CVE-2026-109735

Executive Summary

CVE-2026-109735 is a use-after-free vulnerability (CWE-416) located in the DOM Animation component of Mozilla Firefox, fixed in Firefox 158. The flaw is triggered when the browser processes specially crafted web content that causes an animation-related DOM object to be freed while a reference to it remains accessible, enabling memory corruption. Because the attack surface is the browser's rendering and animation pipeline and exploitation typically requires only that a victim load a malicious or compromised page, successful abuse can result in arbitrary code execution in the context of the Firefox process, with consequences for confidentiality, integrity, and availability on affected endpoints.

Technical Analysis

Browser vulnerabilities in the use-after-free class arise when an object is deallocated while at least one live pointer or reference to that memory still exists. Subsequent dereference of the dangling reference operates on memory that may have been reallocated for a different purpose. In a modern browser this is particularly dangerous because the freed region can be groomed — the attacker arranges for the allocator to reuse the slot with attacker-influenced data — so that the dangling pointer now points at crafted contents. The attacker then attempts to redirect control flow or read/write arbitrary memory.

The affected component is the DOM Animation subsystem (as stated in the description: "Use-after-free in the DOM: Animation component"). This component manages the lifecycle of animation objects bound to DOM elements — their creation, their association with the timing/render machinery, and their teardown. The defining structural risk in such code is a lifetime-ordering error: an animation object, or an internal object the animation holds, is released before all holders of a reference to it have been notified. If script is able to force that ordering — for example through repeated creation/removal of animated elements, timer-driven state changes, or other DOM manipulations that interleave teardown with pending references — a dangling reference is produced.

No public technical write-up, function-level root cause, or specific API surface is available for this CVE. Accordingly, the precise trigger sequence, the exact object freed, and the downstream primitive (arbitrary read/write versus a more constrained confusion) cannot be confirmed from the provided data. What can be stated with confidence is the vulnerability class (use-after-free), the component (DOM Animation), the attack vector (web content processed by the browser), and the remediation (Firefox 158). Analysts should treat any specific implementation detail beyond these facts as unverified until Mozilla's advisory or a public analysis is published.

Use-after-free in a browser's DOM/animation layer is a high-severity class because browser processes are the primary isolation boundary for untrusted web content: escaping the JavaScript sandbox and corrupting native memory is the canonical route to code execution on the host. The absence of a public proof-of-concept, exploit code, or Nuclei template lowers the immediate observed threat but does not reduce the intrinsic risk of the flaw class; weaponization knows no fixed timeline once a fix diff becomes public.

Exploitation Scenarios

The following attack chain describes the vulnerability class and mechanism at a level supportable by the NVD description. No public PoC is available for CVE-2026-109735; the chain below is inferred from the use-after-free class and the stated "DOM: Animation" component, and does not assert specific function names, endpoints, or parameters, none of which are provided in the source data.

1. Reconnaissance / target identification. The attacker identifies targets running an affected Firefox build (versions prior to 158). Unlike server-side flaws, no network scanning of the vulnerable component is needed: the "service" is the victim's browser. Attackers instead profile their audience — for example, serving the payload to visitors of a compromised or attacker-controlled site, to users of a malicious advertisement, or via a phishing link — and may use user-agent or feature-based conditioning to deliver the trigger only to vulnerable Firefox versions.

2. Triggering the flaw (use-after-free in DOM Animation). The victim navigates to attacker-controlled content. That content drives the browser's animation machinery in a way that causes a DOM Animation object to be freed while a reference to it persists. The precise script pattern is not public, but at the class level the attack relies on desynchronizing object lifetime from reference lifetime — manipulating DOM animation state so that teardown occurs while a subsequent operation still holds and uses the stale reference. The effect is dereference of freed memory.

3. Payload / execution method (class level). After the dangling reference is created, the attacker performs heap grooming so that the freed slot is reclaimed with attacker-controlled or attacker-shaped data, converting the dangling pointer into a controlled reference. In browser use-after-free exploitation the objective at this stage is to pivot from the corrupted object into a native memory primitive (an arbitrary read/write or an instruction-pointer/call-target redirection). Because no exploit code or write-up is public, the specific transformation steps cannot be enumerated here without speculation; this section deliberately stops at the class level.

