# CVE-2026-2762: Mozilla Firefox and Thunderbird integer overflow in JavaScript Standard Library

Source: https://junglewise.ai/threats/cve-2026-2762-mozilla-firefox-and-thunderbird-integer-overflow-in-javascript  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-02-24. Updated: 2026-06-30  
License: CC BY 4.0 with a link back

CVE-2026-2762 · Severity: critical · CVSS 9.8 · Published 2026-02-24

A vulnerability exists in the JavaScript engine used by Firefox and Thunderbird that could allow an attacker to execute malicious code on a user's computer. This occurs when the software processes specifically crafted text patterns, potentially leading to unauthorized access to sensitive data or a complete system takeover. Users should update their browser and email client to the latest versions to protect against this threat.

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), [Red Hat Enterprise Linux AppStream](https://junglewise.ai/threats/technologies/enterprise-linux-appstream). Vendors: [Mozilla](https://junglewise.ai/threats/vendors/mozilla), [Red Hat](https://junglewise.ai/threats/vendors/red-hat).

## Quick answers

**Is CVE-2026-2762 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-2762 being exploited in the wild.

**How severe is CVE-2026-2762?** Critical severity, with a CVSS score of 9.8, published 24 February 2026.

**Which products and versions are affected?** Mozilla Firefox < 148; Mozilla Firefox ESR < 140.8; Mozilla Thunderbird < 148, < 140.8; Red Hat Enterprise Linux AppStream affected.

**Is there a fix for CVE-2026-2762?** A patch was published on 2026-02-24: Fixed in Firefox 148 and Firefox ESR 140.8 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-2762 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-2762?** 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.

## Technical details

An integer overflow vulnerability exists in the 'EncodeForRegExpEscape' function within the JavaScript: Standard Library component of Mozilla's Gecko engine. On 32-bit platforms, the output length computation for escaping regular expressions can overflow size_t when processing extremely long strings (e.g., repeating a character approximately 715 million times). This leads to an undersized buffer allocation via 'StringBuilder::reserve()', followed by a heap-based buffer overflow when the actual characters are written. An attacker can leverage this to corrupt memory in the 'js::StringBufferArena' malloc arena, potentially achieving arbitrary code execution. The vulnerability is fixed in Firefox 148, Firefox ESR 140.8, and corresponding Thunderbird releases. 64-bit builds are not affected.

## Affected products

- Mozilla Firefox < 148
- Mozilla Firefox ESR < 140.8
- Mozilla Thunderbird < 148, < 140.8
- Red Hat Red Hat Enterprise Linux Server 7 ELS affected
- Red Hat Red Hat Enterprise Linux AppStream EUS 10.0 affected

## Timeline

- 2026-02-24: advisory: Mozilla Foundation Security Advisory 2026-13 published
- 2026-02-24: patched: Fixed in Firefox 148 and Firefox ESR 140.8

## References

- [https://bugzilla.mozilla.org/show_bug.cgi?id=2011649](https://bugzilla.mozilla.org/show_bug.cgi?id=2011649)
- [https://www.mozilla.org/security/advisories/mfsa2026-13/](https://www.mozilla.org/security/advisories/mfsa2026-13/)
- [https://www.mozilla.org/security/advisories/mfsa2026-15/](https://www.mozilla.org/security/advisories/mfsa2026-15/)
- [https://www.mozilla.org/security/advisories/mfsa2026-16/](https://www.mozilla.org/security/advisories/mfsa2026-16/)
- [https://www.mozilla.org/security/advisories/mfsa2026-17/](https://www.mozilla.org/security/advisories/mfsa2026-17/)
- [https://access.redhat.com/errata/RHSA-2026:3338](https://access.redhat.com/errata/RHSA-2026:3338)
- [https://access.redhat.com/errata/RHSA-2026:3339](https://access.redhat.com/errata/RHSA-2026:3339)

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