CVE-2026-100778: Mozilla Firefox use-after-free (CVSS 9.6)

Description

Sandbox escape due to use-after-free in the DOM: Core & HTML component. This vulnerability was fixed in Firefox ESR 153.4, Thunderbird 157, Thunderbird 140.17, Thunderbird 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17.

CVSS 3.1 breakdown

Attack vectorNetwork

Attack complexityLow

Privileges requiredNone

User interactionRequired

ScopeChanged

ConfidentialityHigh

IntegrityHigh

AvailabilityHigh

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H

Affected products

Mozilla Firefox≥ 115.0.1 < 115.42.0 · ≥ 140.0.0 < 140.17.0 · ≥ 153.0 < 153.4.0 · ≥ 154.0.0 < 157.0.0

Mozilla Thunderbird≥ 140.0 < 140.17.0 · ≥ 153.0 < 153.4.0 · ≥ 154.0 < 157.0

Remediation

Fixed in

Mozilla Firefox≥ 115.42.0

Mozilla Thunderbird≥ 140.17.0

Patch & vendor advisories

References

https://bugzilla.mozilla.org/show_bug.cgi?id=2068406Permissions Required
https://www.mozilla.org/security/advisories/mfsa2026-100/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-101/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-102/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-103/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-97/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-98/Vendor Advisory
https://www.mozilla.org/security/advisories/mfsa2026-99/Vendor Advisory

Related CVEs

Recent high-severity vulnerabilities sharing a weakness type.

Frequently asked

Is CVE-2026-100778 being actively exploited?

CVE-2026-100778 is not in the CISA KEV catalog. Its EPSS exploit probability is 0.32%.

How severe is CVE-2026-100778?

CVE-2026-100778 has a CVSS 3.1 base score of 9.6, rated critical. EPSS estimates a 0.32% probability of exploitation.

Which products are affected by CVE-2026-100778?

Affected products include Mozilla Firefox, Mozilla Thunderbird.

How do I fix CVE-2026-100778?

Upgrade affected software to a fixed version, for example Mozilla Firefox 115.42.0, Mozilla Thunderbird 140.17.0 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2026-100778 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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