CVE-2026-109757

Executive Summary

CVE-2026-109757 is a sandbox escape vulnerability in the Networking component of Mozilla Firefox, caused by a race condition (CWE-362). It affects Firefox versions prior to 158 and Firefox ESR versions prior to 153.5, and was fixed in Firefox 158 and Firefox ESR 153.5. Successful exploitation allows a compromised or malicious renderer to break out of the browser's content sandbox and reach the parent/privileged process, escalating from web-content-level compromise to host-level code execution.

Technical Analysis

The defining characteristics of this issue, per the available data, are: (1) the affected subsystem is the Networking component, (2) the root cause is a race condition, and (3) the security impact is a sandbox escape, not merely a memory-safety crash. A sandbox escape means that code already executing inside the Firefox content/renderer sandbox — for example, script or code that has achieved code execution via a separate memory-corruption or logic bug — can leverage this race to perform operations that should be confined to, or brokered by, a more privileged process. In Firefox's architecture, the Networking component operates at a trust boundary: the content process does not perform privileged I/O directly but communicates with the parent (browser) process, which owns networking, file access, and other privileged capabilities through inter-process communication (IPC). A race condition within that component implies two or more concurrent operations (for example, concurrent IPC requests, asynchronous network callbacks, or state transitions shared between processes) can be interleaved in a way the code does not expect. When a time-of-check-to-time-of-use (TOCTOU) style window is exploited, the attacker can cause the component to act on stale or unauthorized state, bypassing the access controls that normally isolate the content process from the privileged parent.

Because the sandbox boundary in Firefox is enforced through component/IPC design rather than a single check, a race in the networking layer is particularly significant: the networking subsystem holds file descriptors, sockets, and privileged broker connections. A race that lets a content-process request resolve against a privileged resource effectively grants the renderer capabilities it was never intended to hold, which is the essence of a sandbox escape. The vulnerability requires winning a timing window, which raises exploitation complexity (high AC in the CVSS vector) and typically requires user interaction to first reach a state where attacker-controlled content is executing (UI:R). Scope is changed (S:C) because the compromise crosses the privilege boundary from the sandboxed content process to the parent process.

No CVE record in the supplied data includes the specific function, class, or file involved, and no patch diff or vendor advisory was collected. Therefore, the exact interleaving, the specific IPC message pair, and the precise resource whose isolation can be bypassed must be treated as "insufficient technical detail available." Analysts should not attribute a specific root-cause function to this CVE without consulting Mozilla's official Firefox 158 / ESR 153.5 security advisories directly, which were not provided here. The absence of a public PoC, exploit code, or Nuclei template means there is currently no verified reproduction to anchor a more granular code-level analysis.

Exploitation Scenarios

Given no public PoC or exploit code, the following chain is inferred at the vulnerability-class level (race-condition sandbox escape) and should be validated against the official Mozilla advisory before operational use.

1. Reconnaissance / target identification. The attacker identifies a target running an affected Firefox build — Firefox < 158 or Firefox ESR < 153.5. Fingerprinting can be done remotely via JavaScript/WebGL feature detection or network fingerprinting of the browser's HTTP traffic (user agent, TLS and HTTP/2 characteristics). This vulnerability is not directly reachable by a remote unauthenticated scanning tool because it requires a renderer-compromise foothold; it is therefore a post-compromise escalation primitive rather than a standalone RCE.

2. Triggering the flaw class. The attacker must first obtain code execution or arbitrary IPC-request capability inside the content sandbox, typically by chaining a separate renderer bug (memory corruption or logic flaw). With that foothold, the attacker issues rapid, concurrent networking operations designed to interleave with the vulnerable check/use sequence in the Networking component. The core of exploitation is a TOCTOU: racing two operations so that a resource or state verified as "safe/owned by content" is replaced or reassigned before it is consumed by the privileged pathway. Because the race is timing-dependent, exploitation is probabilistic — the attacker loops the racing sequence until the window is hit, which favors local or fast-loopback scenarios and makes deterministic remote exploitation impractical (consistent with high attack complexity).

3. Execution method. At the class level, the attacker abuses the race to cause a privileged networking operation to act on attacker-influenced state, converting a sandboxed request into one processed by the parent process with elevated capabilities. No specific function names, message types, endpoints, or parameters can be cited because none are present in the supplied data; the exploited primitive is the race-induced trust-boundary crossing itself.

4. Post-exploitation impact. Crossing the sandbox boundary places the attacker in the parent (browser) process context, which owns the user's session, filesystem-adjacent APIs, and inter-process broker facilities. From there the attacker can read/modify files the user can access, install persistence, harvest credentials and session tokens, pivot to other local processes, or use the browser as a stepping stone into the local network. The realistic outcome is host-level code execution as the browser user — not necessarily administrative/root, but far beyond the intended confinement of a web page. Post-compromise, the attacker would likely deploy data-exfiltration and persistence tooling commensurate with a userland implant.

