CVE-2026-5734
Memory-safety flaws in Firefox/Thunderbird may allow remote code execution.
Is CVE-2026-5734 being exploited?
Not confirmed. CVE-2026-5734 does not appear in CISA's Known Exploited Vulnerabilities catalog, which records only exploitation that has been observed and reported publicly. That is evidence of absence of a report, not evidence the flaw is unattacked. EPSS currently estimates a 0.33% probability of exploitation in the next 30 days.
How severe is CVE-2026-5734?
CVE-2026-5734 is rated Critical with a CVSS score of 9.8. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2026-5734?
Yes. A fix has been recorded for CVE-2026-5734. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-5734 affect?
CVE-2026-5734 affects Firefox <149.0.2, Firefox ESR <140.9.1, Thunderbird <149.0.2, Thunderbird <140.9.1. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
What should I do about CVE-2026-5734?
Update to the fixed versions ASAP (Firefox 149.0.2+, Thunderbird 149.0.2+; ESR 140.9.1+).
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
Remote code execution via memory corruption; attacker could take control of a user system.
Patch when possible