CVE-2026-109732

Executive Summary

CVE-2026-109732 is a use-after-free vulnerability located in the DOM Streams component of Mozilla Firefox. The flaw was resolved in Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5, meaning any Firefox user on a release prior to those versions is potentially affected. Because use-after-free conditions in a browser DOM engine can corrupt heap memory, successful exploitation may enable arbitrary code execution within the browser's sandboxed content process, making this a high-severity client-side threat.

Technical Analysis

The NVD description states this is a "Use-after-free in the DOM: Streams component." A use-after-free (CWE-416) occurs when an object is freed from memory but a reference to that memory location remains and is later dereferenced, allowing the program to read from or write to memory that has been reallocated for another purpose. In browser DOM implementations, the Streams API manages asynchronous data flows (readable, writable, and transform streams) that are exposed to JavaScript running in web content.

The attack surface for this class of bug is the DOM Streams API as reachable from a web page. Memory lifetime management of stream-related objects can fail to account for timing, garbage collection, or cross-context references, resulting in a dangling pointer. Neither the NVD Description nor the CNA Title supplies the specific function, class, or code path involved. As a result, this report confines its technical description to the vulnerability class and its DOM Streams attack surface; no internal symbol names, file paths, or API parameters are asserted because none were provided in the source data. Insufficient technical detail is available to reconstruct the exact memory-safety defect from the supplied description alone.

The relevant fix boundary is Mozilla's release train: Firefox 158 (stable), Firefox ESR 115.43, and Firefox ESR 153.5. ESR branches indicate that both the legacy 115 ESR line and the newer 153 ESR line received the backport, consistent with Mozilla's practice of patching supported ESR channels alongside the main release.

Exploitation Scenarios

No public proof-of-concept, exploit code, or Nuclei template was identified in the supplied data. The following chain is inferred at the vulnerability-class level and should be treated as hypothetical, not as a confirmed exploit.

1. Reconnaissance / target identification. An attacker identifies victims running Firefox versions earlier than 158, 115.43 ESR, or 153.5 ESR. This may be done via browser user-agent fingerprinting when a victim visits a controlled or compromised web page.

2. Triggering the flaw class. The attacker delivers a crafted web page that exercises the DOM Streams API. Because the precise trigger is not disclosed in the provided data, the exact construction cannot be specified; the general mechanism is a sequence of stream operations that induces the browser to free an object while a live reference to it persists.

3. Execution method at the vulnerability-class level. Dereferencing the freed memory after reallocation can produce memory corruption. In browser exploitation, an attacker typically seeks to convert a use-after-free into control-flow or data corruption to gain arbitrary code execution within the content process. No specific payload technique is described in the source data.

4. Post-exploitation impact. If code execution is achieved in the content process, the attacker would normally attempt to escape the browser sandbox to reach the host. Escalation beyond the sandbox is a separate, additional step not implied by this CVE description.

Impact Assessment

No CVSS score or vector was assigned in the supplied data (CVSS: N/A), so no subscores can be cited. The impact rating above is analyst-derived from the CWE-416 class and the DOM Streams attack surface and should not be treated as an official score. Business impact: use-after-free in a widely deployed browser affects any organization whose users browse the web with an unpatched Firefox build. Blast radius includes the endpoint user's session and, if a sandbox escape is chained, the host itself. Confidentiality, integrity, and availability of the endpoint and any data reachable from the browser context are all at risk. Because exploitation typically requires the victim to visit attacker-controlled content, this is a client-side, user-interaction-driven threat rather than a remotely reachable server vulnerability. CISA KEV status: NO. No evidence of in-the-wild exploitation was provided.

Detection Guidance

Sigma Rule

title: Firefox DOM Streams Use-After-Free Exploitation Attempt (CVE-2026-109732)
id: 7f3c1a92-4b6e-4d2f-9c1a-0a5e6d8b2f11
status: experimental
description: Detects potential client-side exploitation of a use-after-free in Firefox DOM Streams by correlating Firefox process crashes/terminations with preceding browser navigation to untrusted content.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109732
author: valtersit.com
date: 2026/01/01
tags:
  - attack.t1203
logsource:
  category: process_termination
  product: windows
detection:
  selection_firefox:
    Image|endswith:
      - '\firefox.exe'
      - '\firefox'
  selection_status:
    ExitCode|in:
      - 139        # segfault (Linux/POSIX)
      - 3221225477 # 0xC0000005 access violation (Windows)
      - 0xC0000005
  condition: selection_firefox and selection_status
falsepositives:
  - Legitimate browser crashes unrelated to exploitation
level: medium

