CVE-2026-84140 Site isolation issue in the DOM: Navigation component
Critical 9.8
Published
Updated
Check this codebase for CVE-2026-84140 and fix it if it is affected.
What it is: CVE-2026-84140 is a critical vulnerability in Mozilla Firefox, published on 1 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-84140 in the wild and no public exploit code.
Affected product: Mozilla Firefox. No package-level version data is available, so check the vendor advisory linked from the reference page.
Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.
Reference: https://insights.sbomify.com/CVE-2026-84140/ (Markdown: https://insights.sbomify.com/CVE-2026-84140/index.md)
An attacker can bypass site isolation in Firefox and Thunderbird, potentially accessing cross-origin data.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-84140?
CVE-2026-84140 is a critical vulnerability in Mozilla Firefox, published on 1 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Site isolation issue in the DOM: Navigation component. This vulnerability was fixed in Firefox 155, Firefox ESR 153.2, Thunderbird 155, and Thunderbird 153.2.
Weakness: CWE-346
Which versions are affected by CVE-2026-84140?
It affects 2 products.
Products include Mozilla Firefox, Mozilla Thunderbird.
How do I fix CVE-2026-84140?
Upgrade to a fixed version.
Is CVE-2026-84140 being exploited?
There is no known exploitation of CVE-2026-84140 in the wild and no public exploit code.
Other vulnerabilities in Mozilla Firefox
Sources
Also known as GHSA-hg9w-cw2v-4pm9, UBUNTU-CVE-2026-84140.