Description

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.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.3",
  "pubDate": "2026-09-15T10:17:02.640Z",
  "pubdate": "2026-09-15T10:17:02.640Z",
  "executiveSummary": "The self-registration interface is susceptible to username enumeration, a vulnerability that allows unauthenticated actors to verify the existence of specific user accounts within the system.\nBy submitting registration requests with various identifiers, an attacker can distinguish between valid and invalid usernames based on the application's distinct server responses.\nThis vulnerability poses a significant security risk by facilitating targeted reconnaissance, which serves as a prerequisite for more advanced exploitation methods, including credential stuffing, brute-force attacks, and highly personalized social engineering or phishing campaigns.\nThe risk is elevated because the exploitation requires no prior authentication or specialized privileges, allowing for automated, large-scale enumeration of the user database with minimal effort.\nOrganizations should prioritize remediating this information disclosure to reduce the attack surface and harden the application against identity-based threats.",
  "technicalDetails": "The root cause of this vulnerability is improper handling of user-supplied input during the self-registration flow, specifically the application's failure to maintain consistency in server-side feedback when validating account uniqueness.\nWhen a registration request is received, the application performs a lookup against the user database to check for record existence. If the requested username is already associated with an account, the application explicitly triggers an error response—such as a 'Username already exists' message or a specific HTTP status code—which acts as a side-channel for verifying the presence of that user.\nThe exploitation flow is straightforward and does not require authentication. An attacker can craft an automated script to submit a series of registration requests containing a list of potential usernames (e.g., common email addresses or employee naming patterns). By parsing the HTTP response body or monitoring status codes, the attacker can systematically identify which usernames are registered in the system.\nThis vulnerability leverages the application's logic as an oracle. Because the system provides a binary indication of success or failure based on the existence of the username, the attacker can effectively map the target environment without triggering standard authentication logs or lockouts typically associated with login attempts.\nThe impact of successful enumeration is significant. Validated usernames become high-value targets for subsequent attacks. With a verified list of users, an attacker can launch targeted brute-force attacks against specific accounts while avoiding global account lockout thresholds that might be triggered by randomized guessing. Furthermore, the ability to confirm users allows for the construction of credible social engineering lures. For instance, if an attacker confirms a username that corresponds to a high-privileged user or a specific department, they can tailor phishing payloads to match the internal structure of the organization, drastically increasing the likelihood of a successful credential harvest or malware delivery. The lack of rate limiting on these registration requests further amplifies the risk, allowing an attacker to harvest the entire user database efficiently."
}