What’s Vulnerable › Vulnerabilities

CVE-2026-4701: Use-after-free in the JavaScript Engine component

• Severity: critical (CVSS 9.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-03-24 · updated 2026-06-17
• Weakness: CWE-416

Description

Use-after-free in the JavaScript Engine component. This vulnerability was fixed in Firefox 149, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

Affected products

• Mozilla Firefox: before 140.9.0; before 149.0 — fixed in 140.9.0, 149.0

How to fix

Upgrade to a fixed version: Mozilla Firefox 140.9.0, 149.0. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=2009303
• https://www.mozilla.org/security/advisories/mfsa2026-20/
• https://www.mozilla.org/security/advisories/mfsa2026-22/
• https://www.mozilla.org/security/advisories/mfsa2026-23/
• https://www.mozilla.org/security/advisories/mfsa2026-24/

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