URL Redirection to Untrusted Site ('Open Redirect')
Mediawiki before 1.28.1 / 1.27.2 / 1.23.16 has a flaw where Special:UserLogin?returnto=interwiki:foo will redirect to external sites.
CWE-601 - Open Redirect
An open redirect attack employs a URL parameter, HTML refresh tags, or a DOM based location change to exploit the trust of a vulnerable domain to direct the users to a malicious website. The attack could lead to higher severity vulnerabilities such as unauthorized access control, account takeover, XSS, and more.