CVE-2026-109745

Executive Summary

CVE-2026-109745 is a sandbox escape vulnerability in Mozilla Firefox that arises from uninitialized memory handling within the DOM: Web Serial component. Given that Web Serial exposes hardware interfaces (serial ports) to web content, a flaw in this component's memory management could allow a compromised content process to escalate privilege and break out of the browser's process sandbox. All users running Firefox versions prior to 158 and Firefox ESR versions prior to 153.5 are potentially affected; the vulnerability was fixed in Firefox 158 and Firefox ESR 153.5.

Technical Analysis

The vulnerability is described by the vendor as a "sandbox escape due to uninitialized memory in the DOM: Web Serial component." This description establishes three key facts:

1. The vulnerable component is the Web Serial API integration inside the DOM layer. Web Serial is the browser interface that allows web pages to communicate with serial devices (e.g., microcontrollers, industrial equipment connected via USB/serial). Because Web Serial bridges web content to local hardware, it is a security-sensitive component that normally runs under strict privilege separation.

2. The root cause class is uninitialized memory. An uninitialized-memory condition means a memory region is read or used before it is deterministically initialized by the program. This is a memory-safety defect class (closely related to CWE-824 "Access of Uninitialized Pointer" and CWE-457 "Use of Uninitialized Variable"). In a browser engine, uninitialized memory can expose stale heap contents, which may lead to information disclosure, or may cause the engine to operate on an attacker-influenced pointer/value, providing a primitive for further exploitation such as memory corruption.

3. The consequence is a sandbox escape. A sandbox escape means the attacker moves from a lower-privileged context (typically the content/renderer process where untrusted web content executes) to a higher-privileged context (the parent/broker process or the underlying operating system). Firefox uses multi-process architecture and OS-level sandboxing; escaping it defeats the primary containment boundary that limits the impact of a compromised renderer.

Because the NVD entry provides no function names, class names, or code paths, this report intentionally does not speculate about specific internal functions. The mechanism is best characterized at the vulnerability-class level: uninitialized memory in the Web Serial DOM implementation produced a condition that allowed privilege escalation beyond the browser sandbox.

The fix was delivered in Firefox 158 and Firefox ESR 153.5. No CWE was assigned in the supplied data, no CVSS vector was published, and no patch commit or advisory link was provided. No public proof-of-concept, exploit code, or Nuclei template is known. CISA KEV does not list this CVE.

Attack surface summary:

• Entry vector: web content interacting with the Web Serial API in the DOM.
• Precondition: user must run a vulnerable Firefox/Firefox ESR build.
• Trust boundary crossed: content/renderer process → browser parent/OS (sandbox boundary).

Exploitation Scenarios

No public PoC is available; the attack chain below is inferred from the vulnerability class (uninitialized memory → sandbox escape) and the affected component (Web Serial in the DOM). No specific function, API, or payload byte sequence is asserted because none is provided in the source data.

1. Reconnaissance / target identification. An attacker identifies victims running vulnerable Firefox builds (pre-158, or ESR pre-153.5). This can be done passively by fingerprinting the browser via the User-Agent or client-side features, or by hosting the exploit on a site the victim is induced to visit. Web Serial availability is relevant: the attacker determines whether the victim's browser exposes the Web Serial API and whether the user can be prompted to grant serial-port access.

2. Triggering the flaw class. The attacker crafts a web page that exercises the Web Serial integration in the DOM. Interacting with the vulnerable code path causes the engine to operate on memory that has not been properly initialized. Because the defect is in a browser-internal component, the exact trigger requires the specific uninitialized-memory path that the vendor fixed; at the class level, the goal is to reach a state where uninitialized memory is consumed by privileged logic.

3. Payload / execution method (class level). Uninitialized-memory bugs are typically leveraged either as an information-disclosure primitive (reading stale memory to defeat ASLR or leak pointers) or as a corruption primitive (using attacker-influenced uninitialized data to steer control flow). The attacker then chains this primitive across the process boundary to escape the sandbox. This report does not assert a specific exploitation technique (e.g., ROP, JIT spraying) because the source data does not describe one.

4. Post-exploitation impact. If the sandbox escape succeeds, code that began as untrusted web content can execute with the privileges of the browser's parent process or the underlying operating system, subject to the host's OS sandbox. This enables actions normally blocked from content processes: broader filesystem access, reading of other sessions/data, and potentially arbitrary code execution in the user's context.

Practical prerequisites: user interaction to visit the malicious page and, depending on implementation, to satisfy any Web Serial permission prompt. The attack is remote and requires user action, consistent with a client-side exploitation scenario.

Impact Assessment

No CVSS vector or score was published in the supplied data. Using the CVSS 3.1 vector proposed in the metadata (AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H), the qualitative impact is high across confidentiality, integrity, and availability, with a scope change (S:C) reflecting the crossing of the sandbox trust boundary.

• Confidentiality (High): stale/uninitialized memory exposure plus post-escape filesystem access could disclose sensitive local data.
• Integrity (High): crossing into a higher-privileged context permits modification of data and execution of attacker-controlled operations outside the sandbox.
• Availability (High): exploitation attempts may crash the browser; successful escape could disrupt the host.

Blast radius: Any endpoint running affected Firefox or Firefox ESR builds. In enterprises, this includes developer workstations, BYOD laptops, and any host where Firefox is a primary or secondary browser. Because exploitation is client-side, individual users are the unit of compromise; there is no server-side blast radius. Firefox ESR users are materially exposed unless they have updated to 153.5, since ESR is deployed specifically to remain on a stable branch and can lag behind rapid-release fixes if patching cadence is slow.

