CVE-2026-109738

Executive Summary

CVE-2026-109738 is a use-after-free (UAF) memory-safety vulnerability residing in the Audio/Video: Playback component of Mozilla Firefox. According to the NVD description, the flaw was fixed in Firefox 158 and Firefox ESR 153.5, meaning any Firefox or Firefox ESR installation below those versions is potentially affected. Successful exploitation can corrupt heap memory and, in the browser threat model, may lead to renderer compromise; however, no public proof-of-concept, exploit code, Nuclei template, or CISA KEV entry exists, and no CVSS or EPSS score has been assigned, so the practical in-the-wild risk is currently indeterminate.

Technical Analysis

The vulnerability is classified as a use-after-free (CWE-416), a memory-safety defect in which a program continues to reference a heap object after that object has been freed. In the context of a browser media pipeline, UAF conditions typically arise when object lifetimes are mismanaged across asynchronous operations — for example, when a media element, decoder, or playback resource is torn down while another part of the pipeline still holds a pointer to it. Firefox's Audio/Video: Playback component handles the parsing, buffering, decoding, and rendering of media streams, and this subsystem has historically been a source of memory-safety issues because it processes complex, attacker-influenced input asynchronously and across multiple threads.

Based strictly on the supplied NVD description, the following is confirmed: (1) the defect is a use-after-free, (2) it lives in the Audio/Video: Playback component, and (3) it was fixed in Firefox 158 and Firefox ESR 153.5. No function names, class names, source file paths, or API parameters are provided in the available data, and no CWE mapping or CVSS vector was supplied by the source feed. Consequently, the precise trigger conditions, the specific object whose lifetime is mismanaged, and the exact code path that frees and then re-references memory cannot be determined from the available intelligence. What can be stated at the vulnerability-class level is that a dangling pointer to freed memory is dereferenced during playback handling, creating a window in which an attacker who can influence media content or playback state may redirect execution or read/write memory the program no longer owns.

Use-after-free bugs in media playback are typically reachable from web content because the browser loads and plays attacker-supplied media. Handling of malformed or rapid-sequence media events (loading, seeking, switching sources, teardown during playback) is the usual surface where object lifetimes desynchronize. Because the supplied data contains no code-level specifics, this report deliberately avoids attributing the flaw to any particular function or routine; such details are simply not derivable from the description given.

Exploitation Scenarios

No public PoC is available; the attack chain below is inferred from the vulnerability class (use-after-free in a browser media subsystem) and the affected component named in the NVD description.

1. Reconnaissance / target identification. An attacker identifies users running Firefox or Firefox ESR versions below the fixed releases (Firefox < 158, Firefox ESR < 153.5). Delivery is typically opportunistic: any web page, ad, embedded iframe, or other web content the victim loads can exercise the browser's media playback path. Adversaries may fingerprint the browser version via the User-Agent string or feature behavior to avoid triggering on patched clients.

2. Triggering the flaw (vulnerability class). The attacker causes the browser to process media content that drives the Audio/Video: Playback component into a state where an object is freed while a live reference to it remains. In use-after-free media bugs generally, this involves manipulating playback lifecycle transitions (starting, interrupting, navigating away, or rapidly changing media sources) so that cleanup/free logic races or desynchronizes with code still using the object. The exact sequence that reliably triggers CVE-2026-109738 is not specified in the available data; no specific function, event, or parameter names can be asserted.

3. Payload / execution method (class-level). Following the free, the dangling reference is dereferenced. At the vulnerability-class level, an attacker who can influence the contents of the freed allocation may attempt heap grooming so that attacker-controlled data occupies the freed slot, potentially redirecting a pointer or corrupting adjacent structures. No specific exploitation primitive, gadget, or technique is documented for this CVE, and none is invented here.

4. Post-exploitation impact. A successfully weaponized UAF in a browser renderer can lead to memory corruption with the potential for arbitrary code execution within the browser's sandboxed renderer process. Realizing full system compromise would additionally require escaping the browser sandbox, which is a separate exploitation problem and not implied by this CVE alone.

It must be emphasized that no in-the-wild exploitation, public exploit, or PoC is known from the supplied data. The above is a class-based reconstruction, not a validated exploit chain.

Impact Assessment

The impact assessment is constrained by the absence of an assigned CVSS score. The source feed provides no CVSS base score or vector, no EPSS probability, and no CWE mapping. No CVSS subscores can therefore be referenced meaningfully; any numeric severity would be fabricated and is omitted.

Qualitatively, a use-after-free in a browser's media playback subsystem carries potentially high severity because: (a) the attack surface is broad — media playback is exercised by ordinary web browsing, so no special privileges or authentication are required of the victim; (b) memory corruption in the renderer can enable code execution in the renderer process; and (c) the affected products (Firefox and Firefox ESR) are widely deployed, with ESR in particular used in managed enterprise and government environments where patch cadence may lag.

Blast radius is limited to the individual user session and the browser process unless a sandbox escape is chained. No server-side or network-infrastructure component is affected — this is a client-side browser vulnerability. Enterprise exposure is concentrated in endpoints running outdated Firefox or Firefox ESR builds. Because no CVSS/EPSS signal exists and no exploitation has been observed, the current operational priority should be driven by asset inventory and patch status rather than by active-threat indicators.

Detection Guidance

The following rules are behavioral and heuristic detections targeting the vulnerability class (browser media playback memory corruption / crashes) rather than a known exploit signature, because no IOC, PoC, or exploit artifact is available. Analysts should tune thresholds to their environment and correlate with endpoint crash telemetry.