4. Post-exploitation impact. If the memory primitive is successfully leveraged, the attacker achieves code execution within the Firefox process context on the victim host. The severity of that outcome drives the CVSS environmental expectations: full compromise of confidentiality, integrity, and availability (impact subscores C:H/I:H/A:H), subject to process sandboxing on the target platform. Post-exploitation activity could include data theft, installation of malware, or lateral movement if the browser process is not otherwise contained, though the reachable impact is constrained by the host's browser sandbox and OS hardening.

Impact Assessment

The vulnerability is a memory-safety flaw in a widely deployed consumer and enterprise browser. Its impact model is client-side: every workstation, laptop, VM, or endpoint where a user runs an affected Firefox version is potentially reachable. There is no server-side exposure and therefore no internet-facing service to enumerate; the blast radius is defined by endpoint population and user browsing behavior rather than by network topology.

Because CVSS data was not published for this CVE, the vector shown in the metadata (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H) reflects the standard profile for a remotely triggerable browser use-after-free: network vector via web content, low attack complexity once a trigger is known, no privileges required, and user interaction required (the victim must load the page). Availability, integrity, and confidentiality are all rated high because successful exploitation can yield code execution in the user's browser process. This should be treated as an operational estimate of the flaw class, not as a vendor-published score, and it should be re-scored once Mozilla's advisory publishes a CVSS vector.

Residual risk factors:

• Exploitability currently constrained. No public PoC, no exploit code, and no Nuclei template exist as of this report. EPSS has not scored the CVE. This reduces near-term likelihood but is not a durable mitigation — browser use-after-free details become weaponizable quickly after a maintainer publishes the fixing diff.
• No KEV listing. CISA has not flagged this CVE for known exploitation in the wild. Absence of KEV does not mean absence of exploitation; it reflects the absence of confirmed reporting.
• User interaction requirement. Exploitation requires the victim to load attacker-controlled content, which affects the scale of opportunistic attacks but not the severity of a successful one.

Detection Guidance

Because the trigger is client-side and delivered through web content, high-fidelity detection primarily comes from endpoint telemetry (browser crash metrics, process anomalies) rather than from classic network signatures. The rules below are class-level detections for use-after-free exploitation in Firefox and should be tuned to the environment.

Sigma Rule

title: Potential Firefox DOM Animation Use-After-Free Exploitation (CVE-2026-109735)
id: 3fa0b4c2-1d5e-4c7a-9b21-firefox-uaf-dom-anim
status: experimental
description: Detects anomalous browser process behavior consistent with attempted use-after-free exploitation in Firefox DOM Animation component.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109735
author: Threat Intelligence
date: 2026/01/01
tags:
  - attack.t1203
logsource:
  category: process_creation
  product: windows
detection:
  selection_browser:
    Image|endswith:
      - '\firefox.exe'
  selection_anomaly:
    # browser launched with debug/attach-style flags or from unusual parent
    CommandLine|contains:
      - '--contentproc'
      - '--MOZ_CRASH'
  filter_normal:
    ParentImage|endswith:
      - '\explorer.exe'
      - '\firefox.exe'
  condition: selection_browser and not filter_normal
falsepositives:
  - Legitimate Firefox subprocess spawning by the parent browser process
level: medium

YARA Rule

rule CVE_2026_109735_Firefox_DOM_Animation_UAF
{
    meta:
        description = "Heuristic detection of Firefox DOM Animation use-after-free trigger content (CVE-2026-109735)"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109735"
        author = "Threat Intelligence"
        date = "2026-01-01"
        severity = "high"

    strings:
        $anim1 = "requestAnimationFrame" ascii nocase
        $anim2 = "webkitAnimation" ascii nocase
        $dom1  = "createDocumentFragment" ascii nocase
        $heap1 = "ArrayBuffer" ascii nocase
        $heap2 = "Uint32Array" ascii nocase

    condition:
        filesize < 2MB and
        any of ($anim*) and any of ($dom1) and any of ($heap*)
}

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-109735 Firefox DOM Animation use-after-free - suspicious animated-content payload"; flow:to_server,established; http.user_agent; content:"Firefox/"; http.header; content:"Referer"; pcre:"/(requestAnimationFrame|createDocumentFragment)/i"; classtype:attempted-user; sid:2026109735; rev:1; metadata:created_at 2026_01_01, cve CVE-2026-109735;)

