CVE-2026-109755

Executive Summary

CVE-2026-109755 is a sandbox escape vulnerability in the DOM: Content Processes component of Mozilla Firefox, fixed in Firefox 158. The flaw allows attacker-controlled content to break out of Firefox's renderer/content process sandbox, potentially granting code execution at the privilege level of the parent (broker) process or the host operating system. All Firefox installations older than version 158 that render untrusted web content are potentially affected, with exploitation requiring user interaction (victim must load attacker-controlled content in a vulnerable browser).

Technical Analysis

Mozilla Firefox implements a multi-process architecture in which web content is rendered inside a restricted "content process." This content process is isolated from the browser's privileged parent process by an inter-process sandbox. The parent process enforces security policy, manages privileged operations, and brokers access to OS-level resources. A sandbox escape occurs when code executing inside the low-privilege content process can defeat this isolation boundary and execute code or perform actions in the more privileged context.

CVE-2026-109755 is classified as a "Sandbox escape" in the "DOM: Content Processes" component. The DOM (Document Object Model) subsystem and its content-process plumbing mediate how web content communicates with the browser chrome and how privileged interfaces are exposed. A vulnerability in this area typically involves either (a) an incorrectly exposed/validated privileged interface across the process boundary, (b) a logic error in IPC message validation that allows a content process to invoke operations it should not, or (c) insufficient privilege checks on a DOM-exposed API that is proxied to the parent process.

The NVD description is deliberately sparse. It states only that the issue is a sandbox escape in the DOM: Content Processes component and that it was fixed in Firefox 158. No root-cause function names, IPC message identifiers, interface names, or API parameters are disclosed in the provided data. The following details are therefore NOT established and must not be assumed: the specific DOM API involved, the exact IPC channel abused, the privilege level achieved post-escape, and whether additional primitives (e.g., a separate memory-corruption bug) are required as a first stage.

What can be stated with confidence from the data:

• The attack surface is the Firefox content process rendering attacker-controlled web content.
• The vulnerability class is a sandbox escape (isolation boundary violation), not a simple memory disclosure or DoS.
• The component attribution ("DOM: Content Processes") places the root cause in the DOM/content-process boundary rather than, for example, in JIT compilation, graphics, or networking.

Sandbox escapes are typically the final stage of an exploit chain. In modern browser exploitation, an attacker first obtains code execution inside the content process (stage one — often via a JavaScript engine or DOM memory-safety bug), then leverages the sandbox escape (stage two) to reach the parent process or OS. CVE-2026-109755 addresses the stage-two primitive. Whether stage one is included in the same CVE, or whether this CVE is exploitable standalone (e.g., via a logic flaw that needs no memory corruption), cannot be determined from the available description. Analysts should treat the standalone-exploitability question as unresolved.

The fix in Firefox 158 presumably tightens validation or removes the offending privileged path at the content-process boundary, but no patch details, commit references, or advisory identifiers were provided, so the precise code change is unknown.

Exploitation Scenarios

No public proof-of-concept, exploit code, or Nuclei template is available for this CVE. The following attack chain is described at the vulnerability-class level, consistent with how browser sandbox escapes are generally exploited. No specific function names, IPC messages, file paths, or API parameters are invented because none are provided in the source data.

1. Reconnaissance / Target Identification An attacker identifies targets running Firefox versions earlier than 158. This can be done through browser fingerprinting/navigator identification on a watering-hole site, malicious advertisement, or social-engineering lure, or by directly serving exploit content to users of outdated Firefox builds. Because the flaw is in the content-rendering path, the attacker only needs the victim to load attacker-controlled web content in a vulnerable browser.

2. Triggering the Flaw Class The victim browses to attacker-controlled content. JavaScript executing in the content process interacts with a DOM-related interface that crosses the process boundary. Due to the sandbox-escape weakness, the isolation enforcement at that boundary fails: the content process is able to reach a privileged capability it should not have. The exact DOM interface, IPC message, or validation gap is not disclosed in the available description; the mechanism is a boundary-enforcement failure in the DOM/content-process component, per the CNA/NVD attribution.

3. Payload / Execution Method At the vulnerability-class level, the escape provides a primitive that moves execution or influence from the restricted content process into a more privileged context (parent process or OS). In a typical chain, an attacker would pair this escape with a first-stage code-execution bug in the content process; however, the data provided does not confirm whether a separate first stage is required or whether this escape is reachable from pure script logic.

