CVE-2026-109728

Executive Summary

CVE-2026-109728 is a use-after-free (UAF) vulnerability residing in the DOM Animation component of Mozilla Firefox. Successful exploitation allows a remote attacker to corrupt memory and potentially execute arbitrary code within the context of the browser by convincing a user to visit a specially crafted web page. All Firefox users running versions prior to 158, Firefox ESR prior to 115.43, and Firefox ESR prior to 153.5 are affected, posing a significant risk to enterprises with large browser fleets and high-value individual targets.

Technical Analysis

The vulnerability is classified as a use-after-free (CWE-416) within the DOM (Document Object Model) Animation component. Use-after-free flaws occur when a program continues to reference a memory allocation after it has been freed, leading to undefined behavior. In the context of a browser's DOM animation subsystem, this typically involves the lifecycle management of animation-related objects (e.g., animation players, keyframe effects, or timeline objects) that are manipulated via JavaScript and CSS.

The attack surface is the browser's rendering engine, specifically the code path that processes animations. An attacker can trigger the flaw by crafting a web page that manipulates DOM animations—such as creating, modifying, or canceling animations—in a way that causes a required object to be freed prematurely while a stale reference remains. The exact JavaScript API calls or CSS properties involved are not specified in the available description; however, the general mechanism involves a race condition or logic error in the animation component's memory management.

Because the flaw is in the DOM layer, it is reachable from any web content that executes JavaScript, including cross-origin iframes and advertisements. Exploitation typically requires user interaction (e.g., visiting a malicious page), as the browser must execute attacker-controlled script. The use-after-free can lead to memory corruption, which an attacker can leverage to achieve code execution in the renderer process. Modern browsers employ sandboxing to contain such compromises, but a renderer escape may be chained to achieve full system compromise.

The vulnerability was fixed in Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5, indicating a coordinated patch across the rapid release and two extended support release branches. No technical details about the specific fix (e.g., changed functions or classes) are provided in the NVD description.

Exploitation Scenarios

No public PoC or exploit code is available for CVE-2026-109728. The following attack chain is inferred at the vulnerability-class level (use-after-free in DOM animation) and does not rely on specific undisclosed implementation details.

1. Reconnaissance/target identification: The attacker identifies targets running vulnerable Firefox versions (< 158, ESR < 115.43, ESR < 153.5). This can be done via user-agent string analysis, browser fingerprinting, or by hosting malicious content on a website likely to be visited by the target. No special network position is required; the attack is delivered via the web.

2. Triggering the flaw class: The attacker crafts a web page containing JavaScript that interacts with the DOM Animation API. By performing a sequence of operations on animation objects (e.g., creating, starting, canceling, or modifying animations) that races against the browser's garbage collection or animation frame processing, the attacker causes a use-after-free condition. The exact sequence is not public, but the vulnerability class is well understood: a dangling pointer to a freed animation object is dereferenced.

3. Payload/execution method: Once the use-after-free is triggered, the attacker can manipulate the freed memory to control the contents of the reallocated memory region. In a browser exploit, this typically involves heap grooming to place attacker-controlled data in the freed slot, then causing the stale pointer to be used as a function pointer or object reference. This can lead to arbitrary code execution within the renderer process. The attacker may use JavaScript to perform heap spraying or JIT spraying to increase reliability.

4. Post-exploitation impact: If code execution is achieved in the renderer process, the attacker can read sensitive data from the browser (cookies, session tokens, form data), perform actions on behalf of the user, or attempt to escape the sandbox to compromise the underlying operating system. In an enterprise context, this could lead to credential theft, lateral movement, or ransomware deployment.

Impact Assessment

The vulnerability has a CVSS base score of N/A, but based on the use-after-free class and browser context, a typical impact would be high confidentiality, integrity, and availability (C:H/I:H/A:H) with network attack vector (AV:N), high attack complexity (AC:H) due to the need for precise heap manipulation, no privileges required (PR:N), and user interaction required (UI:R). The scope remains unchanged (S:U) as the flaw is confined to the browser process.

Business impact includes:

• Data breach: Attackers can exfiltrate sensitive browsing data, session cookies, and credentials.
• System compromise: Successful sandbox escape can lead to full host takeover, enabling lateral movement within corporate networks.
• Large attack surface: Firefox is widely deployed in enterprises, and unpatched versions are common when update cycles lag.
• Blast radius: Any user browsing to a malicious or compromised website with a vulnerable Firefox is at risk. No authentication is needed beyond user interaction.

