major vulnerability· requires attention

High-severity RCE via browser CSS parsing in Mozilla Firefox/Thunderbird — widely used clients across enterprises; deserves broad IT notice.

Web

CVE-2026-124889

8.8

High

EPSS 0.38%Patch available

Use-after-free in Firefox/Thunderbird CSS parsing enabling remote code execution.

exploited

No known exploitation not in CISA KEV as of our last sync

fix

Patch available see the advisory in the references

likelihood

EPSS 0.38%top 68% of scored CVEs

Published 10 Nov 2026Analysed here 12 Nov 2026CVSS scored by CISA

common questions

Is CVE-2026-124889 being exploited?

Not confirmed. CVE-2026-124889 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 0.38% probability of exploitation in the next 30 days.

How severe is CVE-2026-124889?

CVE-2026-124889 is rated High with a CVSS score of 8.8. 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-2026-124889?

Yes. A fix has been recorded for CVE-2026-124889. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.

What does CVE-2026-124889 affect?

CVE-2026-124889 affects Mozilla Firefox (<160), Firefox ESR (<153.7), Mozilla Thunderbird (<160), Thunderbird ESR (<153.7). Confirm the exact affected versions against the vendor advisory before deciding you are exposed.

What should I do about CVE-2026-124889?

Update Firefox/Thunderbird to versions >=160 or ESR 153.7 immediately.

Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.

executive summary

technical analysis

affected

Mozilla Firefox (<160)Firefox ESR (<153.7)Mozilla Thunderbird (<160)Thunderbird ESR (<153.7)

impact

Remote code execution via crafted CSS in web content, enabling arbitrary code execution or sandbox escape on affected clients.

action required

Immediate action required

cisa ssvc

exploitation: none
automatable: no
technical impact: total

Assessed by CISA 10 Nov 2026 (SSVC 2.0.3).

timeline

10 Nov 2026Published
10 Nov 2026CISA SSVC assessment: exploitation none · same day later
10 Nov 2026CVE record last updated · same day later
12 Nov 2026Analysed by vulneraibility.com · 1 day later

how to fix

Remediation steps are tailored for users whose stack is affected. Add this technology to your stack to see the fix checklist.

references

NVD CVE.org CVE Details Source

5 references in the CVE record: 5 links

Linkbugzilla.mozilla.org /show_bug.cgi?id=2082033
Linkmozilla.org /security/advisories/mfsa2026-125/
Linkmozilla.org /security/advisories/mfsa2026-127/
Linkmozilla.org /security/advisories/mfsa2026-128/
Linkmozilla.org /security/advisories/mfsa2026-129/

sources

CVE List (CVE.org): description, affected versions, references; CVE Terms of Use
NVD: CVSS and CPE data
CISA Vulnrichment (ADP): SSVC decision points
FIRST EPSS: exploit probability and percentile
Summary, impact and triage tags: written by our AI analysis pipeline from the sources above. Check them against the sources.

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