# CVE-2026-109739: Privilege escalation in the WebExtensions component

> Attackers can escalate privileges via the WebExtensions component in Firefox and Thunderbird, requiring an upgrade to the latest versions.

- **Severity:** High 8.8 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 13 Oct 2026
- **Updated:** 14 Oct 2026
- **Also known as:** GHSA-xc46-c8p5-77m7, UBUNTU-CVE-2026-109739

## What is CVE-2026-109739?

CVE-2026-109739 is a high-severity vulnerability in Mozilla Firefox, published on 13 Oct 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Privilege escalation in the WebExtensions component. This vulnerability was fixed in Firefox 158 and Thunderbird 158.

Weakness: CWE-269 (Improper Privilege Management)

## Which versions are affected by CVE-2026-109739?


## How do I fix CVE-2026-109739?

Upgrade to a fixed version.

## Is CVE-2026-109739 being exploited?

There is no known exploitation of CVE-2026-109739 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-109739 and fix it if it is affected.

What it is: CVE-2026-109739 is a high-severity vulnerability in Mozilla Firefox, published on 13 Oct 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-109739 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-2026-109739/ (Markdown: https://insights.sbomify.com/CVE-2026-109739/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-109739
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-109739
- OSV: https://osv.dev/vulnerability/CVE-2026-109739

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-109739/
Summaries are generated and may contain mistakes; the official description above is the record.