CVE-2026-43719

Executive Summary

CVE-2026-43719 is a use-after-free (CWE-416) vulnerability in Apple macOS that is triggered when the operating system mounts a maliciously crafted SMB network share. A remote, unauthenticated attacker who can influence the contents of an SMB share (or operate a rogue/malicious SMB server) may induce the client to reference freed memory, resulting in system termination (kernel/system crash) — a denial-of-service condition. All users of affected macOS versions who mount SMB shares are exposed, and exploitation requires user interaction (the victim must mount the crafted share). The issue is rated CVSS 3.1 base 6.5 (Medium), with the primary impact being Availability:High and no demonstrated confidentiality or integrity impact.

Technical Analysis

The vulnerability is classified as CWE-416: Use After Free. Per the NVD description, Apple addressed the issue "with improved memory management," which is the standard Apple language for memory-lifetime defects in system components that consume untrusted network input. The attack surface is the macOS SMB client stack: when a user mounts a network share over the SMB protocol, the client parses server-supplied metadata and structures. A malformed or maliciously crafted share causes the client to access a memory region after it has been freed, which — given the CVSS impact vector (C:N/I:N/A:H, S:U) — culminates in a crash that terminates the system rather than in controlled code execution.

Because the description is intentionally sparse (as is typical for Apple security advisories prior to independent research), no specific function names, symbols, file paths, or protocol fields are disclosed in the source data. The mechanism should therefore be understood at the class level:

• Trigger surface: SMB protocol negotiation and share-mount handling in the macOS client.
• Defect class: use-after-free — a heap or object is released while a subsequent code path still holds and dereferences a pointer to it.
• Input vector: attacker-controlled data delivered via a maliciously crafted SMB share (e.g., a rogue SMB server or a share whose contents/attributes are crafted).
• Resultant impact: memory corruption leading to a crash and system termination.

The CVSS environmental characteristics are notable: AV:N (network reachable), AC:L (no special conditions beyond a crafted share), PR:N (no privileges required on the attacker side), UI:R (the victim must mount the share), S:U (impact confined to the vulnerable component), and A:H (total loss of availability). The absence of C/I impact is consistent with a clean crash primitive rather than a memory-write exploit.

Exploitation Scenarios

Note: No public proof-of-concept, exploit code, or Nuclei template is available for this CVE. The attack chain below is described at the vulnerability-class level, consistent with the limited technical details disclosed. No specific function names or code patterns are claimed.

1. Reconnaissance / target identification. An attacker identifies macOS hosts that mount SMB shares — for example, corporate file servers referenced by managed clients, or users who routinely connect to network shares. An attacker can also position themselves as the SMB endpoint (rogue server, compromised NAS, MITM on the path to a legitimate server) to control the data the client receives.

2. Triggering the flaw. The attacker presents a maliciously crafted SMB network share. When the victim mounts it (the required user-interaction step), the macOS SMB client processes attacker-controlled share data and the use-after-free condition is reached — a previously freed memory object is subsequently dereferenced by the parsing/mount code.

3. Execution method (class level). This is a client-side memory-safety issue: exploitation depends on the client reaching an inconsistent memory state while handling server-supplied data. Given the CVSS profile, the practical outcome is memory corruption that the system detects/handles as a fatal fault rather than a controlled hijack. No weaponized chain is publicly documented.

4. Post-exploitation impact. The realized impact is availability loss — system termination (kernel/system crash), which can be induced repeatedly. There is no evidence in the CVSS vector of confidentiality or integrity compromise; the flaw is a denial-of-service primitive. Repeated crashes against a target could be used for disruption or as a nuisance/DoS in a targeted or opportunistic campaign.

Because exploitation requires the victim to mount the crafted share, mass exploitation is less likely; realistic threat models are targeted DoS against specific macOS users or opportunistic attacks against users connecting to untrusted networks/shares.

Impact Assessment

• CVSS 3.1 Base: 6.5 (Medium). Vector CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H.
  • AV:N / AC:L / PR:N — remotely reachable with low complexity and no attacker privileges; the barrier is purely the user-interaction requirement.
  • UI:R — the victim (or an automated mount policy) must mount the crafted share; this materially reduces mass-exploitation risk.
  • S:U / C:N / I:N / A:H — impact is confined to the vulnerable component and is purely to availability; the crash is sufficient to terminate the system.
• EPSS 0.00298 (~22.5th percentile). Low predicted exploitation likelihood in the near term, consistent with the absence of public exploit code and the UI requirement.
• CISA KEV: No. No evidence of exploitation in the wild as of the source data.
• Blast radius. Any affected macOS endpoint mounting attacker-influenced SMB shares. High-value targets include knowledge-worker fleets in file-sharing-heavy environments and users on untrusted networks. The blast radius is bounded by the UI requirement — it is not wormable, self-propagating, or remotely triggerable without the mount action.
• Business impact. Primarily operational disruption: forced reboots, loss of unsaved work, and potential destabilization of endpoints during active sessions. No data-exfiltration or integrity impact is implied by the vector.

Detection Guidance

Detection for a client-side memory-safety crash triggered by SMB content is challenging pre-patch. The most reliable signals are crash telemetry (networking/SMB subsystem panics), SMB client connection anomalies, and the identity of the connecting SMB server. There are no public IOCs. The rules below are generic to the vulnerability class (SMB-mount-triggered macOS crashes) and should be tuned to the environment. No IOCs from the source data exist to key on.

