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CVE-2026-4721

Mozilla Firefox buffer overflow

9.8 CRITICALEPSS 0.42%
  1. Published24 Mar 2026

Description

Memory safety bugs present in Firefox ESR 115.33, Firefox ESR 140.8, Thunderbird ESR 140.8, Firefox 148 and Thunderbird 148. 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, Firefox ESR 115.34, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

CVSS 3.1 breakdown

Attack vectorNetwork
Attack complexityLow
Privileges requiredNone
User interactionNone
ScopeUnchanged
ConfidentialityHigh
IntegrityHigh
AvailabilityHigh
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected products

Mozilla Firefox< 149.0 · ≥ 128.0 < 140.9.0 · < 115.34.0
Mozilla Thunderbird< 149.0 · < 140.9.0

Remediation

Fixed in
Mozilla Firefox≥ 149.0
Mozilla Thunderbird≥ 149.0

References

and 24 more references

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Frequently asked

Is CVE-2026-4721 being actively exploited?

CVE-2026-4721 is not in the CISA KEV catalog. Its EPSS exploit probability is 0.42%.

How severe is CVE-2026-4721?

CVE-2026-4721 has a CVSS 3.1 base score of 9.8, rated critical. EPSS estimates a 0.42% probability of exploitation.

Which products are affected by CVE-2026-4721?

Affected products include Mozilla Firefox, Mozilla Thunderbird.

How do I fix CVE-2026-4721?

Upgrade affected software to a fixed version, for example Mozilla Firefox 149.0, Mozilla Thunderbird 149.0 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2026-4721 against the real software inventory on every managed device, every day. Rather than a static severity, each match is scored in context: exploit availability, EPSS, the asset's exposure, and business criticality decide where it lands in the queue. If an affected version is present anywhere across your portfolio, it surfaces automatically with a prioritised remediation path.

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.