Microsoft has released the KB5004945 emergency security update to fix the actively exploited PrintNightmare zero-day vulnerability in the Windows Print Spooler service impacting all Windows versions. However, the patch is incomplete, and the vulnerability can still be locally exploited to gain SYSTEM privileges. The remote code execution bug (tracked as CVE-2021-34527) allows attackers to take over affected servers via remote code execution (RCE) with SYSTEM privileges, as it will enable them to install programs, view, change, or delete data, and create new accounts with full user rights. Detailed instructions on how to install these out-of-band security updates for your operating system, are available on the Microsoft support website. Security updates have not yet been released for Windows 10 version 1607, Windows Server 2016, or Windows Server 2012, but they will also be released soon, according to Microsoft. “Release notes associated with these updates might publish with a delay of up to an hour after the updates are available for download, updates for the remaining affected supported versions of Windows will be released in the coming days,” according to Microsoft. The PrintNightmare vulnerability includes both a remote code execution (RCE) and a local privilege escalation (LPE) vector that can be used in attacks to run commands with SYSTEM privileges on a vulnerable system. After Microsoft released the out-of-band update, security researcher Matthew Hickey verified that the patch only fixes the RCE and not the LPE component. This means that the fix is incomplete and threat actors and malware can still locally exploit the vulnerability to gain SYSTEM privileges.
Microsoft urges customers to install these out-of-band security updates immediately to address the PrintNightmare vulnerability. Those who cannot install these updates as soon as possible should check out the FAQ and Workaround sections in the CVE-2021-34527 security advisory for info on how to protect their systems from attacks exploiting this vulnerability. The available mitigation options include disabling the Print Spooler service to remove printing capability locally and remotely or disabling inbound remote printing through Group Policy to remove remote attack vector by blocking inbound remote printing operations. In the second case, Microsoft says that “the system will no longer function as a print server, but local printing to a directly attached device will still be possible.” CISA has also published a notification on the PrintNightmare zero-day last week encouraging admins to disable the Windows Print Spooler service on servers not used for printing.