# CVE-2026-84134: Mozilla Firefox other issue in Profile Backup component

Source: https://junglewise.ai/threats/cve-2026-84134-mozilla-firefox-other-issue-in-profile-backup-component  
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-2026-84134 · Severity: critical · CVSS 9.8 · Published 2026-09-01

Firefox and Thunderbird's Profile Backup component contained a security issue that could potentially be exploited to affect browser functionality or security. The vulnerability has been patched in Firefox 155, Firefox ESR 153.2, Thunderbird 155, and Thunderbird 153.2.

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-2026-84134 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-84134 being exploited in the wild.

**How severe is CVE-2026-84134?** Critical severity, with a CVSS score of 9.8, published 1 September 2026.

**Which products and versions are affected?** Mozilla Firefox prior to 155; Mozilla Firefox ESR prior to 153.2; Mozilla Thunderbird prior to 155; Mozilla Thunderbird ESR prior to 153.2.

**Is there a fix for CVE-2026-84134?** A patch was published on 2026-09-01: Fixed in Firefox 155, Firefox ESR 153.2, Thunderbird 155, and Thunderbird 153.2 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-84134 attacked?** It is attacked over the network, in a web application.

**Am I affected by CVE-2026-84134?** 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

CVE-2026-84134 is classified as an "other issue" in the Profile Backup component of Firefox and Thunderbird. Limited technical details are available from the referenced advisory, but the vulnerability exists in code responsible for backing up user profile data. The nature of the issue is not fully disclosed in the available documentation. The vulnerability was identified and fixed by Mozilla across multiple product lines and versions. Users running Firefox 155, Firefox ESR 153.2, Thunderbird 155, or Thunderbird 153.2 and later have the fix applied.

## Affected products

- Mozilla Firefox prior to 155
- Mozilla Firefox ESR prior to 153.2
- Mozilla Thunderbird prior to 155
- Mozilla Thunderbird ESR prior to 153.2

## Timeline

- 2026-09-01: disclosed: Vulnerability publicly disclosed via Mozilla Security Advisory MFSA2026-82
- 2026-09-01: patched: Fixed in Firefox 155, Firefox ESR 153.2, Thunderbird 155, and Thunderbird 153.2

## References

- [https://bugzilla.mozilla.org/show_bug.cgi?id=2044882](https://bugzilla.mozilla.org/show_bug.cgi?id=2044882)
- [https://www.mozilla.org/security/advisories/mfsa2026-82/](https://www.mozilla.org/security/advisories/mfsa2026-82/)

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