What’s Vulnerable › Vulnerabilities

CVE-2021-23986

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-03-31 · updated 2026-06-17
• Weakness: CWE-346

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.

Affected products

• Mozilla Firefox: before 87.0 — fixed in 87.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 87.0. Check the vendor advisory for mitigations.

References

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

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