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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