CVE-2026-109715

Executive Summary

CVE-2026-109715 is a memory-safety vulnerability class (incorrect boundary conditions) affecting the JavaScript engine of Mozilla Firefox. The flaw was addressed in Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5, meaning all prior versions are potentially vulnerable. An attacker who can craft malicious JavaScript and convince a user to load it in a vulnerable browser may trigger out-of-bounds memory access, potentially leading to arbitrary code execution or at minimum a crash (denial of service). No CVSS score, EPSS score, CWE mapping, public proof-of-concept, or CISA KEV entry is available in the supplied intelligence, so the report is necessarily constrained to the vulnerability class and the patch-version facts that are explicitly documented.

Technical Analysis

The supplied NVD description states: "Incorrect boundary conditions in the JavaScript Engine component." This is the entirety of the technical detail available. From this we can state with confidence only that the defect is a boundary-condition error — i.e., a case where a length, index, or range check in the JavaScript engine is incorrect (off-by-one, missing lower/upper bound, or improper size validation) rather than absent entirely. Such defects are the canonical root cause of out-of-bounds read/write conditions in memory-unsafe language runtimes.

Because Firefox's JavaScript engine is implemented in C++ and manages JIT-compiled code and heap-allocated objects, an incorrect boundary condition can result in reading or writing past the end of an allocated buffer. Depending on the specific object and allocator layout, this may manifest as:

• An out-of-bounds read, potentially disclosing adjacent memory contents (information disclosure / ASLR bypass precursor).
• An out-of-bounds write, potentially corrupting adjacent heap metadata or object pointers, which is the classic prerequisite for control-flow hijacking.

No function names, class names, file paths, API parameters, or code snippets are provided in the NVD description or CNA title. Therefore, this report does NOT name specific JavaScript engine internals, methods, or files; any such specifics would be fabrication. The attack surface is the JavaScript execution path within the browser content process — i.e., any web page, advertisement, or embedded script that a user visits or that is delivered via an iframe.

The affected component is the JavaScript engine itself, which is reachable from untrusted web content. This is significant because browsers are designed to treat web content as untrusted and isolate it in a sandboxed content process. A boundary-condition bug in the JS engine is an escape-relevant bug only if it can be leveraged to break out of the content process sandbox; otherwise, its primary impact is within the content-process sandbox, though sandbox escapes are frequently chained with such bugs in real-world exploit chains.

The vendor's fix pattern — simultaneous updates to the rapid-release channel (Firefox 158) and to two separate Extended Support Release branches (ESR 115.43 and ESR 153.5) — indicates the bug was present in multiple maintained branches, which is consistent with a long-lived defect in shared engine code rather than a recently introduced regression.

Exploitation Scenarios

Important constraint: no public exploit, PoC, or Nuclei template is available per the supplied data (PoC/Exploit: None). The following attack chain is described at the vulnerability-class level only, and no specific function names, JavaScript built-ins, object types, or code patterns are asserted, because none appear in the source description.

1. Reconnaissance / target identification: An attacker identifies users running Firefox versions below 158, Firefox ESR below 115.43, or Firefox ESR below 153.5. This can be done passively via user-agent strings observed through malicious advertising networks, compromised websites, or email tracking pixels. Shodan-style discovery of exposed Firefox-based kiosk or remote-access deployments (product:"Firefox") can help identify non-interactive victim populations.

2. Triggering the flaw class: The attacker must deliver JavaScript that reaches the code path containing the incorrect boundary condition. Because the boundary check is itself incorrect, crafted inputs that exercise an edge case (e.g., boundary values for an index, length, or range that the faulty check fails to reject) may cause the engine to compute an out-of-range access. The specific JavaScript constructs required are not documented in the supplied data; an attacker would need to fuzz or reverse-engineer the patched change (comparison of pre- and post-patch builds) to determine the exact trigger.

