MantisBT 2.28.3 and earlier contains a critical authentication bypass in the SOAP API's mci_check_login() function. Any user knowing any valid cookie_string can authenticate as any other user (knowing their username), including the administrator, without knowing the target's password. The vulnerability is exploitable with zero prior access on default MantisBT installations because self-registration is enabled by default ($g_allow_signup = ON). A self-registered user can use their own cookie_string (readable from their browser's MANTIS_STRING_COOKIE cookie after login) to impersonate the administrator via the SOAP API. The REST API is NOT affected. The REST API's AuthMiddleware derives the username server-side from the API token or session cookie, so the username cannot be spoofed. The Web UI is NOT affected. The Web UI authenticates via PHP session cookies (PHPSESSID) and validates the MANTIS_STRING_COOKIE against the logged-in user through auth_is_cookie_valid(). The username is derived server-side from the cookie, not supplied by the client.