CVE-2026-109762

Executive Summary

CVE-2026-109762 is a vulnerability in the SVG rendering component of Mozilla Firefox, caused by incorrect boundary conditions that can lead to memory corruption. It affects Firefox versions prior to 158 and Firefox ESR versions prior to 153.5. Successful exploitation could allow an attacker to achieve arbitrary code execution or cause a denial of service when a user views a maliciously crafted SVG document.

Technical Analysis

The NVD description states: "Incorrect boundary conditions in the SVG component. This vulnerability was fixed in Firefox 158 and Firefox ESR 153.5." This indicates a memory-safety flaw in the code responsible for parsing or rendering SVG (Scalable Vector Graphics) content. Incorrect boundary conditions typically manifest as an out-of-bounds read or write, where a length, index, or loop counter is improperly validated, allowing access beyond the allocated buffer. In the context of a browser's SVG engine, such a flaw could be triggered by specially crafted SVG markup — for example, malformed path data, extreme attribute values, or edge-case geometry — that drives the parser or renderer past its intended memory boundaries. Because SVG is a complex, feature-rich format involving path parsing, transformations, and DOM integration, boundary-condition defects are a recurring class of vulnerability in this area. No further technical specifics (e.g., function names, exact malformed inputs, or affected code paths) are provided in the available data; the CWE classification and exploitation details remain unconfirmed in the source material.

The vulnerability is reachable through normal web content: Firefox renders SVG inline in HTML as well as in standalone SVG documents. An attacker does not need to exploit a public-facing service; instead, the attack surface is the browser client, typically reached via a malicious website, a compromised legitimate site, or a malicious advertisement. User interaction (opening a page containing the crafted SVG) is required, which is reflected in the likely CVSS vector.

No CVSS score or vector is provided in the NVD data; the vector used in the metadata is a representative inference based on the vulnerability class and is not an official score. EPSS has not yet assigned a score. CWE is listed as N/A in the source; the metadata uses CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer) as an inferred class, as it is consistent with "incorrect boundary conditions."

Exploitation Scenarios

No public proof-of-concept, exploit code, or Nuclei template is available for CVE-2026-109762. The following attack chain is inferred strictly from the vulnerability class (incorrect boundary conditions in SVG rendering) and must be treated as a theoretical model.

1. Reconnaissance / target identification: The attacker identifies victims running a vulnerable Firefox version (< 158) or Firefox ESR (< 153.5). This is typically opportunistic — drive-by attacks target any browser that visits attacker-controlled or compromised content. No server-side fingerprinting is required; the browser version can be inferred from the user agent or simply assumed for a broad campaign.

2. Triggering the flaw class: The attacker crafts an SVG document (or injects SVG markup into an HTML page) designed to exercise the incorrect boundary condition in the SVG component. Because the exact triggering input is not disclosed, the mechanism is described at the class level: malformed or edge-case SVG data causes the renderer to compute an incorrect boundary, resulting in an out-of-bounds memory access (read or write) during parsing or rendering.

3. Payload / execution method: Exploitation would follow the standard memory-corruption chain. If the boundary error permits an out-of-bounds write, the attacker may attempt to corrupt adjacent heap objects to achieve control-flow hijacking. If it is an out-of-bounds read, the attacker may attempt to leak memory (e.g., to defeat ASLR) or cause a crash. At the vulnerability-class level, this would involve heap grooming and, on modern Firefox builds, bypassing mitigations such as ASLR, stack canaries, and possibly the browser's sandbox. No specific technique, gadget, or offset can be cited because no technical details or PoC are available.

4. Post-exploitation impact: Successful exploitation in the content process could lead to arbitrary code execution within the Firefox sandbox. Without a sandbox escape, impact is limited to the compromised content process; a full system compromise would require an additional sandbox-escape vulnerability. In the absence of successful code execution, the most likely outcome is a browser crash (denial of service).

Because no PoC or in-the-wild exploitation has been observed, the practical likelihood of exploitation is currently low, but the vulnerability should not be dismissed — browser memory-corruption bugs are high-value targets for exploit developers and are often weaponized after patch release.

Impact Assessment

The impact of CVE-2026-109762 depends on whether the boundary condition results in an out-of-bounds write or read. In the worst case, it enables arbitrary code execution in the Firefox content process, which could allow an attacker to steal data, install malware, or pivot to further attacks if combined with a sandbox escape. A more limited outcome is a denial-of-service (browser crash).

Affected systems are endpoints running vulnerable Firefox or Firefox ESR builds. The blast radius is potentially large given Firefox's significant install base, but exploitation requires user interaction (visiting a malicious page). No CVSS score, vector, EPSS score, or CISA KEV listing is provided in the source data; the vector in the metadata is an analytical estimate for a browser memory-corruption flaw requiring user interaction, not an official score. Organizations that rely on Firefox for accessing untrusted web content should treat this as a client-side risk with elevated priority for patching.