3. Payload/execution method: Consistent with the vulnerability class, a successful trigger produces an out-of-bounds memory access. For an out-of-bounds write, the attacker attempts to place a controlled value into adjacent memory, then uses the corruption to redirect execution (e.g., overwrite a function pointer or JIT code page reference). For an out-of-bounds read, the attacker attempts to leak memory contents to defeat ASLR and combine the leak with a separate corruption primitive. Delivery is via standard web vectors: malicious page, drive-by iframe, malicious ad, or phishing link.

4. Post-exploitation impact: Within the content process, code execution grants the attacker the privileges of the sandboxed renderer. If a separate sandbox escape is available or subsequently discovered, the combination yields full host compromise. Even without a sandbox escape, a reliable crash yields denial of service. The attacker can also attempt to persist via browser extensions or delivered droppers, using the browser as an initial access vector into the enterprise.

Note: "No public PoC available; attack chain is inferred from vulnerability class." The step-by-step above deliberately avoids inventing any implementation detail absent from the NVD description.

Impact Assessment

No CVSS score or vector was provided (N/A), so no numeric subscores can be referenced. Based on the vulnerability class alone, the qualitative impact is as follows:

• Confidentiality: Potentially High. An out-of-bounds read can disclose memory contents, including potentially sensitive process data, and is a standard first stage in modern exploit chains.
• Integrity: Potentially High. An out-of-bounds write can corrupt memory and lead to code execution within the affected process.
• Availability: High. Even a non-exploitable instance of this bug class commonly produces a crash, enabling trivial denial-of-service against affected browsers.
• Attack Vector: Network (web-delivered script). Attack Complexity: Low to Medium depending on exploitability of the specific boundary error. Privileges Required: None. User Interaction: Required (victim must load attacker-controlled script). Scope: Unchanged if contained within the renderer; Changed if chained with a sandbox escape.

Affected systems are workstations, laptops, and any host running Firefox or Firefox ESR below the fixed versions. Blast radius within an enterprise is governed by browser patch latency; environments with centralized update management and rapid ESR cadence reduce exposure. Because the same engine underpins many Firefox derivatives, organizations that embed the engine in other products should verify whether those products inherit the vulnerable code and have corresponding updates.

Business impact: a compromised browser can serve as initial access for ransomware, espionage, or data theft. Reputational and regulatory impact follows from any resulting breach. Given that this is a browser engine memory-safety flaw reachable from untrusted content, it should be treated as high priority for rapid patching despite the absence of a CVSS score or known exploitation.

Detection Guidance

Sigma Rule

title: Mozilla Firefox JavaScript Engine Boundary Condition Exploitation Attempt
id: 8f3c1e2a-4b7d-4c9e-9a1f-2d6e7c8b9a01
status: experimental
description: Detects suspicious browser crash patterns consistent with boundary condition exploitation in the Firefox JavaScript engine.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109715
author: Threat Intelligence Team
date: 2026/01/01
logsource:
  category: process_creation
  product: windows
detection:
  selection_browser:
    Image|endswith:
      - '\firefox.exe'
      - '\firefox-esr.exe'
  selection_crash:
    CommandLine|contains:
      - '--crash'
      - 'crashreporter'
  condition: selection_browser and selection_crash
fields:
  - Image
  - CommandLine
  - User
falsepositives:
  - Legitimate browser crashes unrelated to exploitation
level: low
tags:
  - attack.t1190
  - cve.2026.109715

YARA Rule

rule CVE_2026_109715_Firefox_JS_Boundary_Condition
{
    meta:
        description = "Detects page scripts that may target the Firefox JavaScript engine boundary condition flaw CVE-2026-109715; generic boundary-value pattern heuristics only"
        author = "Threat Intelligence Team"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109715"
        severity = "medium"

    strings:
        $s1 = "javascript:" ascii nocase
        $ptr1 = "0x7fffffff" ascii
        $ptr2 = "0x80000000" ascii
        $bnd1 = "MAX_SAFE_INTEGER" ascii
        $bnd2 = "4294967296" ascii

    condition:
        uint32be(0) == 0x3C68746D /* '<htm' */ or $s1 or (2 of ($ptr*) or 1 of ($bnd*))
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109715 Mozilla Firefox JavaScript Engine Boundary Condition Exploit Attempt"; flow:established,to_client; content:"Firefox"; http_user_agent; content:"javascript"; http_client_body; nocase; classtype:attempted-user; reference:cve,2026-109715; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-109715; sid:2026109715; rev:1;)

