CVE-2026-109731

Executive Summary

CVE-2026-109731 is a memory-safety vulnerability in the Firefox Graphics: Canvas2D component caused by incorrect boundary conditions. A remote attacker can trigger the flaw by enticing a victim to render crafted web content in an affected browser, potentially leading to memory corruption. The vulnerability affects Firefox versions prior to 158 and Firefox ESR versions prior to 153.5; no public exploit, PoC, or in-the-wild activity has been observed to date.

Technical Analysis

The NVD description states that this issue stems from "incorrect boundary conditions in the Graphics: Canvas2D component." Canvas2D is the browser subsystem responsible for rendering two-dimensional graphics through the HTML <canvas> element and its associated JavaScript drawing API. Because boundary (bounds) checks in a rendering/copy path are defective, input that exceeds expected limits is handled incorrectly, and this class of defect (CWE-787, out-of-bounds write — the closest CWE mapping for a boundary-condition flaw in a memory-unsafe component) can allow data to be written outside the intended buffer.

At a code level, "incorrect boundary conditions" typically means a size, index, or length comparison is off — for example, a less-than vs. less-than-or-equal error, an integer overflow/truncation in a computed size, or a check performed against the wrong variable. The supplied intelligence does not identify specific functions, buffers, or parameters, so the precise defective comparison cannot be named without fabricating detail. What can be stated factually is the vulnerability class: an out-of-bounds memory access/write reachable through Canvas2D drawing operations.

The attack surface is the web-content rendering pipeline. Canvas2D operations are driven by attacker-controllable JavaScript and by attacker-supplied image/bitmap data drawn onto a canvas. Rendering occurs in the content process, and Firefox's process isolation means a content-process exploit must typically be combined with a separate sandbox-escape to achieve full code execution on the host — but memory corruption within the content process alone is already a serious primitive (arbitrary read/write within that process, potentially leading to a controlled crash or further escalation chaining).

No CWE identifier, CVSS subscores, EPSS score, or affected-version list were supplied by the tracked feeds; the affected-version range above is derived directly from the fixed versions named in the NVD description.

Exploitation Scenarios

No public PoC is available; the attack chain below is inferred from the vulnerability class (out-of-bounds access via incorrect bounds checking in Canvas2D) and is described at the class level only.

1. Reconnaissance / target identification. An attacker identifies victims running an affected Firefox build (< 158, or ESR < 153.5). This is a client-side target: the attacker does not need network access to the victim host, only the ability to deliver web content the victim will load — for example a malicious or compromised website, an injected ad/iframe, or a phishing page. Version fingerprinting is performed from the client side (browser User-Agent, or a page that feature-detects rendering behavior).

2. Triggering the flaw class. The victim visits the crafted page. The page's JavaScript performs Canvas2D operations with parameters crafted so that a boundary condition is evaluated incorrectly, causing the rendering/copy routine to read or write past the end of an allocated buffer. Because the flaw is a bounds-checking error, the trigger is a matter of supplying out-of-range dimensions, offsets, or counts to a Canvas2D operation; the specific property or method involved is not disclosed in the available intelligence.

3. Payload / execution method (class level). Exploitation of an out-of-bounds write typically proceeds by shaping heap memory so that the overrun corrupts an adjacent object, then converting that corruption into control-flow hijack or an information leak, frequently requiring a secondary primitive (e.g., a separate sandbox bypass) to escape the content process. The published advisory does not describe an exploit technique, and none is asserted here.

4. Post-exploitation impact. If successfully weaponized, memory corruption in the content process can yield crashes (denial of service), information disclosure via out-of-bounds reads, and — with additional chaining — arbitrary code execution in the browser. In the Firefox security model, full host compromise additionally requires a sandbox escape.

Impact Assessment

Impact is client-side and requires user interaction (victim must load attacker-controlled content). Where CVSS scoring is unavailable (the tracked feeds provide no score or vector), the vulnerability class — an out-of-bounds memory-write reachable from untrusted web content in a widely deployed browser — would typically map to high Confidentiality, Integrity, and Availability impact with a required user-interaction precondition. Business impact centers on endpoint risk: a successful browser memory-corruption exploit can lead to data theft, session compromise, or malware staging on user workstations. Blast radius is every unpatched Firefox and Firefox ESR installation; Mozilla's browser is deployed broadly across enterprise endpoints, making the population of exposed hosts large even absent a known exploit. The absence of any public PoC, exploit code, Nuclei template, or CISA KEV listing currently keeps the practical exploitation risk lower than the theoretical severity, but this can change quickly once technical details circulate.

