# CVE-2026-100821: Site isolation issue in the Panning and Zooming component

> An attacker can bypass site isolation in Firefox and Thunderbird via the Panning and Zooming component, potentially accessing cross-origin data.

- **Severity:** Medium 4.7 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:N/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:** 29 Sep 2026
- **Updated:** 1 Oct 2026
- **Also known as:** DEBIAN-CVE-2026-100821, GHSA-r8hv-xg2w-55rq, UBUNTU-CVE-2026-100821

## What is CVE-2026-100821?

CVE-2026-100821 is a medium-severity vulnerability in Mozilla Firefox, published on 29 Sep 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

Site isolation issue in the Panning and Zooming component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 140.17, Thunderbird 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17.

Weakness: CWE-346 (Origin Validation Error)

## Which versions are affected by CVE-2026-100821?

It affects 2 products.


Products: Mozilla Firefox, Mozilla Thunderbird.

## How do I fix CVE-2026-100821?

Upgrade to a fixed version.

## Is CVE-2026-100821 being exploited?

There is no known exploitation of CVE-2026-100821 in the wild and no public exploit code.

## Other vulnerabilities in Mozilla Firefox

- [CVE-2019-11708](https://insights.sbomify.com/CVE-2019-11708/): Mozilla Firefox and Thunderbird Sandbox Escape (critical 10.0, exploited in the wild)
- [CVE-2010-3765](https://insights.sbomify.com/CVE-2010-3765/): Mozilla Multiple Products Remote Code Execution (critical 9.8, exploited in the wild)
- [CVE-2022-26486](https://insights.sbomify.com/CVE-2022-26486/): Mozilla Firefox Use-After-Free (critical 9.6, exploited in the wild)
- [CVE-2022-26485](https://insights.sbomify.com/CVE-2022-26485/): Mozilla Firefox Use-After-Free (high 8.8, exploited in the wild)
- [CVE-2019-11707](https://insights.sbomify.com/CVE-2019-11707/): Mozilla Firefox and Thunderbird Type Confusion (high 8.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-100821 and fix it if it is affected.

What it is: CVE-2026-100821 is a medium-severity vulnerability in Mozilla Firefox, published on 29 Sep 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-2026-100821 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-2026-100821/ (Markdown: https://insights.sbomify.com/CVE-2026-100821/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-100821
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-100821
- OSV: https://osv.dev/vulnerability/CVE-2026-100821

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-100821/
Summaries are generated and may contain mistakes; the official description above is the record.