Detection Guidance

Sigma Rule

title: Potential Firefox SVG Memory Corruption Exploitation Attempt (CVE-2026-109762)
id: 8f2a1c7e-4b3d-4e9a-9c1f-2d6e8a0b5f31
status: experimental
description: Detects process crashes or abnormal termination of Firefox processes consistent with exploitation of CVE-2026-109762 in the SVG component.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109762
author: Threat Intelligence
date: 2026-01-01
logsource:
  category: process_creation
  product: windows
detection:
  selection_firefox:
    Image|endswith:
      - '\firefox.exe'
  selection_crash:
    EventID: 1000
  condition: selection_firefox and selection_crash
falsepositives:
  - Legitimate Firefox crashes unrelated to exploitation
level: low
tags:
  - attack.t1190

YARA Rule

rule CVE_2026_109762_SVG_MemoryCorruption_Indicators
{
    meta:
        description = "Detects SVG content with highly abnormal path or attribute values potentially associated with CVE-2026-109762 boundary condition abuse"
        author = "Threat Intelligence"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109762"
        severity = "medium"

    strings:
        $svg_tag = "<svg" ascii nocase
        $path = "<path" ascii nocase
        $d_attr = "d=\"" ascii nocase
        $long_number = /[0-9]{6,}/ ascii

    condition:
        $svg_tag and $path and $d_attr and 2 of ($long_number)
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109762 Potential Firefox SVG Boundary Condition Exploitation Attempt"; flow:to_client,established; http.response_body; content:"<svg"; nocase; content:"<path"; nocase; distance:0; pcre:"/<svg[^>]*>.*<path[^>]*d=\"[^\"]{2000,}/s"; metadata:service http; reference:cve,2026-109762; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-109762; classtype:attempted-user; sid:2026109762; rev:1;)

Note: This rule uses a heuristic pattern for unusually large SVG path data, which may accompany boundary-condition attacks. No exact exploit signature is available.

Elastic Detection

sequence by host.id, process.entity_id with maxspan=5s
  [process where process.name == "firefox.exe" and event.action == "start"]
  [process where process.name == "firefox.exe" and event.action in ("crash", "abnormal-termination")]

This EQL query looks for a Firefox process crash shortly after start, which could correlate with exploitation of memory-corruption vulnerabilities. It is a generic proxy and requires tuning.

Splunk SPL Query

index=windows source="XmlWinEventLog:Application"
EventCode=1000
| search "firefox.exe" AND (Message="*svg*" OR Message="*moz*")
| stats count by host, Message
| where count > 0

This search identifies Firefox application crash events that reference SVG-related modules. It is a best-effort detection given the lack of a public exploit signature.

Wazuh OSSEC Rule

<group name="firefox,cve-2026-109762,">
  <rule id="109762" level="5">
    <if_sid>61600</if_sid>
    <field name="win.system.eventID">^1000$</field>
    <field name="win.eventdata.appName">firefox\.exe</field>
    <description>CVE-2026-109762: Potential Firefox crash related to SVG boundary condition exploitation</description>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

This rule triggers on a Windows application crash event for firefox.exe. It is a low-fidelity indicator.

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED — no patch evidence from any tracked source.

1. Immediate mitigations:

   • Update Firefox to version 158 or Firefox ESR to version 153.5 immediately. These versions contain the fix per the NVD description.
   • If immediate update is not possible, consider isolating high-risk browsing (e.g., using a separate browser or a virtual machine) and enforce a strict content security policy where feasible.
   • Disable JavaScript execution for untrusted sites as a defense-in-depth measure (though SVG rendering may not require JavaScript for trigger).

2. Patch/upgrade steps:

   • For standard Firefox: upgrade to version 158 or later.
   • For Firefox ESR: upgrade to version 153.5 or later.
   • Validate the update via Help > About Firefox or the about:support page.
   • No workaround or vendor patch diff is provided in the source data; rely solely on the version upgrade.

3. Long-term hardening:

   • Maintain a rapid browser patch cadence, prioritizing security releases.
   • Enable automatic updates for browsers where policy allows.
   • Deploy endpoint detection for browser crashes and memory-corruption indicators, and correlate with web proxy logs to identify potential exploit delivery.
   • Monitor Mozilla security advisories and CVE feeds for additional technical details and any updated exploitation status.

References

• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-109762
• Mozilla Foundation Security Advisory (corresponding advisory not yet enumerated in source data; check https://www.mozilla.org/en-US/security/advisories/ for updates)
• CVE.org record: https://www.cve.org/CVERecord?id=CVE-2026-109762
• FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-109762 (not yet scored)
• CISA KEV: not listed (as of data capture)