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

Source: https://junglewise.ai/threats/cve-2026-4699-mozilla-firefox-and-thunderbird-out-of-bounds-write-in-text-and  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-03-24. Updated: 2026-06-30  
License: CC BY 4.0 with a link back

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](https://junglewise.ai/threats/technologies/firefox), [Mozilla Firefox ESR](https://junglewise.ai/threats/technologies/firefox-esr), [Mozilla Thunderbird](https://junglewise.ai/threats/technologies/thunderbird). Vendors: [Mozilla](https://junglewise.ai/threats/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

- [https://bugzilla.mozilla.org/show_bug.cgi?id=2021863](https://bugzilla.mozilla.org/show_bug.cgi?id=2021863)
- [https://www.mozilla.org/security/advisories/mfsa2026-20/](https://www.mozilla.org/security/advisories/mfsa2026-20/)
- [https://www.mozilla.org/security/advisories/mfsa2026-21/](https://www.mozilla.org/security/advisories/mfsa2026-21/)
- [https://www.mozilla.org/security/advisories/mfsa2026-22/](https://www.mozilla.org/security/advisories/mfsa2026-22/)
- [https://www.mozilla.org/security/advisories/mfsa2026-23/](https://www.mozilla.org/security/advisories/mfsa2026-23/)
- [https://www.mozilla.org/security/advisories/mfsa2026-24/](https://www.mozilla.org/security/advisories/mfsa2026-24/)
- [https://access.redhat.com/errata/RHSA-2026:5930](https://access.redhat.com/errata/RHSA-2026:5930)

## Related threats

- [Thunderbird OAuth2 access token leak over HTTP with Exchange accounts](https://junglewise.ai/threats/cve-2026-126136-for-exchange-accounts-using-ews-or-microsoft-graph-with-oauth2)
- [Mozilla Thunderbird SMTP/POP3 response injection via STARTTLS](https://junglewise.ai/threats/cve-2026-126135-if-an-attacker-was-able-to-perform-a-man-in-the-middle-attack-on)
- [Thunderbird NTLM credential leak via feed subscription to file URL](https://junglewise.ai/threats/cve-2026-126134-thunderbird-allowed-feed-subscriptions-to-file-urls-that-point)
- [Thunderbird calendar server credentials sent to HTTP proxy](https://junglewise.ai/threats/cve-2026-126133-when-a-caldav-or-ics-calendar-was-accessed-through-an-http-proxy)
- [Thunderbird attachment replacement disclosure via link files](https://junglewise.ai/threats/cve-2026-126132-when-a-message-was-forwarded-inline-edited-as-new-or-had-an)