CVE-2026-109726

Executive Summary

CVE-2026-109726 is a use-after-free (UAF) vulnerability in the SVG component of Mozilla Firefox. Successful exploitation occurs within the browser process and can lead to memory corruption, potentially enabling arbitrary code execution in the context of the user. All users running Firefox versions prior to 158, Firefox ESR 115.43, or Firefox ESR 153.5 are affected. No public proof-of-concept or in-the-wild exploitation has been observed at this time. The vulnerability class (CWE-416) typically requires user interaction, such as visiting a maliciously crafted web page or opening a specially crafted SVG file.

Technical Analysis

The vulnerability is a use-after-free in the SVG rendering component of Mozilla Firefox. Use-after-free flaws arise when an application frees a memory object but continues to reference it, leading to a dangling pointer. In the context of a browser's SVG engine, this class of bug is commonly triggered when an SVG element (e.g., an animation, a filter, or a scriptable SVG object) is removed or modified while an asynchronous operation (such as layout, rendering, or event dispatch) still holds a pointer to that object. The exact function, class, or code path is not disclosed in the available NVD description or CNA title; therefore, the precise trigger conditions cannot be confirmed, but the mechanism is characteristic of a lifecycle management failure in the SVG DOM or rendering pipeline.

Key technical observations:

• The bug is classified under CWE-416 (Use After Free), a memory safety issue that often results in a wild pointer dereference.
• The browser's multi-process architecture (content process isolation) is the primary mitigation against full system compromise; however, an attacker can still achieve code execution inside the content sandbox and potentially escape it via a separate vulnerability.
• The SVG component is a complex renderer that parses XML-based vector graphics, including animations, filters, and scripting. It is a frequent source of UAF issues due to the asynchronous nature of reflow and repaint.
• No affected versions list, OSV packages, or CVE-specific CVSS score are provided in the source data. The CVSS vector above is a representative estimate based on the vulnerability class (network vector, user interaction required, high confidentiality/integrity/availability impact).

Because the NVD description is sparse, the attack surface is inferred as: a remote attacker supplies a crafted SVG document, either as a standalone file or embedded in a web page, which triggers the UAF during parsing or rendering.

Exploitation Scenarios

The following attack chain is described at the vulnerability-class level. No specific function names, API endpoints, or file paths are available from the provided intelligence; none are invented here.

1. Reconnaissance / target identification: An attacker identifies targets running vulnerable Firefox builds. This can be done via browser fingerprinting (User-Agent strings, JavaScript feature detection) or by broadly targeting users through malicious websites, malvertising, or spear-phishing emails containing a link to a crafted page.

2. Triggering the UAF: The attacker hosts a web page that embeds or references a malicious SVG document. When the victim opens the page in a vulnerable Firefox version, the SVG content causes the browser to free a memory object while a reference to it remains. The exact SVG feature that triggers the bug (e.g., animation, filter, or DOM manipulation) is not specified in the available data, but the flaw is in the SVG component.

3. Payload / execution method (vulnerability-class level): After the dangling pointer is dereferenced, the attacker can influence the freed memory to redirect execution. Because this is a UAF, the exploitation typically involves heap grooming to place attacker-controlled data in the freed memory region, then triggering a call through the corrupted pointer. The result is arbitrary code execution within the Firefox content process. This step requires precise memory layout manipulation, which is non-trivial and not provided by public PoC — no public exploit code exists for this CVE.

4. Post-exploitation impact: Successful exploitation grants the attacker the ability to run code with the privileges of the Firefox content process. On modern Firefox versions, this process is sandboxed, limiting direct access to the operating system. However, an attacker could use the foothold to attempt sandbox escape, exfiltrate browser data (cookies, session tokens, saved passwords), or pivot to further attacks. In the absence of a sandbox escape, the impact is still significant: arbitrary code execution in the browser context can lead to credential theft, data exfiltration, and installation of malware.

No public proof-of-concept or exploit code is available for CVE-2026-109726. The attack chain is inferred strictly from the vulnerability class (use-after-free in a browser component) and the limited NVD description.

Impact Assessment

The business impact is high due to the ubiquity of Firefox in enterprise environments and the potential for remote code execution via a drive-by download or phishing link. The vulnerability requires user interaction (visiting a malicious page or opening a crafted file), which slightly reduces the attack surface but is easily achievable through social engineering. The blast radius includes all endpoints running vulnerable Firefox versions, including Windows, macOS, and Linux desktops. Because the vulnerability is in the SVG component, any web browsing activity where SVG content is rendered can trigger it.

CVSS subscores (estimated): Confidentiality: High, Integrity: High, Availability: High. The attack vector is Network, attack complexity is Low (assuming no special conditions beyond user interaction), privileges required are None, and user interaction is Required. The scope is Unchanged, as the vulnerability affects only the browser process (unless a sandbox escape is chained). These subscores reflect the potential worst-case impact: an attacker who successfully exploits the UAF can read and modify data accessible to the browser process and cause a denial of service (crash).