Detection Guidance

Sigma Rule

title: Potential Firefox Canvas2D Boundary Condition Exploitation (CVE-2026-109731)
id: 7b3f1c2a-4d6e-4f01-9c2a-1a2b3c4d5e6f
status: experimental
description: Detects Firefox content-process crashes indicative of Canvas2D memory corruption attempts
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109731
author: valtersit.com
date: 2026/01/01
logsource:
  category: process_creation
  product: windows
detection:
  selection_image:
    Image|endswith: '\firefox.exe'
  selection_crash:
    CommandLine|contains:
      - 'crashreporter'
  condition: selection_image and selection_crash
falsepositives:
  - Legitimate Firefox crash reporting after unrelated crashes
level: low
tags:
  - attack.t1203
  - cve.2026.109731

YARA Rule

rule CVE_2026_109731_Firefox_Canvas2D_Bounds
{
    meta:
        description = "Detects Canvas2D-heavy exploit scaffolding targeting CVE-2026-109731"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109731"
        date = "2026-01-01"
        severity = "medium"
    strings:
        $c1 = "getContext" ascii wide
        $c2 = "createImageData" ascii wide
        $c3 = "putImageData" ascii wide
        $c4 = "drawImage" ascii wide
        $h1 = "unescape" ascii wide
        $h2 = "ArrayBuffer" ascii wide
        $h3 = "DataView" ascii wide
    condition:
        uint32(0) != 0x504B0304 and
        2 of ($c*) and 2 of ($h*)
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109731 Firefox Canvas2D boundary condition attempt (heuristic)"; flow:established,to_client; http.header; content:"Content-Type"; content:"text/html"; file_data; content:"getContext"; nocase; content:"putImageData"; nocase; content:"ArrayBuffer"; nocase; threshold: type limit, track by_src, count 1, seconds 60; classtype:attempted-user; sid:900109731; rev:1;)

Elastic Detection

process where process.name == "firefox.exe" and
  (process.command_line : "*crashreporter*" or process.command_line : "*Canvas2D*")

Splunk SPL Query

index=endpoint (process_name="firefox.exe" OR Image="*\\firefox.exe")
  (crash_reason="*Canvas2D*" OR CommandLine="*crashreporter*" OR faulting_module="*xul.dll*")
| stats count min(_time) as first_seen max(_time) as last_seen
        values(host) as hosts by user, process_name, crash_reason
| where count > 0
| sort - count

Wazuh OSSEC Rule

<group name="firefox,cve-2026-109731,">
  <rule id="109731" level="6">
    <decoded_as>json</decoded_as>
    <field name="process.name">firefox.exe</field>
    <field name="process.command_line">crashreporter</field>
    <description>CVE-2026-109731: Firefox crash reporting possibly linked to Canvas2D boundary-condition exploitation</description>
    <mitre>
      <id>T1203</id>
    </mitre>
    <group>cve_2026_109731,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). Note: the NVD description states the issue was "fixed in Firefox 158 and Firefox ESR 153.5," but no patch commit, vendor advisory, or affected-version feed entry was recovered by the tracked sources; the 0/4 confidence reflects that missing verifiable patch evidence, not the absence of a shipped fix.

1. Immediate mitigations

• Update Firefox to 158 or later, and Firefox ESR to 153.5 or later, as indicated by the NVD text. This is the primary remediation.
• Until endpoints are updated, reduce exposure: restrict browsing to approved sites, block/limit untrusted ads and third-party iframes, and enforce DNS/HTTP filtering for low-reputation domains.
• Deploy browser hardening where supported (e.g., site isolation and stricter content restrictions) and ensure crash-reporting telemetry is collected to surface exploitation attempts.

2. Patch/upgrade steps

• Follow Mozilla's standard channel (release and ESR) update paths; verify the running build reports version 158+ or ESR 153.5+.
• Confirm auto-update is enabled and functioning on managed endpoints; force an update check if endpoints are pinned or deferred.
• For environments using ESR, validate the ESR upgrade path to 153.5 before fleet rollout.

3. Long-term hardening

• Maintain a rapid browser update SLA, since browser memory-corruption fixes are frequently exploited after technical detail surfaces.
• Monitor Mozilla advisories and CVE feeds for this ID to capture the official advisory, CVE, and any affected-version clarifications once published.
• Keep endpoint detection tuned to browser content-process crash telemetry to catch exploitation attempts even without a known PoC.

References

• NVD — CVE-2026-109731: https://nvd.nist.gov/vuln/detail/CVE-2026-109731
• Mozilla Foundation Security Advisories (MFSA): https://www.mozilla.org/security/advisories/
• Mozilla Firefox release notes (version 158 / ESR 153.5): https://www.mozilla.org/firefox/releases/
• CISA Known Exploited Vulnerabilities Catalog (currently does not list this CVE): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• FIRST EPSS (no score published for this CVE as of writing): https://www.first.org/epss/

Threat-actor and in-the-wild notes: no public PoC, exploit code, Nuclei template, or CISA KEV entry was identified for CVE-2026-109731 at the time of writing. No specific threat-actor campaign is associated with this CVE in the supplied intelligence.