In Jenkins 2.499 and earlier, LTS 2.492.1 and earlier, redirects starting with backslash (`\`) characters are considered safe, allowing attackers to perform phishing attacks by having users go to a Jenkins URL that will forward them to a different site, because browsers interpret these characters as part of scheme-relative redirects.
https://www.jenkins.io/security/advisory/2025-03-05/#SECURITY-3501