How to Fix CVE-2026-5734: Firefox (Bundle Sibling)

*By Sai Kiran Pandrala*

Last verified: 2026-05-25

CVE-2026-5734 is a sibling vulnerability in the same Mozilla Firefox advisory bundle as CVE-2026-5731. The same patched build closes every CVE in the bundle, so the remediation procedure for CVE-2026-5734 matches the primary write-up.

What is different about CVE-2026-5734?

Memory safety bugs present in Firefox ESR 140.9.0, Thunderbird ESR 140.9.0, Firefox 149.0.1 and Thunderbird 149.0.1. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability was fixed in Firefox 149.0.2, Firefox ESR 140.9.1, Thunderbird 149.0.2, and Thunderbird 140.9.1.

Impact is consistent with the bundle: remote code execution on the affected system. The patched build closes every code path in the advisory in one update.

How to fix CVE-2026-5734

Apply the patched build per the primary write-up: How to Fix CVE-2026-5731.

For a quick check, confirm the running version of Firefox:

# Confirm the installed version via your package manager
dpkg -l | grep -i firefox   # Debian/Ubuntu
rpm -qa | grep -i firefox   # RHEL/CentOS/Rocky

Frequently asked questions

Is CVE-2026-5734 being actively exploited?

Not at the time of writing. It is not listed in CISA's Known Exploited Vulnerabilities catalog. That status can change, so monitor the vendor advisory and the KEV catalog if the system is exposed.

How severe is CVE-2026-5734?

CVSS rates it 8.1 (High). Use that score to set your patch priority alongside the other items in your queue.

Do I have to take Firefox offline to apply the patch?

It depends on the deployment. High-availability or clustered installs can usually patch one node at a time with no full outage. Standalone installs typically need a short restart. Always follow the vendor's documented upgrade steps.

What if my vulnerability scanner still flags CVE-2026-5734 after I patch?

Re-run the scan after a service restart, then confirm the scanner's plugin set is up to date. Some scanners detect by banner version only and lag the official fix metadata by a release.

References

• Official vendor advisory: https://bugzilla.mozilla.org/buglist.cgi?bug_id=2022369%2C2023026%2C2023545%2C2023555%2C2023958%2C2025422%2C2025468%2C2025492%2C2025505
• NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2026-5734
• CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• Primary write-up: How to Fix CVE-2026-5731

*Written by Sai Kiran Pandrala. Last verified 2026-05-25. Part of the Mozilla Firefox advisory bundle. Full procedure at how-to-fix-cve-2026-5731.*