Web
CVE-2026-124858
Use-after-free in Firefox/Thunderbird HTML parser enabling potential remote code execution
Major vulnerability. High-severity HTML parser use-after-free enabling potential remote code execution in widely used Mozilla products (Firefox/Thunderbird); impacts many organizations and warrants broad IT notification.
- CVSS
- 8.8Highscored by CISA
- Exploited in the wild
- Not knownnot in CISA KEV as of our last sync
- EPSS
- 0.40%chance of exploitation in the next 30 daystop 66% of scored CVEsFIRST score of 11 Nov 2026
- Fix
- Patch availablesee the advisories in the references
- Published
- 10 Nov 2026analysed here 12 Nov 2026
Summary
Impact
Potential remote code execution via crafted web content rendering (use-after-free), allowing browser compromise or arbitrary code execution.
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
- Mozilla Firefox < 160
- Firefox ESR < 115.45
- Firefox ESR < 153.7
- Thunderbird < 160
- Thunderbird < 153.7
Fix and mitigation
Action required
Immediate action required
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.com1 day later
Common questions
Is CVE-2026-124858 being exploited?
Not confirmed. CVE-2026-124858 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 0.40% probability of exploitation in the next 30 days.
How severe is CVE-2026-124858?
CVE-2026-124858 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-2026-124858?
Yes. A fix has been recorded for CVE-2026-124858. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2026-124858 affect?
CVE-2026-124858 affects Mozilla 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-2026-124858?
Update to fixed Firefox/Thunderbird versions immediately; block or avoid untrusted sites until patched.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
References
6 references in the CVE record, grouped by the tags the record gives them.
Other links (6)
- bugzilla.mozilla.org /show_bug.cgi?id=2074172
- mozilla.org /security/advisories/mfsa2026-125/
- mozilla.org /security/advisories/mfsa2026-126/
- mozilla.org /security/advisories/mfsa2026-127/
- mozilla.org /security/advisories/mfsa2026-128/
- mozilla.org /security/advisories/mfsa2026-129/
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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