CVE-2025-5802 in API Manager
Summary
by MITRE • 09/15/2026
The self-registration flow accepts user-supplied input for usernames without adequately preventing the disclosure of username existence. When a user attempts to register with an existing username, the system responds with an error message that explicitly indicates the username is already in use.
This behavior allows an attacker to discover valid usernames within the system. The discovery of valid usernames can facilitate subsequent attacks such as brute force, social engineering, and targeted phishing campaigns.
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Analysis
by VulDB Data Team • 09/15/2026
The vulnerability described constitutes a classic information disclosure flaw inherent in user registration workflows where username uniqueness is enforced without appropriate obfuscation mechanisms. This specific issue falls under the category CWE-204: Observable Response Discrepancy, as the application behaves differently depending on whether the input corresponds to an existing entity within the system's database. When a legitimate or malicious actor submits a username that already exists in the target environment, the server returns a distinct error message explicitly stating that the username is taken. Conversely, when a non-existent username is submitted, the system likely proceeds with account creation or displays a different generic success indicator. This differential behavior creates a reliable side channel through which an attacker can enumerate valid user identities without needing authentication credentials for those accounts.
From a technical perspective, this flaw arises because developers often prioritize immediate feedback to improve user experience during registration, inadvertently leaking sensitive state information about the backend data store. The application fails to implement consistent error handling that treats all inputs uniformly regardless of their existence in the directory or database. By providing specific feedback regarding username availability, the system violates the principle of least privilege and minimal disclosure, allowing external entities to map out the user base with high precision. This is particularly dangerous because usernames often serve as primary identifiers for subsequent authentication attempts and are frequently reused across multiple platforms by individuals.
The operational impact of this vulnerability extends far beyond simple data enumeration. The ability to accurately identify valid usernames significantly lowers the barrier for initiating more sophisticated attacks against the application's security posture. Attackers can leverage these discovered credentials to launch targeted brute force or credential stuffing campaigns, focusing their efforts only on accounts that are confirmed to exist rather than guessing randomly. This increases the efficiency and success rate of such automated attacks considerably. Furthermore, knowing valid usernames enables highly personalized social engineering and phishing initiatives. An attacker can craft emails that appear legitimate by addressing the recipient by their known username or referencing specific account details, thereby increasing the likelihood of successful credential theft through deception.
This behavior aligns with several techniques documented in the MITRE ATT&CK framework, specifically T1087: Account Discovery, which involves gathering information about accounts on a system or network to aid in follow-on attacks. It also relates to T1596: Search Open Technical Databases if the usernames are cross-referenced against other breached data sources. The vulnerability essentially provides the reconnaissance phase of an attack lifecycle with high-fidelity intelligence, facilitating the transition from passive information gathering to active exploitation phases such as brute force (T1110) or phishing for credentials (T1566).
To mitigate this risk, organizations must ensure that registration endpoints return identical responses regardless of whether the submitted username already exists in the system. The application should display a generic success message indicating that an account has been created and instructing the user to check their email for verification, even if the account creation fails due to duplicate entries. Alternatively, systems can implement rate limiting and CAPTCHA challenges on registration endpoints to slow down automated enumeration attempts while maintaining consistent error messaging. Additionally, implementing strict input validation combined with uniform HTTP status codes and response bodies ensures that no information about existing users is leaked through side channels. Regular security testing should include specific checks for these types of observable discrepancies during the reconnaissance phase of penetration tests to ensure such vulnerabilities are identified and remediated before they can be exploited by malicious actors.