When a link to an external protocol was clicked, a prompt was presented that allowed the user to choose what application to open it in. An attacker could induce that prompt to be associated with an origin they didn't control, resulting in a spoofing attack. This was fixed by changing external protocol prompts to be tab-modal while also ensuring they could not be incorrectly associated with a different origin. This vulnerability affects Firefox < 82.

References

• https://nvd.nist.gov/vuln/detail/CVE-2020-15682
• https://bugzilla.mozilla.org/show_bug.cgi?id=1636654
• https://www.mozilla.org/security/advisories/mfsa2020-45/