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PatchSiren cyber security CVE debrief

CVE-2026-4698 Mozilla CVE debrief

CVE-2026-4698 is a critical Mozilla JavaScript engine JIT miscompilation issue. According to the published advisory data, the flaw was fixed in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9. Because the CVSS score is 9.8 and the vector is network-reachable with no privileges or user interaction required, this should be treated as an urgent update item for any environment running affected Mozilla desktop clients.

Vendor
Mozilla
Product
Firefox
CVSS
CRITICAL 9.8
CISA KEV
Not listed in stored evidence
Original CVE published
2026-03-24
Original CVE updated
2026-07-15
Advisory published
2026-03-24
Advisory updated
2026-07-15

Who should care

Organizations running Firefox or Thunderbird, especially those using ESR channels, should prioritize this immediately. Security teams should also care if these clients are broadly deployed on endpoints where browser-based code execution risk has high business impact.

Technical summary

The issue is described as a JIT miscompilation in Mozilla’s JavaScript Engine JIT component. NVD maps the weakness to CWE-843, indicating a type-confusion style problem. The published CVSS vector is CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H, which indicates a remotely triggerable flaw with no privileges or user interaction required and potential high impact across confidentiality, integrity, and availability.

Defensive priority

Critical. Apply the vendor-fixed releases as soon as possible, with highest priority for internet-facing or high-risk desktop fleets and any systems that routinely process untrusted web content or scripts.

Recommended defensive actions

  • Upgrade Firefox to 149 or later.
  • Upgrade Firefox ESR to 115.34 or later, or 140.9 or later, depending on your deployment track.
  • Upgrade Thunderbird to 149 or later, or 140.9 or later, depending on your deployment track.
  • Prioritize patching on endpoints that browse untrusted content or use web-based services heavily.
  • Verify version compliance across managed desktop fleets and remediate stragglers promptly.

Evidence notes

Facts are limited to the supplied CVE record and NVD source metadata. The record identifies the flaw as a JIT miscompilation in Mozilla’s JavaScript Engine JIT component, assigns CVSS 9.8 with vector AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H, and lists CWE-843. NVD references Mozilla bug 2020906 and Mozilla security advisories mfsa2026-20 through mfsa2026-24. No exploit details or exploitation status were provided in the corpus, so none are asserted here.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-4698 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-4698

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-4698 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-4698

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.