Looking through Twitter recently, I caught a very interesting stream that started with the following message:
What's the deal with the enumeration exclusions on all the @bugcrowd bounties. Clients just don't want to fix?
— Stephen Haywood (@averagesecguy) July 26, 2016
There were quite a few replies, and a good discussion on the topic of the seriousness of username enumeration flaws. 140 characters is difficult to share a lot of thoughts, so I thought this would actually be a great post.
What is Username Enumeration?
Username Enumeration refers to the ability to determine not only if a username is valid within a specific application, but also to automate the process of identifying a multiple valid usernames. The most common example of username harvesting is on the application logon form. The culprit, the unsuccessful login error message. This happens because the application creators want to be helpful and let the user know what went wrong. When the message is not generic, for example “unsuccessful login”, it may indicate if the user exists or not. An example message would be “Username does not exist”. This is bad. A better response would be “Username or password are incorrect”. This is a much more generic answer and doesn’t give away whether or not the username is valid.
Where do we find Username Enumeration?
As I mentioned above, the login screen is one of the most common locations we find username harvesting. However, it is not the only place. Probably the two next biggest offenders are the Forgot Password screen and the User Registration screen. With all three of these screens, it is critical to realize that the display message is not the only way to detect username harvesting. In reality, it is any difference that comes back between an invalid and a valid user. This could be the error message, a status code that is different, timing attacks, or a myriad of other techniques. Don’t just think about the error message. That one is typically the easiest, but not the only method for identification. API calls may also be a source for username harvesting.
The Login Form
The login form is typically vulnerable through the error message. To help protect the login form, make sure that the application returns a generic error message for any error situation. A good example of this is “Your login attempt was unsuccessful. If you continue to have trouble, contact support.” This message is good for invalid username, invalid password, and even account lockout.
The Forgot Password Form
The forgot password screen is usually vulnerable due to poor forget password design. Most systems ask for the username/email address and if correct, do one of the following: 1) Ask for secret security questions, 2) Display a message that an email has been sent to the email address on record. Unfortunately, for an incorrect username, the system returns an error message that the username was not found. To correct this problem, it is possible to either ask for more than just the username (something that is private to the account), or just always display the “email has been sent” message. In this configuration, no matter what, the email is sent form is displayed.
The Registration Form
The registration screen is a bit more tricky because, in most situations, you need to be able to alert the user that their username is already in use. Unlike the other two scenarios, the error message can’t really be fixed. One option on a registration screen is to enable the use of a CAPTCHA to attempt preventing automation attacks. If implemented properly the attacker would be slowed down in their enumeration making things more difficult. It may not make it impossible, but the goal is to reduce the risk.
How is Username Enumeration Used?
There are a few ways that username enumeration becomes important to an attacker. First, it is a good technique to limit down the list of possible usernames to attempt password attacks on. One example on twitter gave the example of taking a list of 100 million usernames and reducing it to 100 thousand to perform actual password guessing on. In this scenario, the attackers are also attempting to take advantage of weak password complexity, or just bad password hygiene. Often times, they may take the top 3 or 4 common passwords and try those against all of the valid usernames to see if anyone is using them.
The second way that username enumeration is used is for social engineering attacks. Or, at least, more targeted social engineering attacks. If the attacker knows that the user has an account on a specific application, it can make phone calls and emails much more convincing and ultimately more successful. Limiting emails to 100,000 users rather than 100 million helps reduce the overhead and reduces the chances of getting detected.
Somewhat related to social engineering attacks, this information could also be used to target individuals based on moral values. Remember the Ashley Madison breach that happened a while back. Depending on what the username is in use, it may be possible to identify people that use a particular site and sort of out them, or blackmail them. Think of other sites people may not want to be public about visiting.
How Serious is it?
I bet you didn’t think we would ever get here. It is important to understand how the attackers may be using a vulnerability to start to determine how serious it really is. Unfortunately, there isn’t an easy answer that fits every application. As I mentioned above, with poor password hygiene, the risk would be raised if someone is using one of those top 4 passwords. It also depends on the type of system, the type of data collected, the functionality the application performs, etc. No systems are created or rated equally.
In the scenario above for the login screen guessing the top 4 passwords, let’s not forget that it is possible to try all 100 million usernames from our list. Sure, it is helpful to reduce it down to only valid usernames, but the attack is still possible without the username enumeration flaw. In the ideal setup, the application would have some form of detection system in place that might detect all of the unsuccessful login attempts, but that is the ideal. It is not common enough to see that type of detection implemented in many applications. Without that detection, and a little crafty rate limiting by an attacker, the difference between 100 million attempts and 100,000 attempts is really just time.
As we all know, social engineering attacks are the latest and greatest. Any help we provide attackers in that arena is a step backwards for us. This could lead to a bigger problem and that is why we need to understand the issues we are trying to solve.
Should we fix it?
If we had all the time and resources in the world, the answer would be an easy “yes”. Unfortunately, we don’t have that. This really comes down to your organization’s risk assessments and priorities. I always like to provide the recommendation to resolve the issue, as it makes for a better application overall. Depending on your situation, the fix may not be straight forward or easy. You may have other higher priority items that need to be worked on. Remember, for the password attack, it is really a combination of a weak password issue as well. It is not just username harvesting on its own.
Take the opportunity to review how you are auditing and logging these types of events within your application. In some cases we may see logging occurring, but then there is no auditing of those logs. Detection and response is critical in helping reduce the risk of these types of flaws.
Username enumeration is a risk to any application. It is up to the organization to understand their business to determine how big or small that risk is. This isn’t something that should just be ignored, but it also doesn’t mean it is something that is a critical finding. Don’t just write the finding off. Use that as an opportunity to discuss the functionality, understand how it works, and determine the best course of action for the organization and the users.
Username enumeration is typically fairly simple to test for and should be worked into the QA testing cycle. Take the time to discuss with the architects, developers, business analysts, and the testers how the organization can test for this and reduce the risk.
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James Jardine is the CEO and Principal Consultant at Jardine Software Inc. He has over 15 years of combined development and security experience. If you are interested in learning more about Jardine Software, you can reach him at email@example.com or @jardinesoftware on twitter.