Squirrelmail 1.4.15 does not set the secure flag for the session cookie in an https session, which can cause the cookie to be sent in http requests and make it easier for remote attackers to capture this cookie.
Cross-site scripting (XSS) vulnerability in search.php in SquirrelMail 1.5.1 and earlier, when register_globals is enabled, allows remote attackers to inject arbitrary HTML via the mailbox parameter.