How severe is CVE-2026-5734?

CVE-2026-5734 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2026-5734 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (25th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-5734?

CVE-2026-5734 primarily affects Mozilla Firefox. In total, 4 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2026-5734?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

Does CVE-2026-5734 have an EU (EUVD) identifier?

Yes. CVE-2026-5734 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-19614.

When was CVE-2026-5734 published?

CVE-2026-5734 was published on 2026-04-07 and last updated on 2026-06-30.

CVE-2026-5734

CVE-2026-5734 is a critical-severity vulnerability in Mozilla Firefox with a CVSS 3.x base score of 9.8. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-787.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
EPSS exploit prediction: 0% (25th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-19614
Weakness: CWE-787
Affected product: Mozilla Firefox
Published: 2026-04-07
Last modified: 2026-06-30

Description

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.

Frequently asked questions

What is 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.
How severe is CVE-2026-5734?: CVE-2026-5734 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2026-5734 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (25th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-5734?: CVE-2026-5734 primarily affects Mozilla Firefox. In total, 4 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2026-5734?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
Does CVE-2026-5734 have an EU (EUVD) identifier?: Yes. CVE-2026-5734 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-19614.
When was CVE-2026-5734 published?: CVE-2026-5734 was published on 2026-04-07 and last updated on 2026-06-30.

References

Affected products (4)

cpe:2.3:a:mozilla:firefox:*:*:*:*:esr:*:*:*
cpe:2.3:a:mozilla:firefox:*:*:*:*:-:*:*:*
cpe:2.3:a:mozilla:thunderbird:*:*:*:*:esr:*:*:*
cpe:2.3:a:mozilla:thunderbird:*:*:*:*:-:*:*:*

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