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# Detecting Usernames Through Abuse of Functionality Vulnerabilities - Websecurity (English translation)

**Виявлення логінів через Abuse of Functionality уразливості - Websecurity** - Author not stated, websecurity.com.ua.

- Title in English: Detecting Usernames Through Abuse of Functionality Vulnerabilities - Websecurity
- Published: date not stated
- Original: <http://websecurity.com.ua/2840/>
- Preserved from: http://websecurity.com.ua/2840/ (live) on 2026-08-10
- Licence: unknown

Rights remain with the original author and publisher. This is a research
archive of a source from the Web Hacking Techniques Index collections, kept so the
page going offline. To read the original, follow the link above.

## Content (translated into English)

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Detecting Usernames Through Abuse of Functionality Vulnerabilities - Websecurity - Web Security

---

## [Detecting Usernames Through Abuse of Functionality Vulnerabilities](http://websecurity.com.ua/2840/)

22:46 30.01.2009

Over the past few years, I have encountered many times a feature on some websites (primarily email services and large projects) that makes it possible to check whether a given username is available. This allows a user to create a unique username when registering on the website. And so, in March 2008, when I developed my Brute force login identifier program (for discovering usernames, something I have to deal with during security audits), I decided to conduct a detailed study of the username-checking feature.

This feature allows an attacker to discover valid usernames in the system (login enumeration). In other words, the presence of this feature on a website gives rise to an Abuse of Functionality vulnerability. I have provided examples of similar vulnerabilities, in particular, on [hulu.com](http://websecurity.com.ua/2834/) and on [www.youtube.com](http://websecurity.com.ua/2837/).

Let us examine the algorithm for discovering a username on YouTube.

If you enter the username being checked in the Username field of the registration form (http://www.youtube.com/signup) and click Check Availability, the system will perform a check and provide a response (this feature is implemented using AJAX). If the response is “Username unavailable,” that username exists in the system; if the response is “Username available!”, that username does not exist in the system.

Thus, you need to check a list of usernames using the Check Availability feature and select those for which the response is “Username unavailable.” You can then create a list of valid usernames.

If this feature has no protection against automated attacks (that is, if there is an Insufficient Anti-automation vulnerability), as is the case most of the time, it allows automated discovery of usernames in the system. This can be done using username brute-forcers, for example, my Brute force login identifier program. The usernames discovered can subsequently be used to determine the passwords of the website's users.