CVE-2026-109706

Executive Summary

CVE-2026-109706 is a use-after-free (UAF) vulnerability in the Storage: IndexedDB component of Mozilla Firefox. It was fixed in Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5. An attacker who can cause a user to load crafted web content can potentially corrupt memory in the browser process, leading to arbitrary code execution or a crash, depending on the exploitable state of the freed object.

Technical Analysis

The vulnerability resides in the Storage: IndexedDB subsystem — the browser component responsible for the IndexedDB asynchronous client-side database API. IndexedDB operations are heavily asynchronous and involve coordination between JavaScript-visible API objects, internal transaction/database handles, and background storage threads.

A use-after-free arises when an object (for example, a transaction, cursor, request, or database connection handle) is freed while a reference to it is still retained and later dereferenced by another asynchronous callback or background task. In the IndexedDB model, this class of bug typically manifests when:

• A transaction, request, or database object is released due to scope teardown, page navigation, or explicit close, while a pending asynchronous completion callback or event still holds a stale pointer.
• Two cooperating asynchronous paths (e.g., an operation scheduled on a background storage thread and a main-thread callback) race such that the object is collected before all consumers have finished using it.

Because this is a UAF, the impact depends primarily on whether the freed memory is reallocated with attacker-influenced content before the stale reference is used. If so, the dereference can act on attacker-controlled data, enabling corruption of adjacent heap objects or control-flow relevant structures. If the freed chunk is not reclaimed in a controlled manner, the outcome is typically an exploitable crash (denial of service). A successful, reliably controlled UAF in a browser storage component is often a precursor to sandbox-defeating exploit chains, though the renderer/content process boundary and Firefox's site isolation and other hardening measures raise the bar significantly.

The NVD description does not disclose the exact function, class, or field involved; therefore, the precise object lifetime bug (for example, which internal handle is prematurely freed) cannot be determined from the available data. No CWE, CVSS vector, or EPSS score was provided by the sources; the CWE class (CWE-416, Use-After-Free) is inferred from the description's explicit wording.

Note: The CVE record for this identifier lists Vendor/Product as "Unknown / Unknown", but the CNA description names Firefox and Firefox ESR explicitly, so the vendor/product is attributed to Mozilla Firefox on the basis of that description. No affected package metadata or version range was supplied.

Exploitation Scenarios

No public proof-of-concept, exploit code, or Nuclei template exists for CVE-2026-109706, and no in-the-wild exploitation has been reported (CISA KEV: NO). The attack chain below is described at the vulnerability-class level and is inferred from the UAF description; no specific implementation details are asserted.

1. Reconnaissance / target identification. An adversary identifies users running an affected Firefox or Firefox ESR build (i.e., below the fixed versions 158, 115.43, or 153.5). Because browser exploitation is usually opportunistic rather than targeted by IP or hostname, this is typically a mass-market or watering-hole step: serving malicious content to any browser that satisfies the version predicate.

2. Triggering the flaw class. The attacker delivers crafted web content that drives the IndexedDB API into a state where an internal object is released while an asynchronous consumer still holds a reference. This is achieved through JavaScript that manipulates database, transaction, and request lifecycles — for example, opening, using, and tearing down IndexedDB connections/transactions in a pattern that exposes the stale reference. The precise triggering sequence is not documented publicly.

3. Payload / execution at class level. Following the free, the attacker attempts to reallocate the freed memory region with controlled content (a classic heap-spray/groom approach used against UAF bugs) so that the dangling reference resolves to attacker-influenced data. A successful reclaim can subvert a function pointer or object method invocation, redirecting execution. If grooming fails, the result is a crash (denial of service).

4. Post-exploitation impact. Successful code execution would run in the context of the browsing component. Whether this yields full system compromise depends on additional sandbox-escape primitives; the NVD data does not establish that escape. At minimum, this is a memory-safety bug whose impact is arbitrary code execution or denial of service, with the reliability dictated by the attacker's ability to control heap layout.

Impact Assessment

The impact of CVE-2026-109706 is consistent with a memory-corruption vulnerability in a widely deployed browser: potential arbitrary code execution in the content/renderer process or, failing controlled corruption, browser crashes (availability impact). The blast radius is large, since Firefox and Firefox ESR are installed on many enterprise and consumer endpoints and ESR is common in managed environments. Browser exploits of this class are frequently the initial-access or user-execution vector in multi-stage intrusions, but exploitation would require additional primitives to escape the content-process sandbox for full host compromise.

No CVSS score or vector was supplied by the sources (CVSS: N/A), so no numeric impact rating can be stated; the vector field above is a standard CWE-416-appropriate vector offered only as context, not a vendor-assigned score. EPSS has not been assigned by FIRST.org, and the CVE is not listed in CISA KEV — indicating no confirmed exploitation at the time of writing.

Detection Guidance

Because this is an in-browser memory-safety bug, reliable network detection is limited; the strongest signals are process-level crash telemetry and host/EDR monitoring rather than payload inspection on the wire. Rules below are representative detection patterns and should be tuned to the environment.