YARA Rule

rule EXPLOIT_CVE_2026_109732_Firefox_DOMStreams_UAF
{
    meta:
        description = "Heuristic detection for Firefox DOM Streams use-after-free exploitation artifacts in memory or crash-adjacent content"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109732"
        cwe = "CWE-416"
        date = "2026-01-01"

    strings:
        $s1 = "ReadableStream" ascii wide nocase
        $s2 = "WritableStream" ascii wide nocase
        $s3 = "TransformStream" ascii wide nocase

    condition:
        any of ($s*) and filesize < 65536
}

Note: This is a heuristic rule that keys on DOM Streams API identifiers and is not a reliable indicator of exploitation on its own; it should only accompany crash correlation.

Suricata IDS Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-109732 Firefox DOM Streams UAF - Suspicious Stream API Content Delivery"; flow:established,to_server; http.header; content:"User-Agent"; nocase; content:"Firefox"; nocase; reference:cve,2026-109732; classtype:attempted-user; sid:2026109732; rev:1;)

Note: Because the trigger content is not specified in the available data, this rule can only flag the transit of stream-API-bearing content to Firefox clients and cannot reliably distinguish benign from malicious traffic. Tune before production use.

Elastic Detection

process where event.type == "end" and
  process.name : ("firefox", "firefox.exe") and
  process.exit_code : (139, 3221225477, 0xC0000005)

Splunk SPL Query

index=endpoint (process_name="firefox" OR process_name="firefox.exe")
(exit_code=139 OR exit_code=3221225477 OR exit_code="0xC0000005")
| stats count min(_time) as first_seen max(_time) as last_seen by host, user, exit_code
| where count > 2
| convert ctime(first_seen) ctime(last_seen)

Purpose: surface hosts with repeated Firefox access-violation crashes, which is a coarse proxy for potential client-side exploitation.

Wazuh OSSEC Rule

<group name="windows,sysmon,">
  <rule id="109732" level="10">
    <if_sid>61603</if_sid>
    <field name="win.eventdata.image">firefox.exe</field>
    <field name="win.eventdata.exitCode">3221225477|0xC0000005</field>
    <description>CVE-2026-109732 possible Firefox DOM Streams use-after-free exploitation (abnormal process termination)</description>
    <mitre>
      <id>T1203</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). Although the NVD description states the vulnerability "was fixed in Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5," no vendor advisory URL, commit reference, or downloadable fix was included in the supplied data; consequently no patch can be independently verified and the vendor fix field remains N/A.

1. Immediate mitigations:

   • Do not rely on the flaw being unexploited: no KEV status and no public PoC were reported, but use-after-free browser bugs are valuable and may be weaponized.
   • Where browser version control is not yet possible, reduce exposure by restricting untrusted web browsing through network policy and web content filtering.
   • Enable and monitor crash reporting so that spikes in Firefox content-process crashes are investigated promptly.
   • Harden the browser ecosystem with exploit mitigations (e.g., OS-level memory protection features) that make memory-corruption exploitation more difficult.

2. Patch/upgrade steps:

   • Verify the actual released version strings for Firefox 158, Firefox ESR 115.43, and Firefox ESR 153.5 against Mozilla's official release channels before deploying, since this data set could not confirm the advisories.
   • Upgrade all Firefox installs to Firefox 158 or the appropriate ESR channel (115.43 or 153.5) as applicable.
   • Enforce automatic updates and centrally audit endpoints for Firefox builds below the fixed thresholds.

3. Long-term hardening:

   • Maintain a patch management process that tracks both rapid-release and ESR channels.
   • Inventory browser versions across the fleet so that exposure windows are measurable.
   • Retain client-side EDR telemetry to detect crash-then-reexecution patterns consistent with exploitation attempts.

References

• NVD — CVE-2026-109732: https://nvd.nist.gov/vuln/detail/CVE-2026-109732
• Mozilla Foundation Security Advisories: https://www.mozilla.org/en-US/security/advisories/ (verify the specific advisory for Firefox 158 / ESR 115.43 / ESR 153.5)
• 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/
• CISA KEV Catalog (CVE-2026-109732 not listed as of the data provided): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No threat actor campaigns or exploit-in-the-wild evidence were identified in the supplied data.