In an enterprise context, successful exploitation could lead to:

• Theft of session cookies and authentication tokens, enabling lateral movement.
• Deployment of malware or ransomware via drive-by download.
• Data exfiltration from web applications accessible through the browser.
• Reputation damage and regulatory consequences (e.g., GDPR) if customer data is compromised.

The EPSS score is not yet available; no CISA KEV listing; no known active exploitation. Nevertheless, browser UAFs are high-value targets and should be prioritized for patching.

Detection Guidance

Sigma Rule

title: Firefox SVG Use-After-Free Exploitation Attempt Detection
id: 3a8f2c1e-9b4d-4e7a-8c3f-1a2b3c4d5e6f
status: experimental
description: Detects potential exploitation of CVE-2026-109726 via crafted SVG content causing Firefox crash or anomalous behavior.
author: Threat Intelligence
date: 2026-05-20
logsource:
  category: process_creation
  product: windows
detection:
  selection:
    Image|endswith: '\firefox.exe'
    CommandLine|contains:
      - '--svg'
      - '.svg'
  condition: selection
falsepositives:
  - Legitimate SVG file opens
level: low
tags:
  - attack.t1190

YARA Rule

rule CVE_2026_109726_SVG_UAF
{
    meta:
        description = "Detects SVG files that may trigger use-after-free in vulnerable Firefox versions"
        author = "Threat Intelligence"
        date = "2026-05-20"
        reference = "CVE-2026-109726"
        hash = "unknown"
    strings:
        $svg1 = "<svg" ascii wide nocase
        $svg2 = "animate" ascii wide nocase
        $svg3 = "filter" ascii wide nocase
        $svg4 = "setAttribute" ascii wide nocase
        $svg5 = "removeChild" ascii wide nocase
    condition:
        $svg1 and 2 of ($svg2, $svg3, $svg4, $svg5)
}

Note: This rule is generic for SVG files that contain features commonly associated with UAF triggers. It is not a specific signature for this CVE, as no unique file-based indicator is available.

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109726 Firefox SVG Use-After-Free Exploitation Attempt"; flow:to_client,established; content:"Content-Type|3a| image/svg+xml"; http_header; content:"<svg"; http_client_body; distance:0; within:100; classtype:attempted-user; sid:10000001; rev:1;)

Elastic Detection

process where process.name == "firefox.exe" and process.command_line : "*.svg*"

This EQL rule detects Firefox processes launched with SVG file arguments. It may indicate a user opening a malicious SVG, but lacks specificity for the vulnerability itself. No network-based EQL is provided due to lack of CVE-specific indicators.

Splunk SPL Query

index=windows (source="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" EventCode=1) Image="*\\firefox.exe" CommandLine="*.svg*"
| stats count by host, user, CommandLine
| where count > 0

Wazuh OSSEC Rule

<rule id="100001" level="5">
  <if_sid>61600</if_sid>
  <field name="win.eventdata.image">firefox.exe</field>
  <field name="win.eventdata.commandLine">\.svg</field>
  <description>CVE-2026-109726: Potential Firefox SVG use-after-free exploitation attempt</description>
  <mitre>
    <id>T1190</id>
  </mitre>
</rule>

Note: The rule IDs used are illustrative; adjust to match your Wazuh decoder and rule set.

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). The NVD description states the vulnerability was fixed in Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5. However, no vendor advisory, patch commit, or official release note is available in the provided data. Treat this as unpatched until a verified vendor update is confirmed.

1. Immediate mitigations:

   • Disable or restrict SVG rendering via browser configuration if feasible (e.g., using about:config to disable svg.enabled). This may break legitimate websites.
   • Enforce strict Content Security Policy (CSP) to block inline SVG and external SVG resources where possible.
   • Use browser isolation or sandboxing technologies to contain potential exploitation.
   • Educate users about phishing and malicious links; avoid opening untrusted SVG files.

2. Patch/upgrade steps:

   • Upgrade to Firefox 158 or later, Firefox ESR 115.43 or later, or Firefox ESR 153.5 or later as soon as official builds are available from Mozilla.
   • Verify the integrity of downloaded installers via GPG signatures.
   • For enterprise environments, use software management tools to push updates and verify version compliance.

3. Long-term hardening:

   • Enable automatic updates for Firefox.
   • Deploy exploit mitigation technologies (e.g., ASLR, DEP, CFG) at the OS level.
   • Monitor for future Mozilla security advisories and apply patches promptly.
   • Consider using a browser with a stronger sandbox or a different rendering engine for high-risk browsing.

References

• NVD entry: https://nvd.nist.gov/vuln/detail/CVE-2026-109726 (placeholder, replace with actual URL once assigned)
• Mozilla Security Advisories: https://www.mozilla.org/security/advisories/ (check for MFSA covering CVE-2026-109726)
• CWE-416: Use After Free — https://cwe.mitre.org/data/definitions/416.html
• MITRE ATT&CK T1190: Exploit Public-Facing Application — https://attack.mitre.org/techniques/T1190/
• FIRST EPSS: https://www.first.org/epss/ (score not yet available)