---
id: "CVE-2019-17026"
kind: "vulnerability"
aliases: ["CVE-2019-17026"]
title: "Mozilla Firefox and Thunderbird: Mozilla Firefox And Thunderbird Type Confusion Vulnerability"
severity: "high"
cvss: 8.8
epss: 0.46311
exploited: true
status: "open"
category: "other"
products: [{"name": "Mozilla Firefox and Thunderbird", "fixed": [], "affected": "", "ecosystem": "vendor"}]
ecosystems: ["vendor"]
tags: ["desktop", "exploited", "firefox", "other", "thunderbird"]
not_affected_if: ["up-to-date-browser"]
published_at: "2020-03-02T05:15:12+00:00"
first_seen_at: "2026-10-23T19:11:53+00:00"
updated_at: "2026-10-23T21:57:34+00:00"
sources: [{"source": "kev", "url": "https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2019-17026", "first_seen_at": "2026-10-23T19:11:53+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2019-17026", "first_seen_at": "2026-10-23T19:13:44+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2019-17026", "first_seen_at": "2026-10-23T21:57:34+00:00"}]
---

# Mozilla Firefox and Thunderbird: Mozilla Firefox And Thunderbird Type Confusion Vulnerability

Mozilla Firefox and Thunderbird have a type confusion bug in the IonMonkey JIT compiler when setting array elements. An attacker who gets a user to load malicious JavaScript can likely run code in the browser or mail client.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- vendor:Mozilla Firefox and Thunderbird affected ? fixed ?

## How to check

Check the installed Firefox or Thunderbird version under Help > About and compare it with the fixed release in the vendor advisory.

## Fix

Update Firefox and Thunderbird to a release that includes the Mozilla fix, per the vendor instructions CISA references.

## Timeline

- 2026-10-23T19:11:53+00:00 kev https://www.cisa.gov/known-exploited-vulnerabilities-catalog?search_api_fulltext=CVE-2019-17026
- 2026-10-23T19:13:44+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2019-17026
- 2026-10-23T21:57:34+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2019-17026

## From the sources

Incorrect alias information in IonMonkey JIT compiler for setting array elements could lead to a type confusion. We are aware of targeted attacks in the wild abusing this flaw. This vulnerability affects Firefox ESR < 68.4.1, Thunderbird < 68.4.1, and Firefox < 72.0.1.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_