CVE-2026-109707

Executive Summary

CVE-2026-109707 is a use-after-free (UAF) vulnerability in the Networking component of Mozilla Firefox that permits a sandbox escape. Because the flaw is reached through the network stack, exploitation requires an attacker to first obtain code execution inside the browser content sandbox (typically via a separate renderer or script bug) and then pivot out of the sandbox to the parent/privileged process. All Firefox builds prior to 158 and all Firefox ESR builds prior to 153.5 are presumed affected based on the NVD fix statement. Impact is critical in the context of a chained exploit: a successful sandbox escape exposes the host to arbitrary code execution at the browser's privilege level, credential theft, and lateral movement from an endpoint into managed enterprise environments.

Technical Analysis

The vulnerability class is a use-after-free (CWE-416) located in the Networking component. A UAF occurs when an object is freed but a reference to it remains reachable and is subsequently dereferenced. In a networking subsystem, freed memory of this kind is typically associated with request/response lifecycle objects — connections, channels, streams, or associated memory buffers — where asynchronous events, cancellation, redirects, or teardown can outlive the object that owns them, leaving a dangling pointer that is later used.

Two properties of this CVE are decisive for risk assessment:

1. Memory-safety primitive in privileged code path. Unlike a UAF confined to a content/renderer process, the Networking component participates in the browser's privileged architecture. This is why the CNA classified the outcome as a "sandbox escape" — successful abuse of the primitive gives a means to execute with permissions beyond the content sandbox boundary.

2. Exploitability requires a multi-stage chain. A UAF in networking code is not, by itself, directly reachable from arbitrary remote input in the way a remote service bug would be. In the browser threat model, the attacker must already possess a first-stage primitive (for example script execution in a content process) and then induce the networking code path to allocate, free, and reuse the target object. The UAF then furnishes the second-stage primitive used to escape the sandbox.

No function names, class names, endpoint paths, or parameter names for this flaw appear in the NVD description or CNA title. The description states only: "Sandbox escape due to use-after-free in the Networking component. This vulnerability was fixed in Firefox 158 and Firefox ESR 153.5." No CWE was supplied by the data source, but the "use-after-free" statement maps to CWE-416; no CVSS vector was published, so the vector shown in METADATA is an analytical placement for chained-sandbox-escape class impact and should not be treated as a vendor-published score. No root-cause commit, bug ID, or memory-corruption detail is available in the supplied intelligence, and none is fabricated here.

The fix boundary is well defined and verifiable by version comparison: the flaw was eliminated in Firefox 158 (rapid release channel) and Firefox ESR 153.5 (extended support channel). Anything earlier than these builds should be treated as vulnerable to the underlying defect, absent evidence a downstream redistributor backported the fix.

Exploitation Scenarios

No public proof-of-concept, exploit code, or Nuclei template exists for CVE-2026-109707. The attack chain below is described at the vulnerability-class level, strictly from the stated "sandbox escape due to use-after-free in the Networking component" characterization. Specific function names, IPC message names, and object layouts are deliberately omitted because they are not present in the source data and cannot be confirmed.

1. Reconnaissance / target identification. The attacker identifies a victim running an unpatched Firefox build below 158 (or ESR below 153.5). Because this is a client-side bug, targeting is typically achieved through lure-based delivery: a malicious or compromised web page, a watering-hole site, or an attacker-controlled ad/iframe. An adversary can narrow the population by fingerprinting the browser, its version, and its platform before deciding to fire.

2. Triggering the flaw class. The attacker must first obtain a foothold inside the content/renderer sandbox — for example via a separate JavaScript or rendering vulnerability. Only then does the networking UAF become reachable. The attacker drives the networking component into a state where a network-related object is freed while a reference to it persists, and then causes that stale reference to be dereferenced. In UAF exploitation at this class, the typical tactics are heap grooming (shaping allocations so the freed slot is reclaimed by attacker-influenced data) and type confusion against the reallocated object. The exact grooming strategy, the object groomed, and the allocation sizes are not disclosed in the available data.

3. Execution method at class level. Successful reuse of the freed object yields a controlled read/write or a controlled call target in the context of the networking component. The attacker converts that primitive into execution that crosses out of the content sandbox — the "sandbox escape" the CNA describes. The concrete mechanism by which the escape is achieved (for example, leveraging the component's privileges to influence a higher-privilege process) is not detailed in the supplied intelligence.

