeably longer to handle.Even in situations where the character is not displayed on the rendered page, a single misplaced character distinguishes the two messages.


In this example, we will first find the username and then the password by using Burpsuite and brute force

first we will enter the username Jennifer and the password 12345

Then let’s retrieve the list of usernames and run the attack to see the responses to the request

After the attack we see that of all the requests sent we received all the same responses except for one, the username root now we go and try to log in to our site again. With root username 

Now we see that the site gives us an error that the password is wrong, so this tells us that we already know the username for sure.

Now we do the same thing with the password, which is to load our password list.

after the attack, the response shows us a 302 status.
This means that the password was found
We look through the request and see that our matching password is andrew

Now we try to access our site with the data we’ve obtained.

Now we’ve checked and we know we’ve picked the right data

Example of Vulnerable Code:

PHP  – Login System

if (isset($_POST['username']) && isset($_POST['password'])) {
    $username = $_POST['username'];
    $password = $_POST['password'];

    // Simulated database check for username
    if (databaseCheckUsername($username)) {
        if (checkPasswordForUsername($username, $password)) {
            echo 'Login successful!';
        } else {
            echo 'Invalid password for user.';
    } else {
        echo 'Username does not exist.';


Python (Flask Web Application)

					from flask import Flask, request

app = Flask(__name__)

@app.route('/login', methods=['POST'])
def login():
    username = request.form.get('username')
    password = request.form.get('password')

    # Simulated database check for username
    if database_check_username(username):
        if check_password_for_username(username, password):
            return 'Login successful!'
            return 'Invalid password for user.'
        return 'Username does not exist.'

if __name__ == '__main__':



In both cases, the username is susceptible to enumeration since the application notifies the client explicitly whether the username exists.

Privilege Escalation Techniques:

Attackers may try privilege escalation after usernames have been listed in order to obtain more privileged access. Typical methods consist of:

Password Attacks: Targeted password guessing or brute-force attacks can be carried out by attackers with a list of legitimate usernames.
Phishing is the practice of tricking enumerated users into divulging passwords or downloading malware by sending them personalized phishing emails.

Exploiting Account Recovery: This involves trying to recover accounts using a list of usernames and taking advantage of predictable or weak password reset questions.

Methodology on Testing for Vulnerability and Checklist

1. Preparation

Assemble Tools: Get ready for automated or semi-automated penetration testing  tools like Burp Suite, OWASP ZAP, or custom scripts.
Determine the Target Endpoints: Pay attention to sections of the application that call for user identification, like the pages for account creation, password resets, and login.

2. Testing

Examine Reactions to Incorrect Usernames: Enter erroneous usernames and record the HTTP status codes, error messages, response times, and other noticeable patterns.
Enter a Valid Username: Try using a well-known, valid username if you can, and see how the application reacts to erroneous entries.
Determine Trends: Keep an eye out for responses that consistently differ between valid and invalid usernames.

Automate Enumeration : Create or use scripts to automatically submit usernames and analyze the responses in order to quickly determine which usernames are valid.

3. Documentation

Record Findings: Keep track of screenshots, HTTP responses, and error messages pertaining to the variations in responses that permitted username enumeration.
Evaluate Impact: Take into account the application’s context and the significance of the listed data when evaluating the vulnerability’s possible effects.

4. Mitigation Recommendations

Uniform Error Messages: Suggest adding general error messages that conceal the validity of a username.
Rate Limiting and Account Lockout: To stop automated attacks, recommend putting rate limiting and account lockout procedures in place.
Suggestions for monitoring and logging suspicious activities should be made in order to analyze and take appropriate action.

CWE Information

The following Common Weakness Enumeration (CWE) entries are frequently linked to username enumeration vulnerabilities:

CWE-203:  This vulnerability arises when an application exhibits inconsistent behaviors or responses that could be exploited to obtain private information, like the presence of a legitimate username

CWE-200: is a broad category that includes username enumeration and any situation in which sensitive information is disclosed to an actor not expressly granted access to it.

CWE-640: Weak Password Recovery Method for Forgotten Password: This pertains to the enumeration of usernames via password recovery tools that validate or invalidate an account based on the entered username

Top CVEs


Because username enumeration is more of an auxiliary or informational vulnerability, its specific CVEs (Common Vulnerabilities and Exposures) may not be as well-known or documented as those for other kinds of vulnerabilities. Nonetheless, the following instances of information disclosure or related problems—which may involve username enumeration—have been noted:


1. CVE-2020-7247:

Due to a vulnerability in OpenBSD’s OpenSMTPD, information could have been exposed as remote attackers could have read any file on the system by enumerating usernames.

