# CVE-2026-109723: Mozilla Firefox integer overflow in Layout: Text and Fonts

Source: https://junglewise.ai/threats/cve-2026-109723-integer-overflow-in-the-layout-text-and-fonts-component-this  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-13. Updated: 2026-10-16  
License: CC BY 4.0 with a link back

CVE-2026-109723 · Severity: medium · CVSS 5.4 · Published 2026-10-13

An integer overflow vulnerability in Firefox's text and font layout engine could allow an attacker to execute arbitrary code or crash the browser by processing specially crafted web content. This affects Firefox, Firefox ESR, Thunderbird, and Thunderbird ESR across multiple versions. The vulnerability was fixed in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5.

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

## Quick answers

**Is CVE-2026-109723 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-109723 being exploited in the wild.

**How severe is CVE-2026-109723?** Medium severity, with a CVSS score of 5.4, published 13 October 2026.

**Which products and versions are affected?** Mozilla Firefox before 158; Mozilla Firefox ESR before 153.5; Mozilla Thunderbird before 158; Mozilla Thunderbird ESR before 153.5.

**Is there a fix for CVE-2026-109723?** Yes: fixed in Firefox 158, Firefox ESR 153.5, Thunderbird 158, Thunderbird ESR 153.5. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-109723 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-109723?** Check whether you run Mozilla Firefox, Mozilla Firefox ESR, Mozilla Thunderbird or one of the others listed 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.

## How to fix

- Fixed in Firefox 158, Firefox ESR 153.5, Thunderbird 158, Thunderbird ESR 153.5

## Technical details

An integer overflow exists in the Layout: Text and Fonts component that can be triggered when processing certain font or text layout operations. The vulnerability allows for potential code execution through memory corruption, with the attack vector being network-based (malicious web content). A fix was released in the versions noted above.

## Affected products

- Mozilla Firefox before 158
- Mozilla Firefox ESR before 153.5
- Mozilla Thunderbird before 158
- Mozilla Thunderbird ESR before 153.5

## Timeline

- 2026-10-13: disclosed
- 2026-10-13: patched: Fixed in Firefox 158, Firefox ESR 153.5, Thunderbird 158, Thunderbird ESR 153.5

## References

- [https://bugzilla.mozilla.org/show_bug.cgi?id=2074171](https://bugzilla.mozilla.org/show_bug.cgi?id=2074171)
- [https://www.mozilla.org/security/advisories/mfsa2026-106/](https://www.mozilla.org/security/advisories/mfsa2026-106/)

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