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9.8

Firefox and Thunderbird let attackers bypass security protections

CVE-2026-109780 · published 22 days ago
Summary

Both Firefox and Thunderbird versions before 158 have a flaw that can let malicious web content avoid built‑in safety checks. This could allow harmful scripts to run or data to be accessed without restriction. Upgrade the browsers to version 158 or later to close the gap.

What to do

The CVE record does not list a fixed version. Check the vendor's site or the advisory links below - a fix may already be released.

Affected software
Ecosystem VendorProductAffected versions
– mozilla firefox All versions
– mozilla thunderbird All versions
Ubuntu:18.04:LTS canonical mozjs52 All versions
Ubuntu:18.04:LTS canonical mozjs38 All versions
Ubuntu:20.04:LTS canonical mozjs68 All versions
Ubuntu:22.04:LTS canonical mozjs102 All versions
Ubuntu:22.04:LTS canonical mozjs78 All versions
Ubuntu:22.04:LTS canonical mozjs91 All versions
Ubuntu:22.04:LTS canonical thunderbird All versions
Ubuntu:24.04:LTS canonical mozjs115 All versions
Original advisory text
Mitigation bypass in the DOM: Core & HTML component
Mitigation bypass in the DOM: Core & HTML component. This vulnerability was fixed in Firefox 158 and Thunderbird 158.
References
Fix within
Internet-facing 3 days
Internal 60 days
  • Not known to be exploited
  • Can be exploited automatically
  • Gives an attacker full control
Severity
9.8 Critical
Exploitation
<1% chance of attack within 30 days
Type
CWE-693Protection Mechanism Failure
Timeline
Published13 Oct 2026
Updated4 Nov 2026
First seen13 Oct 2026
Sources
CVE-2026-109780 · NVD
CVE-2026-109780 · MITRE
UBUNTU-CVE-2026-109780 · OSV
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