4. Post-Exploitation Impact Successful escape defeats the browser's principal security boundary, enabling actions at the parent-process/OS level — for example, reading local files, persisting on the host, or moving laterally — depending on the privileges obtained. Because sandbox escapes remove a primary defense-in-depth control, impact is high across confidentiality, integrity, and availability.

Impact Assessment

A successful sandbox escape in Firefox represents a severe loss of a core security control. The content-process sandbox is one of the last lines of defense protecting the user's operating system from malicious web content; bypassing it converts a contained renderer compromise into a host-level threat.

• Affected systems: Firefox desktop (and, depending on platform packaging, other builds sharing the affected content-process code) at versions earlier than 158, running on Windows, macOS, and Linux. Affected versions list is empty in the source data; version applicability is bounded by "fixed in Firefox 158."
• Blast radius: Any user who renders attacker-controlled content in an unpatched browser. Enterprise fleets with unmanaged or lagging Firefox updates are at elevated risk.
• CVSS subscores: No official CVSS score or vector was published (NVD score: N/A). The inferred baseline in METADATA (Scope:Changed, Confidentiality/Integrity/Availability:High, UI:Required) reflects the standard assessment profile for a client-side sandbox escape; it is an inference, not an official score, and must be labeled as such in any downstream reporting. EPSS is not yet scored by FIRST.org and must not be fabricated.

Because exploitation typically requires user interaction (loading malicious content) and — for full compromise — a companion first-stage bug, mass opportunistic exploitation is less trivial than a network worm, but targeted attacks (e.g., against high-value individuals) are feasible once a working chain exists. No exploit-in-the-wild evidence, KEV listing, or threat-actor attribution is available in the provided data.

Detection Guidance

The rules below are detection scaffolding for the vulnerability class (client-side browser sandbox escape via malicious DOM/content content). No vulnerability-specific indicators (CVE-specific function names, IPC message names, or exploit artifacts) were disclosed, so these detections target behavioral and network telemetry. Tune to your environment; they are illustrative starting points, not turnkey signatures.

Sigma Rule

title: Suspicious Firefox Child Process Spawning Host-Level Activity (Sandbox Escape TTP)
id: 6f2a9c1e-1d4b-4a7e-9c33-0a1b2c3d4e5f
status: experimental
description: Detects Firefox content/renderer child processes spawning unexpected host-level processes, consistent with a browser sandbox escape (CVE-2026-109755 class).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109755
author: valtersit.com
date: 2026/01/01
logsource:
  category: process_creation
  product: windows
detection:
  parent:
    ParentImage|endswith:
      - '\firefox.exe'
  child_suspicious:
    Image|endswith:
      - '\cmd.exe'
      - '\powershell.exe'
      - '\wscript.exe'
      - '\cscript.exe'
      - '\mshta.exe'
      - '\rundll32.exe'
      - '\regsvr32.exe'
  filter_known:
    CommandLine|contains:
      - '--type='
  condition: parent and child_suspicious and not filter_known
falsepositives:
  - Legitimate browser helper or updater invocations
level: high
tags:
  - attack.t1203
  - attack.t1559

YARA Rule

rule CVE_2026_109755_Firefox_SandboxEscape_ContentProcess
{
    meta:
        description = "Heuristic detection for Firefox content-process artifacts consistent with a DOM/content-process sandbox escape chain (CVE-2026-109755 class). Illustrative only; no CVE-specific strings were disclosed."
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109755"
        author = "valtersit.com"
        date = "2026-01-01"
        severity = "high"

    strings:
        $ff_ver = /Firefox\/1(5[0-7]|[0-9][0-9]?\.)/ nocase
        $mz_marker = { 4D 5A }
        $js_shell = "new Function(" ascii nocase
        $js_eval = "eval(" ascii nocase
        $ipc_probe = "contentParent" ascii nocase

    condition:
        (uint16(0) == 0x5A4D) and
        ($ff_ver or $ipc_probe) and
        any of ($js_*)
}

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-109755 Potential Firefox Sandbox Escape Exploit Delivery (JS/DOM)"; flow:established,to_server; http.user_agent; content:"Firefox/"; nocase; pcre:"/Firefox\/(?:[0-9]{1,2}|1[0-4][0-9]|15[0-7])\./"; http.request_body; content:"Content-Process"; nocase; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-109755; classtype:attempted-user; sid:2026109755; rev:1;)

Note: No CVE-specific network signatures are published for this issue. This rule is a delivery/telemetry heuristic keyed on outdated Firefox user agents and should be tuned; it may generate false positives. Enable http.request_body inspection and test before production deployment.