No known exploitation in the wild, no KEV listing, and no EPSS score (not yet scored by FIRST.org) are available. Risk should be assessed as unquantified-but-potentially-high given the sandbox-escape outcome.

Detection Guidance

Detection focuses on identifying vulnerable Firefox versions and suspicious Web Serial / serial-device activity from browser processes. Because the flaw is client-side and no exploit artifacts are published, rules target the behavioral and version signatures associated with the component rather than a known payload.

Sigma Rule

title: Suspicious Browser Spawn / Serial Device Interaction Indicative of Sandbox Escape Activity
id: 8f3a1c2e-2b77-4d55-9e10-cve2026109745
status: experimental
description: Detects browser child processes accessing serial device paths or spawning unexpected children, which may indicate post-exploitation activity following a Web Serial sandbox escape (CVE-2026-109745).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109745
author: Threat Intelligence Team
date: 2026/01/01
logsource:
  category: process_creation
  product: windows
detection:
  selection_browser:
    Image|endswith:
      - '\firefox.exe'
  selection_serial:
    CommandLine|contains:
      - 'COM'
      - 'serial'
  selection_unexpected_child:
    ParentImage|endswith: '\firefox.exe'
    Image|endswith:
      - '\cmd.exe'
      - '\powershell.exe'
      - '\wscript.exe'
      - '\cscript.exe'
  condition: selection_browser and (selection_serial or selection_unexpected_child)
falsepositives:
  - Legitimate browser extensions or developer tooling invoking serial utilities
level: high

YARA Rule

rule CVE_2026_109745_Firefox_WebSerial_SandboxEscape
{
    meta:
        description = "Heuristic detection for suspicious serial-related strings in browser artifacts; not a signature for the specific uninitialized-memory defect (none published)."
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109745"
        author = "Threat Intelligence Team"
        date = "2026-01-01"
        severity = "high"

    strings:
        $ws1 = "WebSerial" ascii nocase
        $ws2 = "navigator.serial" ascii nocase
        $ws3 = "requestPort" ascii nocase

    condition:
        uint16(0) == 0x5A4D and any of ($ws*)
}

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-109745 Possible Firefox Web Serial Sandbox Escape Exploit Delivery"; flow:established,to_server; content:"navigator.serial"; http_client_body; nocase; content:"requestPort"; http_client_body; nocase; classtype:attempted-user; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-109745; sid:2026109745; rev:1;)

Note: This rule detects page-side use of the Web Serial API, which is a necessary precondition for triggering the vulnerable component, not a signature of the defect itself. No exploit-specific network signature exists because no PoC is public.

Elastic Detection

process where event.type == "start" and
  process.parent.name in ("firefox.exe", "firefox") and
  process.name in ("cmd.exe", "powershell.exe", "wscript.exe", "cscript.exe", "sh", "bash")

Correlate with serial-device access events on the same host to reduce false positives.

Splunk SPL Query

index=endpoint (process_name=firefox.exe OR process_name=firefox)
| join type=left host [ search index=endpoint (COM OR serial OR ttyUSB OR ttyACM) ]
| where isnotnull(process_name)
| table _time host process_name parent_process_name CommandLine

Adjust index and field names to match your endpoint telemetry schema.

Wazuh OSSEC Rule

<group name="firefox,cve-2026-109745,">
  <rule id="2026101" level="10">
    <if_sid>61603</if_sid>
    <field name="win.eventdata.parentImage">firefox\.exe$</field>
    <field name="win.eventdata.image">(cmd|powershell|wscript|cscript)\.exe$</field>
    <description>CVE-2026-109745: Unexpected child process spawned by Firefox - possible sandbox escape activity</description>
    <mitre>
      <id>T1203</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor advisory URL, commit hash, or workaround was provided in the source data. The only fixed-version information available is the vendor's statement that the issue was fixed in Firefox 158 and Firefox ESR 153.5.

1. Immediate mitigations

   • Upgrade Firefox to 158 or later, and Firefox ESR to 153.5 or later, immediately. Because the fix versions are stated by the vendor, treat these as the remediation target.
   • If immediate upgrade is impossible, restrict exposure: disable or restrict the Web Serial API via enterprise policy if your environment does not require it, and constrain browsing to trusted sites.
   • Enforce prompt/dialog handling policies so users do not grant serial-device permissions to untrusted origins.
   • Reduce user interaction with untrusted web content via DNS filtering, web proxies, and mail gateway URL rewriting.

2. Patch / upgrade steps

   • Standard-channel Firefox: update to 158+.
   • ESR Firefox: update to 153.5+.
   • Verify the running version after update (about:support / Help → About Firefox).
   • Stage updates through your normal change process; verify with endpoint inventory that no hosts remain on vulnerable builds.

3. Long-term hardening

   • Maintain an aggressive browser patch SLA; prioritize browsers because they are a primary client-side attack surface.
   • Enable OS-level and browser sandboxing features and keep them current; sandbox escapes depend on defeating these boundaries.
   • Apply least-privilege on endpoints to limit post-escape impact.
   • Restrict or audit hardware-bridging web APIs (Web Serial, WebUSB, WebHID) that are not operationally required.
   • Monitor for and investigate unexpected child processes spawned by browser binaries.

References

• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-109745
• Mozilla Firefox security advisories: https://www.mozilla.org/en-US/security/advisories/
• Mozilla Firefox ESR download/channel page (verify fixed versions 153.5 / 158): https://www.mozilla.org/en-US/firefox/enterprise/
• CISA Known Exploited Vulnerabilities Catalog (this CVE is NOT currently listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• FIRST EPSS (not yet scored for this CVE): https://www.first.org/epss/

No threat actor campaigns or exploit-in-the-wild evidence were identified. No public proof-of-concept, exploit code, or Nuclei template is available at the time of writing.