Applications

CVE-2026-124877

Use-after-free in Firefox/Thunderbird layout could enable remote code execution

Major vulnerability. High-severity use-after-free in Mozilla Firefox/Thunderbird (widely deployed browsers/email clients) enabling potential RCE via crafted content — impacts many organizations and warrants broad IT attention.

CVSS: 8.8Highscored by CISA
Exploited in the wild: Not knownnot in CISA KEV as of our last sync
EPSS: No score yet
Fix: Patch availablesee the advisories in the references
Published: 10 Nov 2026analysed here 12 Nov 2026

Summary

Impact

Use-after-free in positioned layout component can lead to remote code execution or crashes when processing crafted web/email content.

Triage

Attack surface: Server-side
Exploitable: At runtime
Attack vector: network
Weakness: CWE-416 Use After Free
Can a WAF block it?: No. See why

The summary, impact and action text on this page are written by our AI analysis pipeline, or taken from the NVD description when a CVE was imported from NVD. They are not the vendor's wording: check them against the sources. A longer AI executive summary and technical analysis are generated for signed-in accounts on the Max plan.

Affected products

Products named in our analysis

Firefox < 160
Firefox ESR < 115.45
Firefox ESR < 153.7
Thunderbird < 160
Thunderbird < 153.7

Fix and mitigation

Action required

Immediate action required

WAF and other mitigations

Can a WAF block it? No. This is a use-after-free memory bug. Generic WAF rules do not recognise the input that triggers it; only a rule written for this specific CVE can, and only if the triggering request has a recognisable shape.

Also consider: patch

Based on the weakness type only; no CVE-specific evidence yet. We check Cloudflare, AWS, Google Cloud Armor and Azure; for Akamai, Imperva, Fastly, F5, ask your vendor.

Remediation steps (AI-written)

Step-by-step remediation is generated for accounts on the Max plan, for CVEs that match a technology in their stack. Add this technology to your stack to see the fix checklist.

Exploitation evidence

CISA KEV catalog: Not listed as of our last sync.
Public exploit code: None on record in Exploit-DB, Metasploit, Nuclei templates or PoC-in-GitHub.
CISA SSVC: exploitation none, automatable no, technical impact totalAssessed by CISA 10 Nov 2026 (SSVC 2.0.3). Exploitation is none, poc (public proof of concept) or active.

Timeline

10 Nov 2026Published
10 Nov 2026CISA SSVC assessment: exploitation nonesame day later
10 Nov 2026CVE record last updatedsame day later
12 Nov 2026Analysed by vulneraibility.com2 days later

Common questions

Is CVE-2026-124877 being exploited?

Not confirmed. CVE-2026-124877 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.

How severe is CVE-2026-124877?

CVE-2026-124877 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-2026-124877?

Yes. A fix has been recorded for CVE-2026-124877. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.

What does CVE-2026-124877 affect?

CVE-2026-124877 affects Firefox < 160, Firefox ESR < 115.45, Firefox ESR < 153.7, Thunderbird < 160, and 1 further product or version. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.

What should I do about CVE-2026-124877?

Update to Firefox/Thunderbird fixed versions (160 / ESR 115.45/153.7) immediately

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

References

NVD CVE.org CVE Details Source

6 references in the CVE record, grouped by the tags the record gives them.

Where the data on this page comes from

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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