OpenMage magento-lts before 20.18.2 and 21.x before 21.0.0-beta3 validates OAuth 1.0a oauth_callback URLs with a string-prefix check and falls back to accepting any well-formed URL. Attackers holding a leaked consumer key and secret can supply host-suffix, userinfo, or arbitrary callback URLs to capture an approving administrator's or customer's request token and verifier, then obtain an access token with the victim's API privileges.