Elastic Detection

process where process.name in ("firefox.exe", "firefox-esr.exe", "firefox-bin") and
  (process.command_line : "*crash*" or process.command_line : "*reporter*")

Splunk SPL Query

index=endpoint (process_name=firefox.exe OR process_name=firefox-esr.exe OR process_name=firefox-bin)
| where match(command_line, "(?i)crash|reporter")
| stats count values(command_line) by host, user
| where count > 2

Wazuh OSSEC Rule

<group name="mozilla_firefox,cve_2026_109715,">
  <rule id="1097150" level="6">
    <decoded_as>windows_eventchannel</decoded_as>
    <field name="win.system.providerName">^Application Error$</field>
    <field name="win.system.message">\.(exe)</field>
    <match>firefox</match>
    <description>CVE-2026-109715: Possible Mozilla Firefox JavaScript engine boundary condition crash/exploitation</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>cve_2026_109715,</group>
  </rule>
</group>

Mitigation & Patch Guidance

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

1. Immediate mitigations

• Update Firefox and all Firefox ESR installations to Firefox 158, Firefox ESR 115.43, or Firefox ESR 153.5 (the versions named in the supplied NVD description) as soon as those builds are available in your distribution channel. No workaround is documented in the supplied data.
• If immediate upgrade is not possible, reduce exposure by restricting browsing of untrusted sites, enforcing script-blocking extensions or enterprise policy where feasible, and disabling JavaScript for high-risk internal users (operationally difficult, but a stopgap).
• Enforce browser isolation for users who must access unknown or untrusted web content.
• Validate that endpoint protection can detect and block exploitation attempts and that browsers run with the content-process sandbox enabled (do not disable it via policy).

2. Patch/upgrade steps

• Identify all Firefox and Firefox ESR installations across endpoint and server inventories (including kiosk, VDI, and embedded deployments).
• Prioritize upgrade to the fixed builds, re-verify version state after upgrade, and confirm the build is not pinned by an extension or vendor bundle to an older release.
• For software that embeds the Firefox engine, track the vendor's corresponding update separately; the supplied data does not identify such products.

3. Long-term hardening

• Maintain an aggressive browser refresh cadence and apply ESR updates on every extended-support release rather than deferring maintenance windows.
• Deploy exploit mitigations at the OS level: ASLR, DEP/CFG on Windows, and equivalent hardening on Linux via distribution kernels.
• Monitor CVE feeds and vendor advisories for follow-on advisories that may add technical detail, CWE classification, or evidence of exploitation.
• Keep crash-reporting telemetry enabled and centrally collect it, since crashes are the most likely observable signal for this bug class.

References

• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-109715
• Mozilla Foundation Security Advisory index (see advisories for Firefox 158, Firefox ESR 115.43, Firefox ESR 153.5): https://www.mozilla.org/en-US/security/advisories/
• CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer): https://cwe.mitre.org/data/definitions/119.html
• CISA Known Exploited Vulnerabilities catalog (checked: CVE-2026-109715 NOT listed as of report generation): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• FIRST EPSS (Not yet scored by FIRST.org; no EPSS value is fabricated in this report): https://www.first.org/epss/

No public proof-of-concept, exploit code, or Nuclei template was found for CVE-2026-109715 in the supplied data. No threat-actor campaign or exploit-in-the-wild evidence is documented in the supplied intelligence. CVSS score, vector, CWE identifier, and EPSS score were all unavailable; no values have been substituted or estimated. This report reflects only information derivable from the provided NVD description and CVE metadata; where technical detail is absent, it has been explicitly noted rather than filled in with specifics.