Impact Assessment

The primary impact is a full bypass of the Firefox content sandbox — a security boundary whose entire purpose is to contain the consequences of renderer compromise. Where a renderer bug alone would be a serious but contained issue, chaining it with CVE-2026-109757 converts it into a host-level compromise. The affected population is large: any environment running Firefox builds older than 158, including consumer endpoints and enterprise fleets on the ESR 153.x channel (below 153.5). ESR users are often the most exposed because ESR installations are deliberately slow to upgrade, giving attackers a longer exploitation window after a fix ships.

Blast radius includes confidentiality, integrity, and availability of the user context and anything the browser process can reach: corporate SaaS sessions, cached credentials, local files, and VPN/intranet reachable through the browser. A swapped CVSS vector assessment (CVSS not provided by the feed; an estimate placed in METADATA) would reflect network attack vector (AV:N), high attack complexity (AC:H) due to the race, user interaction required (UI:R) to first land renderer code, no privileges required (PR:N), scope change (S:C), and high C/I/A impact (C:H/I:H/A:H). No CWE is recorded, though the mechanism maps cleanly to CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization / TOCTOU). CISA KEV status is NO and the EPSS score is unassigned — no exploit-in-the-wild evidence and no FIRST.org prediction data exist for this CVE. These factors mean active exploitation is currently undemonstrated in public data, but the risk is elevated by the ongoing value of browser sandbox escapes to both commodity and targeted threat actors, typically deployed alongside exploit chains delivered via malicious web content or phishing.

Detection Guidance

Because the race is internal to the browser's IPC/network machinery, endpoint-side detection focuses on anomalous process behavior (renderer-to-parent IPC storms, credential/file access by the browser process), while network detection focuses on the delivery phase (exploit-chain staging, suspicious redirects, post-exploitation C2). The following rules are behavioral starting templates and require environment tuning; given no vendor IOCs exist for this CVE, precision will be low and must be paired with browser-process telemetry (telemetry.enabled, content-process crash/abnormal-exit monitoring) and host EDR.

Sigma Rule

title: Firefox Networking Sandbox Escape - Suspicious Browser Child Process Behavior
id: 4b9e2c17-7a3d-4f6e-9c21-2d8a5f1e0b44
status: experimental
description: Detects anomalous Firefox inter-process behavior consistent with a renderer attempting to cross the sandbox boundary via the Networking component (CVE-2026-109757 class, race condition / TOCTOU).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109757
author: valtersit.com Threat Intelligence
date: 2026/01/01
logsource:
  category: process_creation
  product: windows
detection:
  selection_browser:
    Image|endswith:
      - '\\firefox.exe'
      - '/firefox'
  selection_parent:
    ParentImage|endswith:
      - '\\firefox.exe'
      - '/firefox'
  selection_susp_child:
    Image|endswith:
      - '\\cmd.exe'
      - '\\powershell.exe'
      - '\\wscript.exe'
      - '\\cscript.exe'
      - '\\mshta.exe'
      - '\\rundll32.exe'
      - '/bin/sh'
      - '/bin/bash'
  condition: selection_browser and selection_parent and selection_susp_child
falsepositives:
  - Legitimate enterprise extensions or management tooling invoking shells
  - Debugger or development tooling attached to a Firefox build
level: high
tags:
  - attack.t1068
  - attack.t1190

YARA Rule

rule EXPLOIT_Firefox_NetSandboxRace_CVE_2026_109757
{
    meta:
        description = "Generic heuristic hunt for artifacts associated with a Firefox Networking-component sandbox escape (CVE-2026-109757). No public PoC or vendor signature exists; this is a low-confidence behavioral/hunting rule only."
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109757"
        author = "valtersit.com Threat Intelligence"
        date = "2026-01-01"
        severity = "high"
        confidence = "low"

    strings:
        $mz = { 4D 5A }
        $ff = "firefox" ascii wide nocase
        $moz = "Mozilla" ascii wide nocase
        $esr = "ESR 153" ascii wide nocase
        $esc1 = "sandbox escape" ascii wide nocase
        $esc2 = "networking" ascii wide nocase

    condition:
        $mz at 0 and 1 of ($ff, $moz) and 1 of ($esc1, $esc2, $esr)
}

Note: No reliable static signature can be authored for this vulnerability absent a patch diff or PoC; this rule is a hunting aid only and will produce false positives against legitimate Mozilla binaries.

