Enumerating Users
Definition:
User enumeration vulnerabilities occur when a web application provides different responses for valid and invalid inputs at authentication endpoints, allowing attackers to identify valid usernames.
Common Scenarios:
User enumeration often arises in functions like user login, registration, and password reset.
Developers may overlook these vulnerabilities, mistakenly believing usernames are not sensitive. However, usernames can be critical identifiers for authentication across various services (e.g., FTP, RDP, SSH).
User Enumeration Theory:
Protecting against username enumeration can impact user experience. For example, an application may provide feedback on whether a username exists, which can help legitimate users but also assists attackers.
Even established applications (e.g., WordPress) can be vulnerable. Different error messages for valid and invalid usernames can reveal information:
Invalid username: "unknown user. check again or try your email address."
Valid username with incorrect password: "Error. the password you entered for the username [username] is incorrect."
While some user enumeration functionalities (like searching for usernames) are essential for user experience, they can also pose security risks. It's advisable to avoid exposing usernames in login processes, potentially using email addresses instead.
Enumerating Users via Differing Error Messages:
Attackers can use a wordlist of usernames to test against the application. Usernames are typically simpler than passwords and often lack special characters.
Common usernames can be targeted for brute-force attacks or OSINT-based attacks against specific users.
Public information and web crawling can also help gather usernames. A useful resource is the SecLists collection.
Example of User Enumeration:
Attempting to log in with an invalid username (e.g., "abc") returns: "unknown user."
Attempting to log in with a valid username (e.g., "admin") and an invalid password returns: "invalid credentials."
Using Tools for Enumeration:
Tools like
ffufcan automate the enumeration process. For example:ffuf -w /opt/useful/seclists/Usernames/xato-net-10-million-usernames.txt -u http://<SERVER_IP>:<PORT>/index.php -X POST -H "Content-Type: application/x-www-form-urlencoded" -d "username=FUZZ&password=invalid" -fr "Unknown user"This command helps identify valid usernames by filtering out responses containing "Unknown user."
User Enumeration via Side-Channel Attacks:
Side-channel attacks can also be used for user enumeration. These attacks exploit indirect information, such as response timing.
If a web application performs database lookups only for valid usernames, attackers may measure response times to infer valid usernames, even if the responses appear identical.
Summary:
User enumeration vulnerabilities can significantly compromise security by allowing attackers to identify valid usernames through differing error messages or side-channel information. Developers should implement measures to mitigate these vulnerabilities, such as using email addresses for authentication and ensuring consistent error messaging to protect user information.
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