The channel/<ucid>/playlists page was vulnerable to Cross Site Scripting
(XSS), because the different URL parameters were inserted as-is in the URL
meant for instance switching.
This vulnerability could allow an attacker to inject malicious Javascript
in the page by tricking the user to click on a crafted link.
Bug introduced in commit 66e7285108
("Only use /redirect when automatically redirecting").
Thanks to Jack (@testa:cthd.icu on Matrix, @cysea on github) for responsibly
reporting this issue!
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