PrintNightmare official patch is out – update now!

By Paul Ducklin, Sophos

July 7, 2021

Here’s the good news: Microsoft has released an emergency patch for the infamous PrintNightmare bug that showed up just over a week ago.

The patch is what Redmond refers to as an OOB Security Update, where OOB is short for out-of-band.

OOB is a jargon term that refers to communications that are kept separate from the usual channel you use, notably for safety reasons in case the main channel should fail or need overriding in an emergency.

In Windows update parlance, OOB refers to patches that are deemed so important that they can’t wait until the next official Patch Tuesday, which is always the second Tuesday in each calendar month. (This month, that’s 2021-07-13, which is still almost a week away.)

ICYMI, PrintNightmare is an aptly named bug that became a public danger for the unfortunate reason that a team of security researchers jumped to an incorrect conclusion:


What happened?

Briefly put, Microsoft published a Windows Print Spooler patch for a bug dubbed CVE-2021-1675, as part of the June 2021 Patch Tuesday update that came out on 2021-06-08.

Originally, the bug was reported as an elevation of privilege (EoP) vulnerability, meaning that altough attackers already on your computer could exploit the bug to promote themselves from a regular user to a system account, they couldn’t use it to break into your computer in the first place.

In the meantime, Chinese researchers preparing a paper for the 2021 Black Hat conference were working on their own bug in the Windows Print Spooler.

Theirs sounded very similar, except that it was an RCE bug, short for remote code execution, meaning that it could be used for breaking in, not merely for elevating privilege.

Given that the Chinese researchers’ bug was apparently different, they hadn’t disclosed it yet.

Later in the month, however, Microsoft admitted that CVE-2021-1675 could also be used for RCE, and updated its public advisory to say so.

Even though that meant the bug was more serious in theory, no one worried too much in practice.

After all, a patch was already available, and anyone who had installed the patch to close the EoP hole was, ipso facto, protected against the newly announced RCE hole as well.

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