Detection Guidance

Sigma Rule

title: Possible Exploitation of CVE-2026-109728 (Firefox DOM Animation Use-After-Free)
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects suspicious browser crashes in Firefox related to DOM Animation, potentially indicating exploitation of CVE-2026-109728.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109728
author: Valters IT Threat Intelligence
date: 2026/10/23
tags:
  - attack.t1204
logsource:
  category: application
  product: windows
  service: application
detection:
  selection:
    Provider_Name: 'Application Error'
    EventID: 1000
    Data|contains: 'firefox.exe'
  filter_legit:
    Data|contains: 'Firefox'
  condition: selection and not filter_legit
falsepositives:
  - Legitimate browser crashes due to other issues
level: medium

YARA Rule

rule CVE_2026_109728_Firefox_DOM_Animation_UAF
{
    meta:
        description = "Detects potential exploit code targeting CVE-2026-109728 in Firefox DOM Animation"
        author = "Valters IT Threat Intelligence"
        date = "2026-10-23"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109728"
        hash = "not_available"
    strings:
        $js1 = "Animation" ascii wide nocase
        $js2 = "requestAnimationFrame" ascii wide nocase
        $js3 = "cancelAnimationFrame" ascii wide nocase
        $js4 = "getAnimations" ascii wide nocase
        $heap1 = { 90 90 90 90 90 90 90 90 } // NOP sleds common in exploits
    condition:
        uint16(0) == 0x5A4D and // MZ header for Windows executables
        filesize < 10MB and
        (2 of ($js*) or $heap1)
}

Note: This YARA rule is heuristic and targets generic indicators; no specific exploit code is available, so it may produce false positives.

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"Possible Firefox DOM Animation Use-After-Free Exploit Attempt (CVE-2026-109728)"; flow:to_server,established; content:"GET"; http_method; content:"/"; http_uri; pcre:"/(animation|keyframe|requestAnimationFrame)/i"; classtype:attempted-user; sid:10000001; rev:1;)

Note: This is a generic network detection for suspicious animation-related URI patterns; no specific exploit signature exists.

Elastic Detection

sequence by process.entity_id
  [process where event.type == "start" and process.name == "firefox.exe" and process.parent.name == "explorer.exe"]
  [process where event.type == "end" and process.name == "firefox.exe" and process.exit_code != 0]

Note: Detects abnormal Firefox process termination, which could indicate a crash due to exploitation.

Splunk SPL Query

index=windows source="WinEventLog:Application" EventCode=1000
| search "firefox.exe" AND ("access violation" OR "use-after-free" OR "crash")
| stats count by host, _time, Message
| where count > 5

Note: Monitors for repeated Firefox crashes that may indicate active exploitation attempts.

Wazuh OSSEC Rule

<group name="firefox,cve-2026-109728,">
  <rule id="100100" level="10">
    <decoded_as>json</decoded_as>
    <field name="EventID">1000</field>
    <field name="Provider_Name">Application Error</field>
    <field name="Data" type="pcre2">firefox\.exe</field>
    <description>Firefox application crash - possible CVE-2026-109728 exploitation attempt</description>
    <mitre>
      <id>T1204</id>
    </mitre>
  </rule>
</group>

Note: This rule triggers on Firefox crash events; it may require tuning to reduce false positives from legitimate crashes.

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). The vulnerability is reportedly fixed in Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5, but no official vendor advisory or patch details are available in the provided data. The following are general recommendations:

1. Immediate mitigations:

   • Disable JavaScript in Firefox for high-risk browsing (not practical for most users).
   • Use a different browser or a hardened browser profile for critical tasks.
   • Deploy browser isolation or sandboxing solutions to contain potential renderer compromises.
   • Monitor for crashes in firefox.exe and investigate anomalies.

2. Patch/upgrade steps:

   • Upgrade to Firefox 158 or later, or Firefox ESR 115.43/153.5 or later once official patches are available.
   • Verify the version via about:support in Firefox.
   • Ensure automatic updates are enabled for Firefox across all endpoints.

3. Long-term hardening:

   • Enforce browser update policies via Group Policy or MDM.
   • Implement application allowlisting to prevent execution of outdated browser versions.
   • Conduct regular vulnerability scanning to identify unpatched browsers.
   • Educate users about the risks of visiting untrusted websites.

References

• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-109728
• Mozilla Foundation Security Advisory (anticipated): https://www.mozilla.org/en-US/security/advisories/
• MITRE CWE-416: https://cwe.mitre.org/data/definitions/416.html
• CISA KEV Catalog: Not listed (as of report date)
• No known threat actor campaigns or exploit-in-the-wild evidence available at this time.