During one of audits I performed a pentest of custom web platform providing organisational and personnel management functionality. The assessment was conducted with a black box methodology, allowing to interact with the system exclusively through the exposed HTTP interface and without privileged source code access. Introduction During routine brute force checks, I observed strange behaviour in the login endpoint. Subsequent experimentation demonstrated the existence of an embedded backdoor that short circuits credential validation. Any credential pair whose username and password begin with the lowercase character “x” - or consist of that single character - grants immediate access to an authenticated session. Since the backdoor bypasses role mapping entirely, the session automatically gets the highest level of privileges, which exposes personal data (PII) and the full organizational structure. Discovery and verification The first sign of weak authentication appeared when a large wordlist was used against /Login. Surprisingly, adaptive throttling didn’t activate, and many requests returned HTTP 302 redirects, even with clearly wrong credentials.

To better understand the issue, I have created a small set of credentials, changing only the first character. The two requests below show the key findings and are taken directly from the intercepting proxy.

Variant A - arbitrary suffix after the magic prefix Variant B - stand alone “x” character Both submissions triggered the same server response: a 302 redirect to /Dashboard, a new session cookie, and a user context with administrator privileges. This behavior was consistent across different user agents, network addresses, and cookies, showing that the bypass was deterministic, not caused by a race condition or cache issue. Technical exploitation walk through After confirming the existence of the bypass, I created a full exploitation scenario to measure its operational impact. Using Variant A, I authenticated and navigated through the user management panel. Server side rendering immediately revealed tables containing full names, e mail addresses, telephone numbers and departmental assignments for every account in the system.

To validate privilege escalation the same session was directed to the administrative configuration area. The interface exposed CRUD operations for organisational units, role assignment pages and audit log viewers, confirming unrestricted access. Log review demonstrated that each action appeared under an unregistered user identifier created ad hoc by the session bootstrap routine, indicating that the backdoor bypasses not only credential checks but also canonical user existence constraints.

A repeat of the examination with Variant B produced identical results, evidencing that a single sentinel character is treated as a master credential. At no point did the application trigger multifactor challenges, IP lockouts or even warning banners, highlighting a complete absence of compensating controls. Impact assessment and Threat Modelling From a confidentiality perspective, the flaw immediately exposes all stored personal and organizational data. Integrity is also at risk, as an attacker can change records, create fake users, or delete audit logs while posing as a super administrator. This cause that unauthenticated attackers could disable key modules or revoke valid accounts, causing a denial of service for authorized users.

The attack doesn’t need special tools, insider access, or social engineering. It only requires network access to the login endpoint, making the likelihood of exploitation very high and the time to compromise almost zero. Because the bypass is deterministic, traditional security monitoring, like detecting unusual numbers of failed logins won’t catch it, since each attack succeeds on the first attempt. Remediation strategy and concluding remarks Immediate action requires permanently removing the hardcoded backdoor. A thorough code review is needed to find similar patterns in other parts of the system. Automated static analysis checks should be added to the continuous integration pipeline to catch such issues in the future. After removing the vulnerable code, the whole authentication system must go through penetration testing with both typical and edge case scenarios. Finally, the security policy should be updated to clearly forbid non-standard login shortcuts, so developers and QA teams treat them as serious flaws, not helpful features.