4. Post-exploitation impact. Once outside the sandbox, code runs with the browser's host privileges. Typical consequences: reading local files accessible to the user, harvesting saved credentials and session cookies, deploying additional payloads, establishing persistence, and using the endpoint as a pivot into internal network resources. In managed environments the escaped context inherits the user's enterprise identity, making the blast radius significantly larger than a content-only compromise.

Chain dependency is the key operational fact: CVE-2026-109707 is not remotely exploitable in isolation. It is a high-value second stage that raises the severity of any accompanying renderer/script vulnerability from "contained" to "endpoint compromise."

Impact Assessment

No CVSS score or vector was published for this CVE (the only authoritative value available is N/A). Accordingly, the qualitative assessment below is derived from the vulnerability class and the stated sandbox-escape outcome rather than from numeric subscores, and no vendor numeric values are asserted.

Confidentiality: High. Sandbox escape gives access to data outside the browser's content boundary, including local files, browser profile data, and credentials available to the user context.

Integrity: High. Post-escape code execution enables modification of local data, browser state, and potentially security controls reachable at the user's privilege level.

Availability: High in the extreme (full process/host control, potential for destructive payloads); in practice attacker goals are often data- and persistence-oriented rather than denial of service.

Attack complexity and user interaction: In UAF client-side exploitation, complexity is elevated (heap shaping, reliable trigger, bypass of any exploit mitigations), and exploitation generally requires the victim to visit attacker-controlled content (user interaction). Privileges required are None, but scope is changed — the security authority boundary (sandbox) is crossed, which is the defining characteristic of this CVE.

Blast radius: Any endpoint running Firefox below 158 or Firefox ESR below 153.5. High-value targets are knowledge workers with browser-based access to SaaS, webmail, VPN portals, and internal applications; unmanaged personal devices on the corporate network; and jump hosts used for web administration. The lack of a public PoC and absence from CISA KEV (as of the provided data) reduce immediate opportunistic risk, but the defect is a strong candidate for weaponization by sophisticated actors who maintain private exploit chains and combine browser bugs into full escape-and-execute packages.

Detection Guidance

Detection for this CVE is challenging because the exploit chain is client-side and the UAF itself has no network signature. The rules below therefore target the observable side effects of a browser sandbox-escape chain: anomalous child-process behavior, browser crashes at the moment of exploitation attempts, shell/loader activity spawned from the browser process tree, and suspicious outbound network behavior. These are behavioral detections and are expected to produce some benign matches; tune per environment.

Sigma Rule

title: Firefox Sandbox Escape - Suspicious Child Process from Browser (CVE-2026-109707)
id: 5a2c9f41-7d63-4e88-9b21-3f0d1c7e6a54
status: experimental
description: Detects shell or scripting interpreter processes spawned directly from Firefox, which is anomalous and may indicate post-sandbox-escape execution (CVE-2026-109707, use-after-free in Networking component).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109707
author: valtersit.com Threat Intelligence
date: 2026-01-01
tags:
  - attack.t1203
  - attack.t1059
logsource:
  category: process_creation
  product: windows
detection:
  selection_parent:
    ParentImage|endswith:
      - '\firefox.exe'
  selection_child:
    Image|endswith:
      - '\cmd.exe'
      - '\powershell.exe'
      - '\pwsh.exe'
      - '\wscript.exe'
      - '\cscript.exe'
      - '\mshta.exe'
      - '\rundll32.exe'
      - '\regsvr32.exe'
      - '\curl.exe'
      - '\bitsadmin.exe'
  filter_legit:
    CommandLine|contains:
      - '--type=crashpad-handler'
      - 'Default Browser Agent'
  condition: selection_parent and selection_child and not filter_legit
falsepositives:
  - Enterprise launcher or EDR instrumentation wrapping the browser
  - Legitimate application-launch integrations (rare)
level: high

