macOS CUPS Flaw Lets Local Attackers Write Arbitrary Files as Root

A recently disclosed privilege-related vulnerability in the Common UNIX Printing System (CUPS) on macOS could allow an unprivileged local user to create attacker-controlled files in arbitrary locations outside the protection of System Integrity Protection (SIP), thereby gaining root ownership.

This flaw, tracked as CVE-2026-39875, affects Apple devices running macOS Sonoma, Sequoia, and Tahoe versions before specific updates: Sonoma 14.8.8, Sequoia 15.7.8, and Tahoe 26.6.

According to a public proof-of-concept (PoC) repository published by security researcher Dallas Dubs, the vulnerability lies within cupsd, the privileged daemon responsible for managing printers and print jobs on macOS.

While the PoC demonstrates the ability to create arbitrary files as root, its author clarified that it does not provide a complete root shell or a standalone path for privilege escalation.

macOS CUPS Flaw

The reported attack exploits two separate logic weaknesses in the CUPS printer registration process.

First, when CUPS probes a newly registered printer, the daemon forwards a valid local authentication token to the backend endpoint specified in the printer’s device URI.

An attacker with local access can register a printer that points to an attacker-controlled listener, causing the privileged service to send an authentication token outside its expected trust boundary.

This design assumes that a printer backend is trusted; however, allowing a non-privileged user to define the backend URI undermines this assumption and creates an opportunity to capture a token associated with the local CUPS authentication process.

The second issue involves a timing condition during the printer registration. According to the researcher, cupsd can initiate root-privileged file input/output (I/O) before completing authentication validation.

At the time of the write operation, both the destination path and the written content can still be influenced by the attacker.

The PoC combines token capture and replay techniques to configure a second printer using a file:// device URI. An attacker can then submit a print job containing controlled data, causing cupsd to write the content to a selected location with root privileges.

This attack requires local access but does not need user interaction once it starts. While it cannot overwrite paths protected by SIP, which limits its immediate impact on core macOS system files, arbitrary root-owned file writes can be significant, especially in enterprise environments.

Writable startup locations, application directories, scheduled task paths, or configuration files could be exploited in a separate attack chain.

Security teams should consider this flaw as a potential enabler for local privilege escalation rather than a complete method for root compromise on its own.

Affected Versions and Mitigation

The researcher’s repository includes demonstrations against macOS Tahoe 26.4.1, Sequoia 15.7.5, and Sonoma 14.8.5.

Apple has credited Dallas Dubs along with Aaron Grattafiori of NVIDIA AI Red Team, XBreach.ai, and Andreas Jaegersberger and Ro Achterberg of Nosebeard Labs.

Organizations should ensure that affected devices are updated to macOS Tahoe 26.6, Sequoia 15.7.8, or Sonoma 14.8.8 or later.

Administrators should also restrict local access, monitor for unexpected printer registrations and changes in CUPS configurations, and investigate any unusual root-owned files created in connection with printing activity.

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