Sigma Rule

title: Firefox IndexedDB Use-After-Free Crash (CVE-2026-109706) - Suspicious Child Process
id: 7e3a1c2f-4b9d-4e21-9a10-8f1c2d3e4a5b
status: experimental
description: Detects Firefox crashing and spawning unexpected child processes, indicative of memory corruption exploitation attempts against Firefox IndexedDB (CVE-2026-109706).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109706
author: valtersit.com
logsource:
  category: process_creation
  product: windows
detection:
  selection_parent:
    ParentImage|endswith:
      - '\firefox.exe'
  selection_child:
    Image|endswith:
      - '\cmd.exe'
      - '\powershell.exe'
      - '\wscript.exe'
      - '\cscript.exe'
      - '\mshta.exe'
      - '\rundll32.exe'
      - '\regsvr32.exe'
  condition: selection_parent and selection_child
falsepositives:
  - Legitimate Firefox-launched helper applications
  - Enterprise extensions launching local tools
level: high

YARA Rule

rule EXPLOIT_CVE_2026_109706_IndexedDB_UAF
{
    meta:
        description = "Detects crash-dump / memory strings consistent with Firefox IndexedDB use-after-free abuse (CVE-2026-109706)"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109706"
        date = "2026-01-01"
        tlp = "WHITE"

    strings:
        $mz = { 4D 5A }
        $ff  = "firefox" ascii wide nocase
        $idb = "IndexedDB" ascii wide
        $idb2 = "indexedDB" ascii
        $uaf = "use-after-free" ascii nocase
        $ipc = "dom::indexedDB" ascii wide nocase

    condition:
        $mz at 0 and filesize < 50MB and
        ( $ff or $idb or $idb2 or $uaf or $ipc )
}

Suricata IDS Rule

alert http any any -> any any (msg:"WEB-CLIENT Mozilla Firefox IndexedDB use-after-free attempt (CVE-2026-109706)"; flow:established,to_client; http.user_agent; content:"Firefox/"; http.response_body; content:"indexedDB"; nocase; http.response_body; content:"indexeddb.open"; nocase; threshold: type limit, track by_src, count 1, seconds 60; classtype:attempted-user; reference:cve,2026-109706; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-109706; sid:2026109706; rev:1;)

Note: this signature only flags network-observable indicators (Firefox user-agent plus IndexedDB script delivered in the response body). Because the vulnerability is triggered in-process, absence of an alert does not imply absence of exploitation.

Elastic Detection

process where event.type == "start" and
  process.parent.name == "firefox.exe" and
  process.name in ("cmd.exe", "powershell.exe", "wscript.exe", "cscript.exe", "mshta.exe", "rundll32.exe", "regsvr32.exe")

Splunk SPL Query

index=sysmon EventCode=1
| where match(ParentImage, "(?i)firefox\\.exe$")
| where match(Image, "(?i)(cmd|powershell|wscript|cscript|mshta|rundll32|regsvr32)\\.exe$")
| stats count values(Image) as child_procs values(CommandLine) as cmdlines by Computer, ParentImage, User
| sort - count

Wazuh OSSEC Rule

<group name="windows,sysmon,firefox,">
  <rule id="2026101" level="12">
    <if_sid>61603</if_sid>
    <field name="win.eventdata.parentImage">firefox\.exe$</field>
    <field name="win.eventdata.image">(cmd|powershell|wscript|cscript|mshta|rundll32|regsvr32)\.exe$</field>
    <description>Firefox spawned a suspicious child process - possible browser memory-corruption exploitation (CVE-2026-109706)</description>
    <mitre>
      <id>T1203</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source).

1. Immediate mitigations.

   • Restrict or block untrusted web content on high-value systems where feasible; use managed allow-listing for critical endpoints.
   • Harden the browser: enable/retain site isolation, keep OS-level exploit mitigations (ASLR, DEP, CFG/CET where available) enabled, and enforce EDR memory-protection features.
   • Monitor for Firefox crashes and unexpected child processes (see detection section); treat repeated IndexedDB-related crashes as a potential exploitation signal.
   • Because IndexedDB is a normal web platform API, disabling it broadly is not a practical mitigation and may break legitimate sites.

2. Patch / upgrade steps.

   • Although the NVD description states the issue is fixed in Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5, no vendor advisory or patch source is present in the tracked sources; treat these as the stated fixed versions and validate against the official Mozilla advisory before relying on them.
   • Upgrade all affected Firefox and Firefox ESR installations to the versions at or above those named, or later. Managed environments using ESR should prioritize update channels and pin to the fixed ESR branches.
   • Verify the deployed version after updating (e.g., via about: or the vendor's supported means), and ensure auto-update is functioning.

3. Long-term hardening.

   • Maintain a rapid browser patch cadence for both standard and ESR channels; browser memory-safety bugs are routinely weaponized in exploit kits and targeted campaigns.
   • Layer network and endpoint detections (as above) and retain process/crash telemetry for retroactive hunting.
   • Consider reducing reliance on a single browser version and segment critical workloads.

References

• NVD — CVE-2026-109706: https://nvd.nist.gov/vuln/detail/CVE-2026-109706
• Mozilla Foundation Security Advisories (consult the advisory corresponding to Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5): https://www.mozilla.org/en-US/security/advisories/
• MITRE ATT&CK T1203 — Exploitation for Client Execution: https://attack.mitre.org/techniques/T1203/
• CWE-416 — Use After Free: https://cwe.mitre.org/data/definitions/416.html
• CISA Known Exploited Vulnerabilities Catalog (verify current listing status): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No known threat actor campaigns or exploit-in-the-wild evidence were identified for CVE-2026-109706 in the provided data; the CVE is not listed in CISA KEV, no public PoC or exploit code is available, and EPSS has not been scored by FIRST.org.