Service workers can use redirection to avoid the tainting of cross-origin resources in some instances, allowing a malicious site to read responses which are supposed to be opaque. This vulnerability affects Firefox < 61.
https://bugzilla.mozilla.org/show_bug.cgi?id=1467852
https://security.gentoo.org/glsa/201810-01
https://usn.ubuntu.com/3705-1/