If the attack can be automated (by comparing the response to when a username is not in use), it allows an attacker to whittle down a large list of potential usernames to a smaller list of confirmed usernames.

An app signup error message saying “username already in use” would be one example. User enumeration flaws provide attackers with a method to determine whether a specified username exists. But first, a quick recap for readers that are less familiar with the terminology: What is user enumeration?

Let’s take a look at the attack scenarios these bugs create, and explore why they should be considered a security risk, particularly when it comes to Microsoft products. They are generally ignored by the industry and are under the radar of most security professionals but is this an acceptable situation or should you be working to find and fix these issues in your infrastructure? Most organisations won’t be aware that these flaws exist they are not detected by the leading vulnerability scanners, documented by Microsoft or even assigned a CVE reference. Over the last decade or so multiple user enumeration flaws have been discovered in a range of popular Microsoft products that are commonly exposed to the Internet, such as Outlook Web Access and Skype for Business. The flaws have been exposing internal corporate networks to attacks for years, yet are undetected by leading vulnerability scanners. Intruder’s latest research reveals that up to 13,000 organisations are affected by little-known user enumeration flaws in a range of popular Microsoft products.

How to use nessus enum