# CVE-2026-109721: Incorrect boundary conditions in the JavaScript Engine: JIT component

> Incorrect boundary conditions in the JavaScript Engine JIT allow attackers to execute arbitrary code via crafted web content.

- **Severity:** Medium 5.4 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 13 Oct 2026
- **Updated:** 19 Oct 2026
- **Also known as:** DEBIAN-CVE-2026-109721, GHSA-h7f3-g97w-8vxg, UBUNTU-CVE-2026-109721

## What is CVE-2026-109721?

CVE-2026-109721 is a medium-severity memory corruption vulnerability in Mozilla Firefox, published on 13 Oct 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

Incorrect boundary conditions in the JavaScript Engine: JIT component. This vulnerability was fixed in Firefox 158, Firefox ESR 153.5, Thunderbird 158, and Thunderbird 153.5.

Weakness: CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer)

## Which versions are affected by CVE-2026-109721?


## How do I fix CVE-2026-109721?

Upgrade to a fixed version.

## Is CVE-2026-109721 being exploited?

There is no known exploitation of CVE-2026-109721 in the wild and no public exploit code.

Public exploit code:

- Write-up for CVE-2026-109721 (github_poc): https://github.com/Scyrix-LLC/CVE-2026-109721

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-109721 and fix it if it is affected.

What it is: CVE-2026-109721 is a medium-severity memory corruption vulnerability in Mozilla Firefox, published on 13 Oct 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-109721 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-2026-109721/ (Markdown: https://insights.sbomify.com/CVE-2026-109721/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-109721
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-109721
- OSV: https://osv.dev/vulnerability/CVE-2026-109721

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-109721/
Summaries are generated and may contain mistakes; the official description above is the record.