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.
[
{
"product": "mediawiki",
"vendor": "mediawiki",
"versions": [
{
"status": "affected",
"version": "n/a"
}
]
}
]