Microsoft’s emergency 'PrintNightmare' patch fails to fix critical exploit

An emergency patch released to address the PrintNightmare remote code execution (RCE) vulnerability in Windows is said to have been unsuccessful, with hackers still being able to infect targeted devices, researchers have warned.

Microsoft released the patch this Tuesday outside of its routine Patch Tuesday wave of updates given the severity of the PrintNightmare vulnerability, as well as the fact that exploit code has been circulating online. The flaw has been assigned CVE-2021-34527 and a CVSS threat severity score of 8.8 out of ten.

However, Researcher Benjamin Delpy found that he could still demonstrate successful exploitation on a Windows Server 2019 deployment with the patch installed, and the ‘point and print’ feature enabled.

Point and print is a tool that makes it easier for users within a network to obtain the printer drivers, and queue documents to print.

Microsoft acknowledged in its security alert that the feature isn’t directly related to the flaw, but that the technology “weakens the local security posture in such a way that exploitation will be possible”.

The patch purporting to fix CVE-2021-34527 seemingly hasn’t addressed this particular shortcoming, Delpy’s demonstration shows, with hackers potentially able to bypass the fix and attack victim’s machines, if they have point and print enabled.

The threat stemmed from a vulnerability in the Print Spooler component in Windows systems, which allows print functionality remotely within local networks. Microsoft patched a similar Print Spooler flaw on 8 June, which was initially deemed to be a privilege escalation bug but the company then upgraded weeks later to an RCE vulnerability.

Following that 8 June patch, researchers with Sangfor published what they believed to be a proof-of-concept exploitation for the same Print Spooler RCE flaw, however, it was later discovered to be an entirely different flaw that hadn’t been previously disclosed.

Although the researchers promptly removed their work, the gaffe led to the exploit code being downloaded and republished elsewhere, with Microsoft confirming a few days later that hackers had exploited the flaw.

Microsoft previously recommended that businesses disable the Print Spooler service or inbound remote printing through their group policy - until a patch became available. The first mitigation deactivates the ability to print locally or remotely, while the second one blocks the remote attack vector by preventing inbound remote printing operations. Local printing would still be possible, though.