Microsoft’s Emergency Patch Fails to Absolutely Repair PrintNightmare RCE Vulnerability

At the same time as Microsoft expanded patches for the so-called PrintNightmare vulnerability for Home windows 10 model 1607, Home windows Server 2012, and Home windows Server 2016, it has come to gentle that the patch for the distant code execution exploit within the Home windows Print Spooler service will be bypassed in sure eventualities, successfully defeating the safety protections and allowing attackers to run arbitrary code on contaminated methods.

On Tuesday, the Home windows maker issued an emergency out-of-band update to deal with CVE-2021-34527 (CVSS rating: 8.8) after the flaw was by chance disclosed by researchers from Hong Kong-based cybersecurity agency Sangfor late final month, at which level it emerged that the problem was totally different from one other bug — tracked as CVE-2021-1675 — that was patched by Microsoft on June 8.

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“A number of days in the past, two safety vulnerabilities have been present in Microsoft Home windows’ current printing mechanism,” Yaniv Balmas, head of cyber analysis at Examine Level, advised The Hacker Information. “These vulnerabilities allow a malicious attacker to realize full management on all home windows environments that allow printing.”

“These are principally working stations however, at instances, this pertains to total servers which can be an integral a part of very talked-about organizational networks. Microsoft categorised these vulnerabilities as vital, however once they have been printed they have been capable of repair solely considered one of them, leaving the door open for explorations of the second vulnerability,” Balmas added.

PrintNightmare stems from bugs within the Home windows Print Spooler service, which manages the printing course of inside native networks. The principle concern with the risk is that non-administrator customers had the flexibility to load their very own printer drivers. This has now been rectified.

“After putting in this [update] and later Home windows updates, customers who should not directors can solely set up signed print drivers to a print server,” Microsoft said, detailing the enhancements made to mitigate the dangers related to the flaw. “Administrator credentials shall be required to put in unsigned printer drivers on a printer server going ahead.”

Publish the replace’s launch, CERT/CC vulnerability analyst Will Dormann cautioned that the patch “solely seems to deal with the Distant Code Execution (RCE through SMB and RPC) variants of the PrintNightmare, and never the Native Privilege Escalation (LPE) variant,” thereby permitting attackers to abuse the latter to realize SYSTEM privileges on weak methods.

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Now, additional testing of the replace has revealed that exploits focusing on the flaw may bypass the remediations completely to realize each native privilege escalation and distant code execution. To realize this, nonetheless, a Windows policy referred to as ‘Point and Print Restrictions‘ have to be enabled (Pc ConfigurationPoliciesAdministrative TemplatesPrinters: Level and Print Restrictions), which will be probably used to put in malicious printer drivers.

“Be aware that the Microsoft replace for CVE-2021-34527 doesn’t successfully forestall exploitation of methods the place the Level and Print NoWarningNoElevationOnInstall is about to 1,” Dormann said Wednesday. Microsoft, for its half, explains in its advisory that “Level and Print will not be immediately associated to this vulnerability, however the know-how weakens the native safety posture in such a approach that exploitation shall be doable.”

Whereas Microsoft has advisable the nuclear possibility of stopping and disabling the Print Spooler service, an alternative workaround is to allow safety prompts for Level and Print, and restrict printer driver set up privileges to directors alone by configuring the “RestrictDriverInstallationToAdministrators” registry worth to forestall common customers from putting in printer drivers on a print server.

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