Elastic Detection

sequence by process.entity_id with maxspan=10s
  [process where process.name == "firefox.exe"]
  [file where event.action == "creation" and wildcard(file.name, "*.dmp")]

Splunk SPL Query

index=endpoint (process_name=firefox.exe OR Image="*\\firefox.exe")
| eval suspicious=if(match(CommandLine, "(?i)(--contentproc|moz_crash|attach)"), 1, 0)
| where suspicious=1 OR match(process_name, "(?i)firefox")
| stats count min(_time) as first_seen max(_time) as last_seen by host, user, process_name, CommandLine
| where count > 3
| convert ctime(first_seen) ctime(last_seen)

Wazuh OSSEC Rule

<group name="windows,firefox,cve-2026-109735,">
  <rule id="109735" level="10">
    <if_sid>60000</if_sid>
    <field name="win.eventdata.image">firefox\.exe$</field>
    <field name="win.eventdata.commandLine">(?i)(--contentproc|moz_crash)</field>
    <description>CVE-2026-109735: Suspicious Firefox process invocation consistent with DOM Animation use-after-free exploitation</description>
    <mitre>
      <id>T1203</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence is 0/4 — UNPATCHED. No patch evidence was returned from any tracked source for this CVE, and the metadata fields for vendor fix, workarounds, and affected version ranges are empty. The only remediation datum supported by the source data is that the vulnerability "was fixed in Firefox 158." Treat the guidance below as version-aware operational advice and confirm everything against Mozilla's official advisory before change execution.

1. Immediate mitigations (reduce exposure before upgrade validation completes):

   • Restrict or sandbox browsing to trusted sites for users on affected builds; content-driven use-after-free requires the victim to load attacker-controlled content, so reducing exposure to untrusted/untested web content reduces opportunistic risk.
   • Enforce browser process sandboxing and OS hardening (e.g., platform exploit mitigations) to limit the consequences of a successful memory-corruption exploit.
   • Enable crash reporting and centralize Firefox crash telemetry; a spike in content-process crashes on endpoints may be an early indicator of exploitation attempts.
   • Where policy permits, use browser isolation / remote rendering for high-risk browsing to move the vulnerable process off the endpoint.

2. Patch / upgrade steps:

   • Upgrade to Firefox 158 or later once release availability is confirmed against Mozilla's official channel. Because patch confidence is 0/4 and no vendor advisory is on file here, do not treat "158" as verified until Mozilla's release notes/advisory confirm the CVE mapping.
   • Verify the installed build version on endpoints (about:preferences or via software inventory) and confirm the deployed version resolves CVE-2026-109735.
   • On managed fleets, push the update through the standard software distribution pipeline and confirm adoption before closing the change; account for enterprise channels (ESR, MSI/policy-managed installs) which may lag the rapid-release build.
   • Re-scan endpoints after rollout to confirm the vulnerable version is no longer present.

3. Long-term hardening:

   • Maintain an aggressive browser update cadence; client-side memory-safety flaws are remediated primarily by staying current.
   • Monitor Mozilla security advisories and the NVD entry for a vendor-published CVSS vector and any updated exploitation status; re-score risk when that data lands.
   • Keep exploit mitigations (ASLR, DEP, sandbox policies) standardized across the endpoint fleet to raise the cost of any future browser memory-corruption exploit.
   • Retain endpoint detection for browser child-process anomalies so that post-compromise activity is visible if a future weaponized exploit emerges.

References

• NVD — CVE-2026-109735: https://nvd.nist.gov/vuln/detail/CVE-2026-109735
• Mozilla Foundation Security Advisories (check the Firefox 158 advisory for the CVE mapping and any stated impact): https://www.mozilla.org/en-US/security/advisories/
• Mozilla Firefox Release Notes (version 158 and later): https://www.mozilla.org/en-US/firefox/releases/
• CWE-416: Use After Free — https://cwe.mitre.org/data/definitions/416.html
• CISA Known Exploited Vulnerabilities Catalog (currently does NOT list CVE-2026-109735): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No public proof-of-concept, exploit code, or Nuclei template is known for CVE-2026-109735 as of this report. EPSS has not yet scored the CVE, and no threat-actor campaign or exploit-in-the-wild evidence is associated with this CVE in the available data.