MantisBT before 1.3.11, 2.x before 2.3.3, and 2.4.x before 2.4.1 omits a backslash check in string_api.php and consequently has conflicting interpretations of an initial \/ substring as introducing either a local pathname or a remote hostname, which leads to (1) arbitrary Permalink Injection via CSRF attacks on a permalink_page.php?url= URI and (2) an open redirect via a login_page.php?return= URI.