Cross-site scripting (XSS) vulnerability in blocks/html/block_html.php in Snoopy 1.2.3, as used in Moodle 1.6 before 1.6.9, 1.7 before 1.7.7, 1.8 before 1.8.8, and 1.9 before 1.9.4, allows remote attackers to inject arbitrary web script or HTML via an HTML block, which is not properly handled when the "Login as" feature is used to visit a MyMoodle or Blog page.