# CVE-2026-5734: Mozilla Firefox and Thunderbird memory safety bugs

Source: https://junglewise.ai/threats/cve-2026-5734-mozilla-firefox-and-thunderbird-memory-safety-bugs  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-07. Updated: 2026-06-30  
License: CC BY 4.0 with a link back

CVE-2026-5734 · Severity: critical · CVSS 9.8 · Published 2026-04-07

Mozilla Firefox and Thunderbird are affected by multiple memory safety vulnerabilities that could allow an attacker to execute malicious code on a user's system. These issues occur when the software incorrectly handles data in memory, potentially leading to system instability or unauthorized access to sensitive information. While Thunderbird is generally less susceptible when reading standard emails, the risk remains high in browser-like contexts or if malicious scripts are executed. Users should update to the latest versions immediately to protect their data and operations.

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-2026-5734 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-5734 being exploited in the wild.

**How severe is CVE-2026-5734?** Critical severity, with a CVSS score of 9.8, published 7 April 2026.

**Which products and versions are affected?** Mozilla Firefox < 140.9.1; Mozilla Firefox ESR < 140.9.1; Mozilla Thunderbird < 140.9.1; Mozilla Thunderbird ESR < 140.9.1.

**Is there a fix for CVE-2026-5734?** A patch was published on 2026-04-07: Fixed versions released by Mozilla Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-5734 attacked?** It is attacked over the network, in a network appliance.

**Am I affected by CVE-2026-5734?** 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

This advisory covers a collection of memory safety bugs (CWE-787, CWE-120) identified by the Mozilla Fuzzing Team and other researchers. The vulnerabilities involve memory corruption issues, including out-of-bounds writes and buffer overflows, within the browser engine. An unauthenticated remote attacker could exploit these flaws by enticing a user to visit a specially crafted website or interact with malicious content, potentially leading to arbitrary code execution (RCE). While Thunderbird disables scripting by default when reading email, the vulnerabilities are exploitable in browser-like contexts. The issues are resolved in Firefox 149.0.2, Firefox ESR 140.9.1, Thunderbird 149.0.2, and Thunderbird 140.9.1.

## Affected products

- Mozilla Firefox ESR < 140.9.1
- Mozilla Firefox < 149.0.2
- Mozilla Thunderbird ESR < 140.9.1
- Mozilla Thunderbird < 149.0.2

## Timeline

- 2026-04-07: advisory: Mozilla Foundation Security Advisory published
- 2026-04-07: patched: Fixed versions released by Mozilla
- 2026-04-29: advisory: Red Hat security advisory issued for affected packages

## References

- [https://bugzilla.mozilla.org/buglist.cgi?bug_id=2022369%2C2023026%2C2023545%2C2023555%2C2023958%2C2025422%2C2025468%2C2025492%2C2025505](https://bugzilla.mozilla.org/buglist.cgi?bug_id=2022369%2C2023026%2C2023545%2C2023555%2C2023958%2C2025422%2C2025468%2C2025492%2C2025505)
- [https://www.mozilla.org/security/advisories/mfsa2026-25/](https://www.mozilla.org/security/advisories/mfsa2026-25/)
- [https://www.mozilla.org/security/advisories/mfsa2026-27/](https://www.mozilla.org/security/advisories/mfsa2026-27/)
- [https://www.mozilla.org/security/advisories/mfsa2026-28/](https://www.mozilla.org/security/advisories/mfsa2026-28/)
- [https://www.mozilla.org/security/advisories/mfsa2026-29/](https://www.mozilla.org/security/advisories/mfsa2026-29/)
- [https://access.redhat.com/errata/RHSA-2026:11805](https://access.redhat.com/errata/RHSA-2026:11805)
- [https://access.redhat.com/errata/RHSA-2026:11813](https://access.redhat.com/errata/RHSA-2026:11813)

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