Rhino Security Labs recently identified two critical username enumeration vulnerabilities in the AWS Web Console that could expose IAM (Identity and Access Management) users to potential risks. These vulnerabilities, which fall under AWS’s shared responsibility model, were discovered to affect the security of the cloud platform itself, rather than being a customer-side issue. While one of the vulnerabilities (CVE-2025-0693) was quickly patched by AWS, the other was considered an “accepted risk” by the company. The security flaws highlight how seemingly minor issues, such as inconsistencies in error messaging and server response times, can pose significant threats to cloud environments.
The first vulnerability arises when Multi-Factor Authentication (MFA) is enabled for an IAM user. During the login process, the application reveals whether a username exists based on different states: if the username exists, the user is prompted for an MFA code, and if it does not exist, an error message explicitly states that the user doesn’t exist. This discrepancy provides attackers with a simple method of confirming valid usernames without needing specialized tools.
Such behavior is particularly alarming in a major cloud platform like AWS, where consistent error messaging is expected to avoid these kinds of risks.
The second vulnerability involves a timing attack that affects non-MFA users. When a single-factor user attempts to log in, the server’s response times differ depending on whether the username is valid. For valid usernames, the system spends more time validating the password, resulting in a slower response, while for invalid usernames, the server exits early, leading to a quicker response. Researchers demonstrated that even a small delay of around 100ms could be enough to indicate the validity of a username, allowing attackers to pinpoint valid accounts with minimal effort. AWS has patched this vulnerability by introducing uniform delays for all authentication failures.
Despite AWS’s quick response to patch the CVE-2025-0693 vulnerability, organizations must remain vigilant. The vulnerabilities generate distinct CloudTrail events, which can help identify enumeration attempts through repeated login failures or other patterns. AWS’s patch, while effective, highlights the importance of securing IAM login flows, using MFA, and regularly monitoring for suspicious activity. Organizations are advised to monitor logs and ensure authentication systems provide consistent server behavior to mitigate the risks of username enumeration attacks in the future.
