CVE-2026-4699: Mozilla Firefox and Thunderbird out-of-bounds write in Text and Fonts

CVE-2026-4699 · Severity: high · CVSS 7.5 · Published 2026-03-24

A vulnerability exists in the text and font processing component of Mozilla Firefox and Thunderbird. This flaw could allow a remote attacker to cause a service outage or application crash by providing specially crafted content. This impact primarily affects the availability of the browser or email client during operation.

Technologies: Mozilla Firefox, Mozilla Firefox ESR, Mozilla Thunderbird. Vendors: Mozilla.

Quick answers

Is CVE-2026-4699 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-4699 being exploited in the wild.
How severe is CVE-2026-4699?
High severity, with a CVSS score of 7.5, published 24 March 2026.
Which products and versions are affected?
Mozilla Firefox < 149; Mozilla Firefox ESR < 115.34, < 140.9; Mozilla Thunderbird < 149, < 140.9.
Is there a fix for CVE-2026-4699?
A patch was published on 2026-03-24. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-4699 attacked?
It is attacked over the network, in a network appliance.
Am I affected by CVE-2026-4699?
Check whether you run Mozilla Firefox, Mozilla Firefox ESR or Mozilla Thunderbird at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

The vulnerability is caused by incorrect boundary conditions (CWE-754) within the 'Layout: Text and Fonts' component of Mozilla browsers and mail clients. Red Hat's analysis further classifies this as an out-of-bounds write (CWE-787). An attacker can exploit this over the network without authentication or user interaction to trigger a denial-of-service (DoS) condition, typically manifested as an application crash. The issue is resolved in Firefox 149, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

Affected products

  • Mozilla Firefox < 149
  • Mozilla Firefox ESR < 115.34, < 140.9
  • Mozilla Thunderbird < 149, < 140.9
  • Red Hat Lightspeed Red Hat Enterprise Linux Server 7 ELS

Timeline

  • 2026-03-24: disclosed
  • 2026-03-24: patched
  • 2026-03-24: advisory

References

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