2. CVE-2018-1312:

An enumeration vulnerability existed in the mod_authnz_ldap module of Apache HTTP Server prior to 2.4.33, whereby an attacker could utilize a specially crafted request to trigger an LDAP search that would return all valid usernames.

3. CVE-2019-1010315:

Before GitLab CE/EE 12.0.3, there existed a vulnerability known as the Username Enumeration Vulnerability, which allowed an attacker to use an error message to enumerate usernames.

Scanners and tools

Manual Tools

1. Burp Suite 

a potent instrument for testing the security of web applications. By tailoring requests and examining responses, its Intruder and Repeater features can be used to test for username enumeration.


An open-source web application scanner that can be used by hand to examine the way the program reacts to manipulated requests in order to check for different vulnerabilities, such as username enumeration.

Automated Tools

1. Nmap with NSE Scripts

By examining web server responses, Nmap’s scripts like http-enum—which work in tandem with its scripting engine (NSE)—can assist in locating files, directories, and usernames.

2. WPScan

One of the many features of this WordPress security scanner is the ability to count users, which is helpful for WordPress-powered websites.

3. Metasploit

Metasploit is well-known for its exploitation features, but it also has auxiliary modules for information gathering, such as username enumeration.

Courses and Books


1. “Web Application Penetration Testing” by eLearnSecurity

focuses on different web application vulnerabilities and teaches practical skills for finding and taking advantage of them, such as username enumeration.

2. The Complete Web Application Hacking & Penetration Testing” (Available on platforms like Udemy)

provides a thorough examination of web application vulnerabilities and includes methods for response analysis and username enumeration.

3. “Advanced Web Attacks and Exploitation” by Offensive Security (AWAE) 

A challenging course that explores the nuances of web application vulnerabilities and is appropriate for students who want to learn intricate enumeration methods.

4. OWASP Top 10 Web Application Security Risks courses (Available on Coursera, Pluralsight, etc.)

These courses often cover information leakage and enumeration techniques in addition to the top 10 web security risks.


1. The Web Application Hacker’s Handbook: Finding and Exploiting Security Flaws by Dafydd Stuttard and Marcus Pinto

Thorough manual that covers methods for identifying and taking advantage of different web application vulnerabilities, such as username enumeration.

2. Hacking: The Art of Exploitation by Jon Erickson

Offers a thorough analysis of exploitation methods, including those useful for enumerating usernames.

3. Black Hat Python: Python Programming for Hackers and Pentesters by Justin Seitz

Provides information on how to use Python for security testing and provides scripts that could be modified to count the number of usernames.

4. Penetration Testing: A Hands-On Introduction to Hacking by Georgia Weidman

contains sections on web application penetration testing that are useful for comprehending and taking advantage of vulnerabilities caused by username enumeration.


A serious security flaw in ethical hacking and penetration testing that exposes the subtleties of how web applications handle and react to authentication requests is username enumeration through response analysis. This vulnerability occurs when an application unintentionally divulges information about the legitimacy of usernames in response to login attempts, password reset requests, or any other type of user identification query. These disparities in replies can give attackers important information, enabling them to pinpoint genuine user accounts for additional malevolent actions.

This vulnerability has wide-ranging consequences. An attacker can increase the success rate of targeted attacks like spear-phishing, social engineering campaigns, and brute-force password guessing by using a list of verified usernames. In addition to jeopardizing individual user accounts, this can lead to more widespread security lapses that affect the system’s availability, confidentiality, and integrity.

A sophisticated strategy that strikes a balance between security and user experience is needed to address username enumeration. The use of CAPTCHAs to thwart automated enumeration tools, rate-limiting authentication attempts to prevent automated attacks, and uniform error messages that do not distinguish between incorrect usernames and passwords are examples of best practices.

Improving awareness and education are essential for reducing this vulnerability. It is imperative for developers, security experts, and system administrators to embrace secure coding practices and be cognizant of the intricacies of attacker response analysis. Permeability testing, regular security audits, and adherence to security frameworks such as OWASP can assist in locating and fixing such vulnerabilities prior to their potential for exploitation.

Finally, Username Enumeration Through Response Analysis serves as a helpful reminder of the careful balance that must be maintained between security and user interface design. Organizations can maintain the security posture of their systems against potential attacks, safeguard the identities of their users, and preserve trust by comprehending and mitigating this vulnerability.

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