---
id: "CVE-2026-117921"
kind: "vulnerability"
aliases: ["CVE-2026-117921"]
title: "Firefox: use-after-free in CSS parsing lets attacker run code"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["browser", "desktop", "firefox", "runtime"]
not_affected_if: ["already-patched-version"]
published_at: "2026-10-27T14:16:47+00:00"
first_seen_at: "2026-10-27T14:32:22+00:00"
updated_at: "2026-10-27T14:37:18+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-117921", "first_seen_at": "2026-10-27T14:32:22+00:00"}]
---

# Firefox: use-after-free in CSS parsing lets attacker run code

A use-after-free bug in Firefox's CSS Parsing and Computation component lets memory be reused after it is freed. The sources do not say what an attacker can achieve beyond that, and Mozilla's fixed releases are Firefox 159, Firefox ESR 115.44 and Firefox ESR 153.6.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the installed Firefox version in Help > About Firefox, or run `firefox --version` on Linux.
- Compare it against `159`, `115.44` (ESR) and `153.6` (ESR); anything older on the matching branch is affected.

## Fix

- Update Firefox to `159` or later.
- Update Firefox ESR to `115.44` or `153.6` on the matching ESR branch.

## Timeline

- 2026-10-27T14:32:22+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-117921

## From the sources

Use-after-free in the CSS Parsing and Computation component. This vulnerability was fixed in Firefox 159, Firefox ESR 115.44, and Firefox ESR 153.6.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_