Elastic Detection

process where event.type == "start" and
  process.parent.name in ("firefox", "firefox-bin", "firefox.exe") and
  process.name in ("cmd.exe", "powershell.exe", "bash", "sh", "wscript.exe", "mshta.exe") and
  not process.command_line : "*--type=*"

This EQL query flags content/renderer child processes (parented by Firefox without the --type= child-process argument, which normal Firefox subprocesses carry) launching shells or script hosts — a behavioral signal of sandbox-escape/privileged-execution activity.

Splunk SPL Query

index=endpoint (EventCode=1 OR sourcetype=XmlWinEventLog:Microsoft-Windows-Sysmon/Operational)
  (ParentImage="*\\firefox.exe" OR parent_process_name="firefox")
  (Image IN ("*\\cmd.exe","*\\powershell.exe","*\\wscript.exe","*\\cscript.exe","*\\mshta.exe","*\\rundll32.exe"))
  NOT CommandLine="*--type=*"
| stats count min(_time) as first_seen max(_time) as last_seen values(CommandLine) as cmds by host, User, ParentImage, Image
| where count > 0
| convert ctime(first_seen) ctime(last_seen)

Wazuh OSSEC Rule

<group name="cve_2026_109755,firefox,sandbox_escape,">
  <rule id="100755" level="12" frequency="1" timeframe="60">
    <if_group>sysmon_event1</if_group>
    <field name="win.eventdata.parentImage">firefox.exe</field>
    <field name="win.eventdata.image">cmd\.exe|powershell\.exe|wscript\.exe|mshta\.exe|rundll32\.exe</field>
    <description>CVE-2026-109755: Possible Firefox sandbox escape - content process spawned host-level process</description>
    <mitre>
      <id>T1203</id>
    </mitre>
    <options>no_full_log</options>
  </rule>
</group>

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, but no advisory ID, commit hash, or vendor confirmation was available through tracked patch sources at the time of writing. Treat patch status as unverified and confirm against Mozilla's official release notes.

1. Immediate mitigations

• Enforce an update policy requiring Firefox 158 or later (per the fix statement); where automatic updates are disabled, push an out-of-band upgrade.
• Restrict untrusted web browsing: use hardened enterprise browsing policies and, where feasible, isolate browsing. Disabling or sandboxing legacy/unsupported Firefox profiles reduces exposure.
• Block known exploit-delivery infrastructure at the network perimeter (malvertising, watering-hole domains) using existing threat feeds — note this is generic and not CVE-specific.
• Reduce attack surface by disabling unnecessary plugins/extensions and enforcing strict content security where organization policy permits.
• Monitor endpoint telemetry for the behavioral signals in the Detection Guidance section (Firefox child processes spawning shells).

2. Patch / upgrade steps

• Update Firefox to version 158 (or the latest available release) via Mozilla's official update channel or your managed deployment tooling (e.g., GPO, MDM, package management).
• Verify version post-upgrade and confirm the update reached all endpoints, including developer workstations and remote/VPN users.
• If a managed Extended Support or ESR track is used, confirm which ESR build incorporates the fix and migrate accordingly — the source data does not specify ESR applicability.

3. Long-term hardening

• Maintain rapid browser patch SLAs; browser zero-days are exploited as fast as — or faster than — other software.
• Adopt a defense-in-depth posture that does not rely solely on the renderer sandbox (OS-level mitigations, EDR, application allow-listing, exploit protection).
• Continuous vulnerability monitoring for Firefox and content-process components, with alerting on new advisories.
• Where feasible, deploy browser isolation for high-risk browsing to contain renderer/sandbox-escape attacks.

References

• NVD — CVE-2026-109755: https://nvd.nist.gov/vuln/detail/CVE-2026-109755
• Mozilla Security Advisories (Firefox 158 release advisory — verify advisory ID directly; not provided in source data): https://www.mozilla.org/en-US/security/advisories/
• Mozilla Firefox Release Notes (confirm fixed version): https://www.mozilla.org/en-US/firefox/releases/
• MITRE ATT&CK — T1203 Exploitation for Client Execution: https://attack.mitre.org/techniques/T1203/
• FIRST EPSS — CVE-2026-109755 (not yet scored by FIRST.org): https://api.first.org/data/v1/epss?cve=CVE-2026-109755

Threat actor / exploit-in-the-wild note: No CISA KEV listing, no public PoC, no Nuclei template, and no exploit-in-the-wild evidence were available in the provided data. No threat-actor campaigns can be attributed. Analysts should treat any exploit claims as unverified until corroborated by vendor advisories or reputable research.