JIT Miscompilation in JavaScript Engine Affects Firefox and Firefox ESR
CVE-2026-4702

9.8CRITICAL

Key Information:

Vendor

Mozilla
Status
Firefox
Firefox Esr
Thunderbird
Vendor
CVE Published:
24 March 2026

What is CVE-2026-4702?

A vulnerability exists in the JavaScript Engine component of Mozilla Firefox that may lead to JIT miscompilation. This issue affects users running Firefox versions prior to 149 and Firefox ESR versions before 140.9, potentially compromising browser security and performance. Users are encouraged to update to the latest versions to mitigate this vulnerability.

Affected Version(s)

Firefox < 149

Firefox ESR < 140.9

Thunderbird < 149

References

https://bugzilla.mozilla.org/show_bug.cgi?id=2013560
https://www.mozilla.org/security/advisories/mfsa2026-20/
https://www.mozilla.org/security/advisories/mfsa2026-22/

CVSS V3.1

Score:
9.8
Severity:
CRITICAL
Confidentiality:
High
Integrity:
High
Availability:
High
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
None
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

Evyatar Ben Asher, Keane Lucas, Nicholas Carlini, Newton Cheng, Daniel Freeman, Alex Gaynor, and Joel Weinberger using Claude from Anthropic
.