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"EXPLOIT-POSSIBLE Firefox Content Compromise Follow-on - CVE-2026-109757 Post-Exploitation C2 Pattern"; flow:established,to_server; http.user_agent; content:"Firefox/"; pcre:"/Firefox\/(1[0-4][0-9]|15[0-7])\./"; threshold:type both, track by_src, count 5, seconds 60; classtype:attempted-user; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-109757; sid:2026109757; rev:1;)

Elastic Detection

process where process.parent.name == "firefox" and
  process.name in ("powershell.exe", "cmd.exe", "wscript.exe", "cscript.exe",
                   "mshta.exe", "rundll32.exe", "sh", "bash") and
  not process.parent.executable : ("*\\Program Files*", "*\\extensions\\*") and
  not process.args : ("*--marionette*", "*--debug*", "*--profile*")

Splunk SPL Query

index=endpoint (source="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" OR source="WinEventLog:Security")
(EventCode=1 OR EventCode=4688)
| where (match(lower(ParentImage), "firefox(\.exe)?$") OR match(lower(ParentImage), "\/firefox$"))
| where match(lower(Image), "(cmd|powershell|wscript|cscript|mshta|rundll32|sh|bash)(\.exe)?$")
| stats count min(_time) as first_seen max(_time) as last_seen values(CommandLine) as cmd by ComputerName, User, ParentImage, Image
| where count >= 1
| convert ctime(first_seen) ctime(last_seen)
| sort - count

Wazuh OSSEC Rule

<group name="windows,sysmon,exploitation,">
  <rule id="100757" level="12">
    <if_sid>61603</if_sid>
    <field name="win.eventdata.parentImage">firefox\.exe$</field>
    <field name="win.eventdata.image">(cmd|powershell|wscript|cscript|mshta|rundll32)\.exe$</field>
    <description>Possible Firefox sandbox escape follow-on: shell spawned by browser process (CVE-2026-109757 class)</description>
    <mitre>
      <id>T1068</id>
    </mitre>
    <group>attack_exploitation,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). Despite the NVD description naming the fixed versions, no patch reference, vendor advisory link, or diff was collected in the underlying data; treat patch status as unverified until Mozilla's official advisory is retrieved and matched.

1. Immediate mitigations.

• Upgrade Firefox to version 158 or later, and Firefox ESR to 153.5 or later, as stated in the NVD description. These are the versions in which the issue was fixed; if the organization cannot validate the fix yet, isolate affected hosts from sensitive resources as an interim control.
• Enforce Enterprise policy (policies.json / GPO) to restrict extension sources, block unsigned extensions, and keep the browser on the supported update channel with forced, rapid updates. Because exploitation requires an initial renderer-compromise foothold, reducing exposed attack surface (disable unnecessary plugins/features, restrict untrusted web access on high-value hosts) directly lowers the chance of reaching the escalation step.
• For high-value or high-risk users (executives, admins, finance), consider temporary non-Firefox browsing for untrusted web content until the fleet is confirmed on fixed builds, or route browsing through a disposable virtualized/container browser so a sandbox escape lands in a throwaway environment.
• Apply endpoint detection (rules above) with attention to shells or scripting engines spawned as children of Firefox, and to abnormal browser-process file/credential access.

2. Patch/upgrade steps.

• Identify all Firefox and Firefox ESR installations via software inventory (e.g., endpoint management, package manager: dnf/apt/winget/MSI inventory). Flag any build < 158 (release channel) or < 153.5 (ESR).
• Deploy Mozilla's official updates for those channels. Verify the resulting version on a sample of endpoints. Prioritize internet-exposed endpoints and users with access to sensitive data.
• Re-audit after upgrade: confirm no pinned or sideloaded older versions remain, and that ESR update policy is not blocked by local configuration.

3. Long-term hardening.

• Maintain a tracked browser-patching SLA: internet-facing browsers patched within days, not weeks. Browser sandbox escapes are routinely chained with renderer bugs, so keeping the browser current is the single most effective control.
• Harden the browser sandbox and IPC layer via Enterprise policies (site isolation / process isolation where applicable), reduce rendering of untrusted content on high-value hosts, and segregate browsing from privileged workloads.
• Deploy anti-exploitation controls at the OS level (e.g., Windows Defender Exploit Guard / ACG / CIG on browser processes) so a renderer or parent-process code-execution attempt has additional hurdles.
• Monitor for the exploit chain: content-process crashes/abnormal exits followed by elevated child-process activity, plus post-exploitation C2 from browser-tied processes. Because no PoC or in-the-wild evidence exists yet, rely on behavioral coverage and apply this CVE's fixes proactively.

References

• NVD — CVE-2026-109757: https://nvd.nist.gov/vuln/detail/CVE-2026-109757
• Mozilla Foundation Security Advisories (Firefox 158 / ESR 153.5 advisories — verify the specific advisory entry for this CVE; not collected in source data)
• Mozilla Firefox download/update page (release channel): https://www.mozilla.org/firefox/
• Mozilla Firefox ESR download/update page: https://www.mozilla.org/firefox/enterprise/
• CWE-362 — Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition'): https://cwe.mitre.org/data/definitions/362.html
• CISA Known Exploited Vulnerabilities Catalog (CVE-2026-109757 currently NOT listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Threat-actor note: No public PoC, exploit code, Nuclei template, or exploit-in-the-wild evidence exists for CVE-2026-109757 in the supplied data; no threat actor campaign is attributed. EPSS is unscored by FIRST.org and CISA KEV status is NO as of this report. Treat the above exploitation scenarios as vulnerability-class inference pending retrieval of Mozilla's official advisory and any proof-of-concept.