YARA Rule

rule CVE_2026_109707_Firefox_SandboxEscape_Indicators
{
    meta:
        description = "Heuristic detection of artifacts associated with the Firefox Networking use-after-free sandbox escape (CVE-2026-109707). Targets stale/old Firefox builds and appended shellcode-style loader characteristics."
        author = "valtersit.com Threat Intelligence"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109707"
        tlp = "WHITE"

    strings:
        $mz = { 4D 5A }

        // Firefox product/version strings found in binaries
        $ff_prod = "Firefox" ascii wide
        $ff_ver1 = "Mozilla/5.0 (Windows NT" ascii wide
        $ff_ver2 = "Gecko/20100101 Firefox/" ascii wide

        // Common post-exploitation shellcode loader markers (heuristic)
        $sc1 = { 48 8B ?? ?? ?? ?? ?? 48 85 C0 74 ?? FF D0 }   // resolve-and-call stub pattern
        $sc2 = { 65 48 8B 04 25 60 00 00 00 }                  // gs:[60] PEB access (x64)
        $sc3 = { 64 8B 30 8B 76 18 8B 76 20 }                  // fs:[0] PEB walk (x86)
        $sc4 = "VirtualAlloc" ascii
        $sc5 = "CreateThread" ascii

    condition:
        uint16(0) == 0x5A4D and
        $ff_prod and
        (
            any of ($sc1, $sc2, $sc3) or
            (2 of ($sc4, $sc5))
        )
}

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-109707 Possible Firefox Sandbox-Escape Chain - Browser User-Agent to Uncategorized Host with Suspicious Stage Response"; flow:established,to_server; http.user_agent; content:"Firefox/"; http.header; content:"|0d 0a|"; content:"Accept:"; pcre:"/Firefox\/(1[0-4][0-9]|15[0-7])\./"; pkt_data; classtype:attempted-user; reference:cve,2026-109707; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-109707; metadata:created_at 2026_01_01, updated_at 2026_01_01; sid:2026109707; rev:1;)

Note: this rule is a correlation aid only — it flags traffic from outdated Firefox builds below the fixed versions (pre-158) and must be combined with severity scoring and reputation data to avoid excessive alert volume. The underlying UAF has no direct network signature.

Elastic Detection

sequence by process.entity_id with maxspan=2s
  [process where event.type == "start" and process.name : ("firefox", "firefox.exe") and process.parent.name != null]
  [process where event.type == "start" and process.parent.name : ("firefox", "firefox.exe") and
     process.name : ("cmd.exe", "powershell.exe", "pwsh.exe", "wscript.exe", "cscript.exe", "mshta.exe", "rundll32.exe", "regsvr32.exe", "curl.exe")]

Query intent: a browser process that immediately spawns a command interpreter or LOLBin is a high-fidelity indicator of sandbox-escape execution. Complement with a crash-correlation query: process where process.name : ("firefox", "firefox.exe") and event.action == "crash" and process.exit_code != 0 to identify the final stage of a failed or triggering UAF attempt.

Splunk SPL Query

index=endpoint (source="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" OR source="WinEventLog:Security")
(EventCode=1 OR EventCode=4688)
| eval parent=coalesce(ParentImage, Creator_Process_Name)
| eval child=coalesce(Image, New_Process_Name)
| eval cl=coalesce(CommandLine, Process_Command_Line)
| where match(lower(parent),"(firefox\.exe|firefox)$")
  AND match(lower(child),"(cmd\.exe|powershell\.exe|pwsh\.exe|wscript\.exe|cscript\.exe|mshta\.exe|rundll32\.exe|regsvr32\.exe|curl\.exe|bitsadmin\.exe)$")
| where NOT match(cl,"(?i)(crashpad-handler|default browser agent|-contentproc)")
| stats count min(_time) as first_seen max(_time) as last_seen values(cl) as command_lines by ComputerName, parent, child, User
| convert ctime(first_seen) ctime(last_seen)
| sort - count

Companion query for crash bursts that may correspond to repeated trigger attempts: search the Firefox crash reporter or Sysmon ProcessTerminate/Application Error logs for firefox.exe faulting events with rapidly repeating timestamps on the same host.

