The Dutch National Cyber Security Centre (NCSC) issued a warning after being notified of several incidents where a vulnerability in Apple’s Screen Sharing feature was exploited to install Monero cryptominers.

The vulnerability, tracked as CVE-2026-65400, was patched by Apple on August 6. It is an authentication-bypass flaw in macOS Screen Sharing that can let an attacker on the network connect without valid credentials.

macOS’s built-in Screen Sharing service is a remote-control feature commonly associated with port 5900. Successful exploitation can allow a remote attacker on a reachable network to authenticate to the service without legitimate credentials.

Apple said the bug was fixed through “improved state management,” which suggests an authentication-flow or session-state validation failure rather than a cryptographic break.

The patch was issued for macOS Tahoe 26.6.1, Sequoia 15.7.9, and Sonoma 14.8.9. Practical exposure requires Screen Sharing to be enabled, so the highest-risk systems are those where port 5900 is internet-accessible, typically through a router port-forward, public IP assignment, or hosting-provider setup. Hosts reachable only from an internal network are still potentially exposed, but attackers would have to gain a position on that network.

An attacker could view and control the Mac remotely because that is the function Screen Sharing provides. NCSC says active cases involved attackers gaining root access and installing cryptomining software, specifically for Monero mining.

The criminals likely chose Monero mining because it does not depend on heavily specialized, application-specific integrated circuits (ASICs), but can be done with any CPU or GPU.

Cryptomining isn’t necessarily the worst an attacker could do. With a root-level compromise an attacker could enable persistence, data theft, credential and key harvesting, deployment of additional malware, and lateral movement.

How to stay safe

Install the update

The best way to protect your Mac is to install the update.

To update macOS on any supported Mac, use the Software Update feature, which Apple designed to work consistently across all recent versions. Here are the steps:

• Click the Apple menu in the upper-left corner of your screen.
• Choose System Settings (or System Preferences on older versions).
• Select General in the sidebar, then click Software Update on the right. On older macOS, just look for Software Update directly.
• Your Mac will check for updates automatically. If updates are available, click Update Now (or Upgrade Now for major new versions) and follow the on-screen instructions. Before you upgrade to macOS Tahoe 26, please read these instructions.
• Enter your administrator password if prompted, then let your Mac finish the update (it might need to restart during this process).
• Make sure your Mac stays plugged in and connected to the internet until the update is done.

Make sure Screen Sharing is disabled

If you can’t update immediately, check whether Screen Sharing is enabled and turn it off if you don’t use it.

1.  Click the Apple menu in the top-left corner of the screen.
2. Select System Settings.
3. In the left sidebar, click General.
4. Click Sharing on the right; you may need to scroll down.
5. Find Screen Sharing:
   • If the switch is off/grey, it is disabled.
   • If the switch is on/colored, click it to switch it off.

Also check Remote Management on that same Sharing page. It provides another remote-control route and should be off unless the owner knowingly uses it for work or IT support.

Macs need protection too

Malwarebytes Premium Security for Mac stops threats and protects your Mac and personal files from hackers and cybercriminals.

Hackers are exploiting a macOS Screen Sharing flaw to gain root access and install Monero miners on Macs with port 5900 exposed online.

The Dutch National Cyber Security Centre confirmed active exploitation of a critical macOS authentication flaw, tracked as CVE-2026-65400 (CVSS score of 9.8), less than two weeks after Apple shipped the fix.

The bug sits in macOS’s built-in Screen Sharing feature, the remote desktop tool baked into every Mac. Apple’s fix improved how the system manages authentication state, closing a gap that let attackers on the network authenticate to Screen Sharing without valid credentials at all.

Apple patched this issue with the release of  macOS Tahoe 26.6.1, macOS Sequoia 15.7.9, and macOS Sonoma 14.8.9, crediting researcher Alfredo Pesoli (@__rev) at Bynario Atlas (bynar.io) for the discovery.

“An attacker on the network may be able to authenticate to Screen Sharing without valid credentials” reads the advisory.

That’s a fast, coordinated fix by industry standards. It just wasn’t fast enough to beat whoever started scanning for exposed systems.

NCSC-NL says it received reports of active abuse hitting multiple systems where port 5900, the port Screen Sharing runs on, was reachable directly from the internet.

“The vulnerability concerns an authentication issue in the Screen Sharing functionality where network attackers can gain access without valid credentials. This is made possible by insufficient state management during the authentication process. As a result, unauthorized individuals can perform authentication attempts that would normally not be accepted.” reads the advisory. “The NCSC has received a security advisory indicating that active exploitation of this vulnerability has been observed on multiple systems where port 5900 was accessible from the internet. In all these cases, root access was obtained on the affected system and a Monero crypto miner was placed.”

In every case documented so far, attackers gained root access and dropped a Monero cryptocurrency miner on the compromised machine. Cryptomining is a relatively boring payload compared to what root access on a Mac could actually enable, which makes this look more like opportunistic scanning than a targeted campaign, for now.

This flaw sits in the same source code file as two other Screen Sharing bugs Apple patched a month earlier in macOS 26.6, one of them a genuinely pre-authentication flaw that a researcher going by @osxreverser described needing nothing but a target’s IP address to exploit, no password, no username, nothing.

That researcher claimed to have found around 40,000 exposed Screen Sharing hosts on the internet during a scan, nearly half of them in the US, spanning residential connections, university networks, and at least a few corporate servers.

What ties both bugs together is how mechanically simple they are to trigger. Security firm Calif, which analyzed the flaws, found no memory corruption, no exploitation trickery, no race condition to win, just logic errors that let a couple of correctly ordered packets walk straight past authentication. Calif also said it built a working exploit for both vulnerabilities in about four hours using an AI coding agent, which is the detail that should worry defenders more than the Monero miner itself: the gap between a patch note and a working exploit keeps shrinking, and it’s shrinking because building the exploit barely takes effort anymore.

If you’re running a Mac with Screen Sharing enabled and haven’t updated yet, do it now rather than after finishing this article. And if updating isn’t possible immediately, turn Screen Sharing off entirely under General, Sharing, until you can; leaving port 5900 open to the internet at this point is less a risk than an open invitation.

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Pierluigi Paganini

(SecurityAffairs – hacking, Monero)