CVE-2026-6755 Mitigation bypass in the DOM: postMessage component
Medium 6.5
Published
Updated
Check this codebase for CVE-2026-6755 and fix it if it is affected.
What it is: CVE-2026-6755 is a medium-severity vulnerability in Mozilla Firefox, published on 21 Apr 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-2026-6755 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-2026-6755/ (Markdown: https://insights.sbomify.com/CVE-2026-6755/index.md)
Attackers can bypass DOM postMessage mitigations in Firefox and Thunderbird, potentially enabling cross-origin attacks.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-6755?
CVE-2026-6755 is a medium-severity vulnerability in Mozilla Firefox, published on 21 Apr 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Mitigation bypass in the DOM: postMessage component. This vulnerability was fixed in Firefox 150 and Thunderbird 150.
Weakness: CWE-352
Which versions are affected by CVE-2026-6755?
It affects 2 products.
Products include Mozilla Firefox, Mozilla Thunderbird.
How do I fix CVE-2026-6755?
Upgrade to a fixed version.
Is CVE-2026-6755 being exploited?
There is no known exploitation of CVE-2026-6755 in the wild and no public exploit code.
Other vulnerabilities in Mozilla Firefox
Sources
Also known as GHSA-g672-5549-fj5x, UBUNTU-CVE-2026-6755.