(From https://bugzilla.mozilla.org/show_bug.cgi?id=1753838)

Summary:
There was an open redirection vulnerability in the path of:

https://pollbot.services.mozilla.com/ and https://pollbot.stage.mozaws.net/

Description:
An attacker can redirect anyone to malicious sites.

Steps To Reproduce:
Type in this URL:

https://pollbot.services.mozilla.com/%0a/evil.com/

It redirects to that website

evil.com

evil.com was used as an example but this could be any website. Note, the /%0a/ and trailing / are required.

Supporting Material/References:
https://cheatsheetseries.owasp.org/cheatsheets/Unvalidated_Redirects_and_Forwards_Cheat_Sheet.html

Impact

Attackers can serve malicious websites that steal passwords or download ransomware to their victims machine due to a redirect and there are a heap of other attack vectors.