The company also published a FAQ and workarounds for the freshly issued CVE while it investigated the critical new vulnerability for which exploit code is already publicly available. "Microsoft is aware of and investigating a remote code execution vulnerability that affects Windows Print Spooler and has assigned CVE-2021-34527 to this vulnerability," the company said July 1. "This is an evolving situation and we will update the CVE as more information is available."
Microsoft's move came after the CERT Coordination Center, the US Cybersecurity and Infrastructure Security Agency (CISA), and others urged organizations to immediately disable Print Spooler services on critical systems like domain controllers and Active Directory admin systems, citing exploits targeting PrintNightmare.
The CERT CC alert pointed to the update that Microsoft had issued for CVE-2021-1675 on June 8 and warned that it was ineffective against PrintNightmare exploits. "While Microsoft has released an update for CVE-2021-1675, it is important to realize that this update does NOT address the public exploits that also identify as CVE-2021-1675," CERT CC's vulnerability note said. The CERT alert — like one from CISA — urged organizations to disable Print Spooler on critical systems because of the risk it presented. The PrintNightmare flaw, if successfully exploited, would allow an authenticated attacker to gain complete system-level access to vulnerable systems, it warned.
Microsoft's update on July 1, however, sought to separate PrintNightmare from the vulnerability it had patched on June 8, though both affect the exact same Print Spooler function — RpcAddPrinterDriverEx(). That would officially make PrintNightmare a new zero-day in Print Spooler for which exploits are currently available. Fortinet has released IPS signatures for the vulnerability.
According to the company, the PrintNightmare vulnerability was present in Print Spooler before the June 2021 update and involves a different attack vector from CVE-2021-1675. All Windows versions are affected by the PrintNightmare flaw, but Microsoft is still investigating if the exploits against it will work on all versions, the company said.
Domain controllers — one of the most critical resources on any network — are affected, Microsoft said. "We are still investigating if other types of roles are also affected."
As workarounds for PrintNightmare, the company advised organizations to either disable the Print Spooler service or to disable inbound remote printing using Group Policy. Disabling the Spooler would disable the ability to print both locally and remotely, while disabling inbound remote printing would allow local printing to a directly attached device, the company said.
Boris Larin, a security researcher at Kaspersky, says that while Microsoft's official position is that PrintNightmare is a separate vulnerability from CVE-2021-1675, both likely stem from the same root cause. "CVE-2021-1675 was aimed to patch the scenario of local elevation of privilege, but PrintNightmare allows remote code execution when attackers have access to Print Spooler service," he says. "It is possible that the remote scenario for exploitation was not considered during the patch."
PrintNightmare is only the latest in a long list if vulnerabilities that have been uncovered in Print Spooler over the years. The most infamous of these vulnerabilities was a privilege escalation issue that enabled the US-led Stuxnet attack against Iran's uranium enrichment facility at Natanz back in 2010.
A trio of researchers at China's Sangfor Technologies uncovered PrintNightmare during a months-long inspection of the technology recently. The researchers are scheduled to detail the flaw — and other vulnerabilities they discovered in Print Spooler during their investigation — at the upcoming Black Hat USA. The researchers released proof-of-exploit code for PrintNightmare on GitHub but quickly deleted it after blowback from other researchers. However, in the brief window of time it was available on GitHub, the exploit code was copied and, as a result, is available in the wild.
The Microsoft advisory noted that the company has detected exploit activity against the flaw.