Open redirect vulnerability (CWE-601) in the _safe_redirect function of the click-tracking endpoint (/c//) in Mailerup <1.0.0 on all platforms allows remote unauthenticated attackers to redirect victims to arbitrary external sites and conduct phishing attacks via a crafted u query parameter, because the URL scheme is validated (blocking javascript: and data:) but the destination host is not restricted to an allowlist, and a signing.BadSignature exception is silently caught so a valid signed token is not required.
URL Redirection to Untrusted Site The web application accepts a user-controlled input that specifies a link to an external site, and uses that link in a redirect.