Sigma Rule

title: macOS SMB Client Crash Indicative of Use-After-Free (CVE-2026-43719)
id: 7f3c1b2a-4d5e-4f6a-9b1c-2e8a9c0d4f21
status: experimental
description: Detects macOS SMB client crashes and mounts involving untrusted SMB servers that may indicate exploitation of CVE-2026-43719 (use-after-free in SMB share handling leading to system termination).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-43719
author: valtersit.com
date: 2026/01/01
tags:
  - attack.t1203
logsource:
  product: macos
  service: system
detection:
  selection_crash:
    message|contains:
      - 'smb'
      - 'SMB'
    message|contains|all:
      - 'panic'
  selection_mount:
    message|contains:
      - 'mount_smbfs'
      - 'smbfs'
      - 'SMB mount'
  condition: selection_crash or selection_mount
falsepositives:
  - Legitimate SMB client crashes unrelated to exploitation
  - Routine SMB mounts by users (mount events alone are noisy)
level: medium

YARA Rule

rule macos_smb_uaf_crash_cve_2026_43719
{
    meta:
        description = "Heuristic detection for macOS SMB client crash artifacts associated with CVE-2026-43719 use-after-free in SMB share handling"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-43719"
        date = "2026-01-01"
        cve = "CVE-2026-43719"
    strings:
        $smb1 = "smbfs" ascii
        $smb2 = "mount_smbfs" ascii
        $panic = "panic" ascii
        $uaf = "use-after-free" ascii nocase
        $smbproto = { 00 00 00 85 FF 53 4D 42 } /* SMB/2 negotiation header fragment */
    condition:
        ($smb1 or $smb2) and $panic
        or ($smbproto and ($smb1 or $smb2))
}

Suricata IDS Rule

alert tcp any 445 -> any any (msg:"CVE-2026-43719 Possible macOS SMB Client Exploitation Attempt - Malicious SMB Share Response"; flow:from_server,established; content:"|FF 53 4D 42|"; offset:4; depth:4; sid:90043719; rev:1; reference:cve,2026-43719; classtype:attempted-dos; metadata:service smb;)

Note: SMB responses are typically encrypted/authenticated and content matching is limited; this rule targets cleartext SMB negotiation traffic and should be used as a coarse signal only. There is no published byte-level exploit signature.

Elastic Detection

process where event.type == "start"
  and process.name in ("mount_smbfs", "mount", "smbutil")
  and process.args : "*smb://*"

event.category: "process" and process.name : "mount_smbfs"

Kernel panic correlation (macOS unified logs forwarded to Elastic):

any where event.category : "process" and process.name : "mount_smbfs"
  and wildcard(process.args, "*smb://*")

Splunk SPL Query

index=macos_logs (sourcetype=macos:system OR sourcetype=macos:kernel)
| search ("smbfs" OR "mount_smbfs" OR "SMB mount" OR "panic")
| where match(_raw, "(?i)(smb)") AND match(_raw, "(?i)(panic|assert|will be terminated)")
| stats count min(_time) as first_seen max(_time) as last_seen values(host) as hosts by signature
| sort - count

Wazuh OSSEC Rule

<group name="macos,smb,cve-2026-43719,">
  <rule id="100719" level="10">
    <decoded_as>json</decoded_as>
    <field name="sourcetype">macos_system</field>
    <field name="message">smbfs</field>
    <field name="message">panic</field>
    <description>CVE-2026-43719: macOS SMB client crash/mount anomaly (possible use-after-free exploitation)</description>
    <mitre>
      <id>T1203</id>
    </mitre>
    <group>cve_2026_43719,smb_abuse,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence is 2/4 (CLAIMED) — a single source (NVD) references availability of fixes; this has not been independently confirmed.

1. Immediate mitigations.

   • Avoid mounting SMB shares from untrusted or unverified servers. Treat SMB mounts on untrusted networks as hostile.
   • Where feasible, restrict/disable auto-mounting of SMB shares and block outbound/ inbound SMB (TCP 445, 139) at network boundaries to untrusted networks.
   • Monitor for repeated kernel panics/system terminations correlated with SMB mount events on macOS endpoints. Ensure unsaved work protection.
   • Because exploitation requires mounting a crafted share (UI:R), user awareness and endpoint policies limiting arbitrary SMB mounts reduce exposure.

2. Patch / upgrade steps.

   • Apply Apple's fixes: update to macOS Golden Gate 27, macOS Sequoia 15.8, or macOS Tahoe 26.7 (or later within each track), per the NVD description.
   • Verify the installed build after updating and confirm the fix in Apple's security release notes. (Patch status CLAIMED, verify against the vendor advisory.)
   • Reboot affected endpoints to ensure the corrected memory management path is active.

3. Long-term hardening.

   • Enforce a vulnerability-management process that tracks Apple security releases for macOS endpoint fleets.
   • Restrict SMB access to trusted, managed servers; segment file-sharing traffic on internal networks.
   • Maintain endpoint crash telemetry and forward it to the SIEM for correlation (see Detection Guidance).
   • Treat client-side network parsers (SMB, etc.) as high-attack-surface and monitor for recurring memory-safety advisories.

References

• NVD — CVE-2026-43719: https://nvd.nist.gov/vuln/detail/CVE-2026-43719
• Apple Security Releases (per NVD description, fixes ship in macOS Golden Gate 27, macOS Sequoia 15.8, macOS Tahoe 26.7): https://support.apple.com/en-us/HT201222
• 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

Threat actor / exploit-in-the-wild evidence: None. CISA KEV status is NO, EPSS is low (0.00298), and no public PoC, Nuclei template, or exploit code exists in the provided data. No known campaigns are associated with this CVE at publication time.