A malicious extension with the 'search' permission could have installed...

Vulnerability Description

A malicious extension with the 'search' permission could have installed a new search engine whose favicon referenced a cross-origin URL. The response to this cross-origin request could have been read by the extension, allowing a same-origin policy bypass by the extension, which should not have cross-origin permissions. This cross-origin request was made without cookies, so the sensitive information disclosed by the violation was limited to local-network resources or resources that perform IP-based authentication. This vulnerability affects Firefox < 87.

References

• https://www.mozilla.org/security/advisories/mfsa2021-10/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1692623