# CVE-2026-84637 — Mozilla Thunderbird

> As of 2026-09-02, CVE-2026-84637 is a CRITICAL-severity vulnerability in Mozilla Thunderbird, CVSS v3.1 9.8, EPSS 0.1% (4.5th percentile). No Threadlinqs-tracked threat campaign has been attributed to CVE-2026-84637 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-2026-84637?

Malicious calendar invitations could use file URI attachments to launch local or network-hosted executables on Windows, bypassing Thunderbird's normal executable attachment protections. With the new invitation display enabled, the attachment could also appear under a misleading filename. This vulnerability was fixed in Thunderbird 154 and Thunderbird 153.2.

The record classifies CVE-2026-84637 under weakness class [CWE-434](https://intel.threadlinqs.com/cwe/CWE-434). 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, integrity, availability. 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:** 9.8 — CRITICAL (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **EPSS (FIRST):** 0.1% probability of exploitation in the next 30 days, higher than 4.5% of all scored CVEs
- **CISA KEV:** Not listed in the CISA Known Exploited Vulnerabilities catalog
- **Threadlinqs priority:** 5.1/10 — 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-2026-84637 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-2026-84637

No vendor patch reference has been recorded for CVE-2026-84637 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-2026-84637

No threat campaign in the Threadlinqs corpus currently references CVE-2026-84637, 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, GitHub Security Advisories, FIRST EPSS. Last verified by Threadlinqs on 2026-09-03.

**Other references**

- [bugzilla.mozilla.org](https://bugzilla.mozilla.org/show_bug.cgi?id=2043878)
- [mozilla.org](https://www.mozilla.org/security/advisories/mfsa2026-78/)
- [mozilla.org (mfsa2026 88)](https://www.mozilla.org/security/advisories/mfsa2026-88/)

Canonical: https://intel.threadlinqs.com/cve/CVE-2026-84637
Full detection coverage and IOCs for threats exploiting CVE-2026-84637 via the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp