# CVE-2026-2778: Mozilla Firefox and Thunderbird sandbox escape in DOM Core & HTML

Source: https://junglewise.ai/threats/cve-2026-2778-mozilla-firefox-and-thunderbird-sandbox-escape-in-dom-core-html  
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-2026-2778 · Severity: critical · CVSS 10 · Published 2026-02-24

A critical security vulnerability has been identified in Mozilla Firefox and Thunderbird that could allow an attacker to escape the browser's security sandbox. This flaw occurs when the software incorrectly handles specific data structures used for web content, potentially allowing a malicious website to execute code outside of the restricted browser environment. If exploited, this could lead to full system compromise, unauthorized data access, or the installation of malware on a user's computer.

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-2026-2778 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-2778 being exploited in the wild.

**How severe is CVE-2026-2778?** Critical severity, with a CVSS score of 10, published 24 February 2026.

**Which products and versions are affected?** Mozilla Firefox < 148; Mozilla Firefox ESR < 115.33, < 140.8; Mozilla Thunderbird < 148, < 140.8.

**Is there a fix for CVE-2026-2778?** A patch was published on 2026-02-24: Fixed in Firefox 148 and ESR releases Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-2778 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2026-2778?** 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

A heap-based out-of-bounds (OOB) read vulnerability exists in the 'DOM: Core & HTML' component of Mozilla products. The flaw is located in 'StructuredCloneBlob::Holder::ReadStructuredCloneInternal' within 'StructuredCloneBlob.cpp', where 'blobOffset' and 'blobCount' are read from an attacker-controlled buffer without sufficient source bounds checking. When 'AppendElements' is called, it performs an uninitialized copy of elements based on these controlled values. An attacker can provide a crafted 'blobCount' to trigger an OOB read on the heap; because the copied elements are treated as 'RefPtr<BlobImpl>', the subsequent 'AddRef()' call on garbage memory can lead to a sandbox escape and arbitrary code execution. The vulnerability is reachable via Inter-Process Communication (IPC) from a compromised content process to the parent process.

## Affected products

- Mozilla Firefox < 148
- Mozilla Firefox ESR < 115.33, < 140.8
- Mozilla Thunderbird < 148, < 140.8
- Red Hat Red Hat Enterprise Linux Server 7 ELS

## Timeline

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

## References

- [https://bugzilla.mozilla.org/show_bug.cgi?id=2016358](https://bugzilla.mozilla.org/show_bug.cgi?id=2016358)
- [https://www.mozilla.org/security/advisories/mfsa2026-13/](https://www.mozilla.org/security/advisories/mfsa2026-13/)
- [https://www.mozilla.org/security/advisories/mfsa2026-14/](https://www.mozilla.org/security/advisories/mfsa2026-14/)
- [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)

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