Sigma Rule

title: Potential Firefox Media Playback Use-After-Free Exploitation Attempt
id: 3f9c1a72-0000-4a10-9d0e-cve2026109738
status: experimental
description: Detects Firefox child/content process crashes or abnormal terminations potentially associated with a use-after-free in the Audio/Video Playback component (CVE-2026-109738).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-109738
author: valtersit.com
date: 2026/01/01
logsource:
  category: process_creation
  product: windows
detection:
  selection_image:
    Image|endswith:
      - '\firefox.exe'
  selection_crash:
    CommandLine|contains:
      - 'crashreporter'
      - 'MOZ_CRASH'
  condition: selection_image and selection_crash
falsepositives:
  - Legitimate Firefox crash reporter invocations after genuine crashes
level: medium
tags:
  - attack.t1190
  - cve.2026.109738

YARA Rule

rule CVE_2026_109738_Firefox_Media_UAF
{
    meta:
        description = "Heuristic detection of Firefox media playback use-after-free indicators (CVE-2026-109738). Behavioral/contextual - no known exploit sample."
        author = "valtersit.com"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-109738"
        severity = "medium"
    strings:
        $ff = "firefox" ascii nocase
        $esr = "esr" ascii nocase
        $media = "Audio/Video: Playback" ascii
        $uaf = "use-after-free" ascii nocase
    condition:
        uint16(0) == 0x5A4D and $ff and ($media or $uaf)
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-109738 Firefox Media Playback UAF - Suspicious Firefox UA to media content"; flow:established,to_client; http.user_agent; content:"Firefox/"; nocase; http.header; content:"Accept"; content:"video"; nocase; threshold: type limit, track by_src, count 1, seconds 60; classtype:attempted-user; reference:cve,2026-109738; sid:2026109738; rev:1;)

Elastic Detection

process where event.type == "start" and
  process.name : "firefox.exe" and
  process.command_line : ("*crashreporter*", "*MOZ_CRASH*")

Splunk SPL Query

index=endpoint (process_name="firefox.exe" OR process_name="firefox")
| search (CommandLine="*crashreporter*" OR CommandLine="*MOZ_CRASH*" OR EventID=1000)
| stats count min(_time) as first_seen max(_time) as last_seen by host, user, process_name, CommandLine
| where count > 0
| sort - count

Wazuh OSSEC Rule

<group name="firefox,cve-2026-109738,">
  <rule id="100738" level="6">
    <if_sid>5920</if_sid>
    <field name="win.eventdata.image">firefox.exe</field>
    <match>crashreporter|MOZ_CRASH</match>
    <description>CVE-2026-109738: Possible Firefox media playback use-after-free crash/exploitation indicator</description>
    <mitre>
      <id>T1190</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No patch reference, commit hash, vendor advisory URL, or workaround was provided in the supplied intelligence. The only remediation fact available is that the vulnerability "was fixed in Firefox 158 and Firefox ESR 153.5" per the NVD description.

1. Immediate mitigations. Given no vendor advisory was supplied, treat the fixed versions as the boundary and prioritize upgrade over workaround. In the interim: (a) inventory endpoints running Firefox and Firefox ESR and identify any build below 158 / 153.5 respectively; (b) where possible, restrict or disable untrusted rich media/plugin content via enterprise policy and reduce exposure from untrusted sites; (c) ensure browser crash reporting and endpoint EDR telemetry are enabled so that any exploitation attempts (which would typically manifest as renderer crashes) are captured; (d) monitor Mozilla's security advisories directly for the official advisory accompanying the 158 / 153.5 releases, which the source feed did not include.

2. Patch / upgrade steps. Upgrade Firefox to version 158 or later and Firefox ESR to version 153.5 or later. For managed fleets, deploy via the enterprise update mechanism (e.g., Firefox ESR MSI/enterprise deployment, package manager, or MDM). Verify build versions post-deployment. Because no patch artifact, source commit, or advisory link is available in the supplied data, the exact release notes should be confirmed against Mozilla's official advisory channel before closing the remediation ticket.

3. Long-term hardening. Maintain a rapid browser-update cadence and an accurate software inventory of both standard Firefox and ESR channels. Consider application allow-listing, network egress filtering, and sandbox integrity controls (e.g., OS-level exploit mitigations such as ASLR/DEP/CFG) to reduce the impact of any renderer memory-corruption class. Adopt a standing process to ingest vendor browser advisories into vulnerability management, since media-subsystem UAFs recur across browser release cycles.

References

1. NVD — CVE-2026-109738: https://nvd.nist.gov/vuln/detail/CVE-2026-109738
2. Mozilla Foundation Security Advisories (Firefox / Firefox ESR): https://www.mozilla.org/en-US/security/advisories/
3. Mozilla Security Advisories for Developers / release notes for Firefox 158 and Firefox ESR 153.5 (confirm official advisory): https://www.mozilla.org/en-US/firefox/releases/
4. CWE-416 — Use After Free (MITRE): https://cwe.mitre.org/data/definitions/416.html
5. CISA Known Exploited Vulnerabilities Catalog (this CVE is NOT listed as of the supplied data): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No known threat actor campaigns or exploit-in-the-wild evidence were identified for CVE-2026-109738 in the supplied intelligence. No public PoC, exploit code, or Nuclei template exists per the provided data, and the CVE is not in CISA KEV. No EPSS score has been assigned by FIRST.org.