A recently disclosed vulnerability in the Common UNIX Printing System (CUPS), identified as CVE-2026-39875, has been addressed by Apple in macOS versions 14.8.8, 15.7.8, and 26.6. This flaw allowed unprivileged local users to perform arbitrary file writes with root privileges, posing a significant security risk.
CUPS is an open-source printing system widely used in Unix-like operating systems, including macOS. It manages print jobs and queues, facilitating communication between computers and printers. Given its integral role, vulnerabilities within CUPS can have far-reaching implications for system security.
Security researcher Dallas Dubs released a proof-of-concept (PoC) exploit demonstrating how two logic flaws in the privileged cupsd printing daemon could be chained to achieve arbitrary file writes with root privileges. The exploit involves registering a malicious printer and capturing a valid CUPS authentication token during printer probing. This token is then replayed to register another printer with a file:// device URI targeting a local path not protected by System Integrity Protection (SIP). Subsequently, a print job containing controlled data is submitted to the malicious printer, resulting in the cupsd daemon writing the supplied content to the selected location as root.
While the PoC demonstrates the ability to create a file with root ownership, it does not directly provide a full privilege escalation mechanism. However, arbitrary root file writes are a high-impact primitive. An attacker with access to a standard local account could exploit this condition to alter configuration files, scheduled tasks, application support files, or other writable locations, potentially leading to persistent root-level code execution.
Apple has addressed this vulnerability in macOS 14.8.8, 15.7.8, and 26.6. Organizations are urged to prioritize deploying these updates, especially on shared Mac systems, developer endpoints, and devices where users can execute untrusted local software. Additionally, security teams should monitor for unusual local printer registrations, unexpected changes to printer device URIs, and suspicious print jobs targeting file-based destinations. Restricting unnecessary printer administration access and monitoring CUPS-related activity can provide additional detection coverage while patches are rolled out.
The release of this PoC increases the likelihood of active testing and attempted exploitation, making prompt patching essential. This incident underscores the importance of timely vulnerability management and the need for organizations to stay vigilant against emerging threats.
