# CVE-2026-84642 — Mozilla Thunderbird

> As of 2026-09-02, CVE-2026-84642 is a HIGH-severity vulnerability in Mozilla Thunderbird, CVSS v3.1 7.5, EPSS 0.1% (4.3th percentile). No Threadlinqs-tracked threat campaign has been attributed to CVE-2026-84642 as of 2026-09-02; the identifier is re-checked against the Threadlinqs threat corpus on every daily ingest.

**Last updated:** 2026-09-02

## What is CVE-2026-84642?

The values of the mail.allowed_attachment_hostnames advanced config setting were used in a regular expression without escaping. For some possible valid hostnames, this could allow certain unintended hostnames to also match and serve remote attachments. This vulnerability was fixed in Thunderbird 155 and Thunderbird 153.2.

The record classifies CVE-2026-84642 under weakness class [CWE-1333](https://cwe.mitre.org/data/definitions/1333.html). Its CVSS v3 base vector states that the flaw is reachable remotely over the network, needs no prior authentication, needs no user interaction, and has high impact on confidentiality. 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 34 days ago.

## Severity and exploitation probability

- **CVSS v3.1 base score:** 7.5 — HIGH (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N`)
- **EPSS (FIRST):** 0.1% probability of exploitation in the next 30 days, higher than 4.3% of all scored CVEs
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 3.9/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- **Published:** 2026-09-01, last modified 2026-09-02

## Is CVE-2026-84642 being exploited?

It currently carries a trending score of 31 in the Threadlinqs vulnerability feed.

## Affected products and versions

- [Mozilla](https://intel.threadlinqs.com/vendors/mozilla): Thunderbird

## How to fix CVE-2026-84642

No vendor patch reference has been recorded for CVE-2026-84642 in the tracked sources. Follow the references below for a fix, and treat the products listed above as exposed until the vendor states otherwise.

## Threat activity tracking CVE-2026-84642

No threat campaign in the Threadlinqs corpus currently references CVE-2026-84642, in its CVE list or as an indicator. The linkage is recomputed from the live corpus on every daily ingest, so this section fills in as soon as a tracked campaign cites the identifier.

## Sources

Enriched from CVE.org, NVD, GitHub Security Advisories, FIRST EPSS. Last verified by Threadlinqs on 2026-09-03. This product uses the NVD API but is not endorsed or certified by the NVD.

**Other references**

- [bugzilla.mozilla.org](https://bugzilla.mozilla.org/show_bug.cgi?id=2060462)
- [mozilla.org](https://www.mozilla.org/security/advisories/mfsa2026-86/)
- [mozilla.org (mfsa2026 88)](https://www.mozilla.org/security/advisories/mfsa2026-88/)

Canonical: https://intel.threadlinqs.com/cve/CVE-2026-84642
Full detection coverage and IOCs for threats exploiting CVE-2026-84642 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp