Description

The HTTP Basic Authentication modules in ((OTRS)) Community Edition 6 and prior accept the authenticated username from either the REMOTE_USER or HTTP_REMOTE_USER environment variable. In CGI and mod_perl based deployments, HTTP request headers may be mapped to corresponding HTTP_* environment variables. As a result, an unauthenticated remote attacker can supply a crafted Remote-User request header which is exposed to the application as HTTP_REMOTE_USER. When HTTP Basic Authentication is configured as the authentication backend, OTRS trusts this value as an already authenticated username and does not perform an additional password verification. An attacker may therefore authenticate through the Generic Interface as an existing agent or customer user without knowing the corresponding credentials. The resulting privileges depend on the account being impersonated. Successful exploitation may allow access to confidential information and modification of data or configuration with the permissions of the affected user. Exploitation requires HTTP Basic Authentication to be configured as an authentication backend and a web server environment in which attacker-controlled HTTP headers are exposed through the corresponding HTTP_* environment variables. OTRS 7 and later are not affected by this attack vector due to the use of Mojolicious/Hypnotoad and does not expose arbitrary request headers through %ENV in the manner required for exploitation. The supported REMOTE_USER mechanism additionally requires validation through the configured SSO shared secret. The unused HTTP_REMOTE_USER fallback has nevertheless been removed as a security-hardening measure. Please note that ((OTRS)) Community Edition 6.x and before are vulnerable. Products based on the ((OTRS)) Community Edition also very likely to be affected

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "9.4",
  "pubDate": "2026-10-12T07:16:37.717Z",
  "pubdate": "2026-10-12T07:16:37.717Z",
  "executiveSummary": "The vulnerability involves an authentication bypass flaw in the HTTP Basic Authentication module of ((OTRS)) Community Edition 6 and prior versions.\nThe issue stems from the application's reliance on unsanitized environment variables, specifically HTTP_REMOTE_USER, to establish user identity.\nAn unauthenticated remote attacker can exploit this by injecting a malicious 'Remote-User' HTTP header, which is then mapped to the HTTP_REMOTE_USER environment variable by CGI and mod_perl web server deployments.\nUpon successful exploitation, the application erroneously trusts this header, granting the attacker full access to the target account without requiring credentials.\nThe impact is significant, potentially allowing unauthorized access to confidential information, modification of internal data, and configuration changes, contingent upon the privileges of the impersonated user.\nExploitation requires the target environment to have HTTP Basic Authentication configured as the authentication backend and a web server architecture that exposes arbitrary headers via %ENV.",
  "technicalDetails": "The root cause of this vulnerability lies in the improper processing of authentication inputs within the HTTP Basic Authentication modules of ((OTRS)) Community Edition 6.x and earlier.\nThe application logic attempts to determine the current user identity by checking environment variables, specifically looking for either REMOTE_USER or HTTP_REMOTE_USER. In common web server configurations using CGI or mod_perl, the server maps arbitrary client-supplied HTTP headers into the environment space using the 'HTTP_' prefix.\nAn attacker can leverage this behavior by sending a crafted HTTP request containing a 'Remote-User' header. The web server environment then populates the HTTP_REMOTE_USER variable with the attacker-provided value.\nWhen the OTRS authentication backend processes this request, it prioritizes or fails to validate the source of this variable, treating the value as a pre-authenticated identity provided by the web server's authentication layer. Because the application assumes that the web server has already performed the necessary password verification, it bypasses its own authentication routines entirely.\nThe attack flow follows a predictable pattern: First, the attacker identifies an OTRS instance utilizing HTTP Basic Authentication. Second, the attacker crafts a malicious HTTP request by inserting a 'Remote-User: [target_username]' header. Third, as the request is processed by the CGI/mod_perl interface, the application inspects the environment and retrieves the 'target_username' from the HTTP_REMOTE_USER variable. Finally, the application session is initialized with the privileges of the impersonated user account.\nThis vulnerability is limited to environments where these specific web server interface mappings exist; however, where present, it allows for total account takeover. Post-exploitation impact varies based on the impersonated account, but in the case of administrative accounts, this results in full system compromise. OTRS 7 and subsequent versions are immune, as they utilize the Mojolicious/Hypnotoad framework, which prevents the leakage of arbitrary headers into the %ENV environment and mandates validation against an SSO shared secret for the REMOTE_USER mechanism."
}