Wazuh OSSEC Rule

<group name="windows,sysmon,exploit,">
  <rule id="109707" level="12">
    <if_sid>61603</if_sid>
    <field name="win.eventdata.parentImage">firefox.exe$</field>
    <field name="win.eventdata.image">(cmd\.exe|powershell\.exe|pwsh\.exe|wscript\.exe|cscript\.exe|mshta\.exe|rundll32\.exe|regsvr32\.exe|curl\.exe|bitsadmin\.exe)$</field>
    <description>CVE-2026-109707: Firefox spawned a shell/LOLBin - possible sandbox escape post-exploitation</description>
    <mitre>
      <id>T1203</id>
      <id>T1059</id>
    </mitre>
    <group>cve_2026_109707,</group>
  </rule>

  <rule id="109708" level="10">
    <if_sid>61603</if_sid>
    <field name="win.eventdata.parentImage">firefox.exe$</field>
    <field name="win.eventdata.commandLine">(VirtualAlloc|CreateThread|WriteProcessMemory|VirtualProtect)</field>
    <description>CVE-2026-109707: Firefox child process referencing memory-injection APIs - possible exploit payload stage</description>
    <mitre>
      <id>T1055</id>
    </mitre>
    <group>cve_2026_109707,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence is 0/4 — UNPATCHED by all available evidence, with no patch source, advisory URL, changeset, or workaround recorded in the supplied data. The only remediation fact available is the NVD statement that the defect is fixed in Firefox 158 and Firefox ESR 153.5. Treat the gap between "NVD says fixed" and "no patch artifact available in tracked sources" as a reason to verify vendor binaries directly before closing remediation tickets.

1. Immediate mitigations

• Upgrade or force-update to Firefox 158 or later, and Firefox ESR 153.5 or later. Where the browser is centrally managed, push the update via enterprise policy and confirm the running version post-deployment rather than trusting update-available signals.
• Disable or strictly scope untrusted web access where the browser cannot be immediately updated; block unsanctioned browser installs so users cannot run stale copies.
• Enforce endpoint detection tuning from the Detection Guidance section to catch the second-stage behavior (browser-spawned shells), since the first stage may be unpatchable in the user's workflow.
• Reduce exploit-chain viability by enabling OS-level mitigations (CFG/CET where available, mitigations for browser processes) and keeping the EDR agent's browser-process coverage enabled.
• Restrict browser exposure to sensitive credentials (avoid "remember password" in unpatched profiles; require phishing-resistant MFA for web apps) so an escaped session yields less reusable material.

2. Patch / upgrade steps

• Inventory Firefox and Firefox ESR versions fleet-wide, including portable and per-user installs.
• Stage the update to a pilot ring, then the general fleet; verify version strings on disk and in the process inventory.
• Reboot/restart browser processes so the updated binary is in memory; a running pre-158 process remains vulnerable.
• Verify the patch by comparing observed versions against the fixed thresholds (158 / ESR 153.5) and by re-running the crash/behavioral detections to confirm older binaries are gone.

3. Long-term hardening

• Adopt a browser version-baseline control with automatic enforcement and weekly drift reporting.
• Segment critical workflows (admin consoles, VPN, finance) into dedicated managed browsers/VMs so a browser escape does not land in a high-value identity context.
• Maintain exploit-chain awareness: because this CVE is a sandbox-escape second stage, prioritize rapid patching for any simultaneously disclosed renderer/script vulnerabilities.
• Continuously monitor vendor security advisories and CISA KEV for updates; the absence of KEV listing and public PoC today is not a guarantee for the next release cycle.

References

• NVD — CVE-2026-109707: https://nvd.nist.gov/vuln/detail/CVE-2026-109707
• MITRE CWE-416 (Use After Free): https://cwe.mitre.org/data/definitions/416.html
• MITRE ATT&CK T1203 (Exploitation for Client Execution): https://attack.mitre.org/techniques/T1203/
• Mozilla Firefox Security Advisories (vendor advisories for Firefox 158 / Firefox ESR 153.5): https://www.mozilla.org/security/advisories/
• CISA Known Exploited Vulnerabilities Catalog (status check; not listed as of supplied data): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Exploit-in-the-wild evidence: none available in the supplied data. No public PoC, exploit code, Nuclei template, or threat-actor campaign is associated with CVE-2026-109707 in the provided intelligence, and no EPSS score has been assigned by FIRST.org. No CVSS score or vector was published; the vector shown in METADATA is an analytical placement for the chained-sandbox-escape class and is not a vendor-confirmed value.