# CVE-2026-84142: Mozilla Thunderbird memory corruption and security defects

Source: https://junglewise.ai/threats/cve-2026-84142-mozilla-thunderbird-memory-corruption-and-security-defects  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-01. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-84142 · Severity: critical · CVSS 9.8 · Published 2026-09-01

Thunderbird 154 contains multiple internally discovered bugs that demonstrate evidence of memory corruption and other security-relevant defects. These defects could potentially be exploited by an attacker to compromise the email client. The vulnerabilities were addressed in Thunderbird 155.

Technologies: [Mozilla Firefox](https://junglewise.ai/threats/technologies/firefox), [Mozilla Thunderbird](https://junglewise.ai/threats/technologies/thunderbird). Vendors: [Mozilla](https://junglewise.ai/threats/vendors/mozilla).

## Quick answers

**Is CVE-2026-84142 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-84142 being exploited in the wild.

**How severe is CVE-2026-84142?** Critical severity, with a CVSS score of 9.8, published 1 September 2026.

**Which products and versions are affected?** Mozilla Thunderbird 154.

**Is there a fix for CVE-2026-84142?** A patch was published on 2026: Fixed in Thunderbird 155 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-84142 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-84142?** Check whether you run Mozilla Firefox 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

Multiple memory corruption and security-relevant defects were identified in Thunderbird 154 through internal testing. The vulnerabilities span several bug reports and represent memory safety issues or other exploitable security flaws. The exact attack vectors and preconditions vary by individual bug, but collectively they posed enough risk that Mozilla prioritized fixes in Thunderbird 155. No evidence of active exploitation in the wild has been reported.

## Affected products

- Mozilla Thunderbird 154

## Timeline

- 2026-09-01: disclosed
- 2026: patched: Fixed in Thunderbird 155

## References

- [https://bugzilla.mozilla.org/buglist.cgi?bug_id=2029421%2C2040889%2C2045313%2C2045435%2C2053578%2C2058621%2C2058626](https://bugzilla.mozilla.org/buglist.cgi?bug_id=2029421%2C2040889%2C2045313%2C2045435%2C2053578%2C2058621%2C2058626)
- [https://bugzilla.mozilla.org/buglist.cgi?bug_id=2048796%2C2053150%2C2057356](https://bugzilla.mozilla.org/buglist.cgi?bug_id=2048796%2C2053150%2C2057356)

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