THE THREAT
On July 6th and 7th, 2021, Microsoft announced Out-of-Band (OoB) security patches for the Windows Print Spooler vulnerability CVE-2021-34527 (aka PrintNightmare). eSentire is aware of reports from July 7th, stating that the patches can be bypassed under certain conditions. eSentire has confirmed that patch bypass is possible only if non-default configurations are in place.
As of July 7th, Microsoft has released security patches for all impacted products.
The PrintNightmare vulnerability is classified as Remote Code Execution (RCE). Exploitation could allow a threat actor to take full control of vulnerable systems. Proof-of-Concept (PoC) exploit code was publicly released on June 29th. The public release of PoC code significantly reduces the barriers for exploitation. Microsoft has stated that exploitation of PrintNightmare has been identified.
Organizations are strongly recommended to deploy the July OoB security patches and confirm Point and Print settings are not modified from the default configuration [see below].
What we’re doing about it
- MVS will automatically add the relevant plugins for this vulnerability once details are made available
- esENDPOINT has rules in place to detect exploitation of CVE-2021-34527
- eSentire security teams continue to track this topic for additional details and detection opportunities
What you should do about it
- On July 6th & 7th Microsoft released OoB patches for the PrintNightmare vulnerability:
- Security patches are available for Windows 10, version 21H1, 20H2, 2004, 1909, 1507, 1607, 1809, Windows Server 2019, Windows 8.1 and Windows Server 2012, Windows 7 SP1 and Windows Server 2008 R2 SP1, Windows Server 2008 SP2, Windows Server 2016, and Windows Server 2012
- For the patches to be effective, organizations must confirm that the following settings are set to 0 (zero) or are not defined
- HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows NT\Printers\PointAndPrint
- NoWarningNoElevationOnInstall = 0 (DWORD) or not defined (default setting)
- NoWarningNoElevationOnUpdate = 0 (DWORD) or not defined (default setting)
- If patching is not possible, organizations should disable the Windows Print Spooler service on Domain Controllers and any system that do not require printing capabilities
- Microsoft has released guidance for disabling the print spooler service
- As another option, organizations may disable inbound remote printing through Group Policy
- It should be noted that both of these options may impact standard printing procedures
Additional information
eSentire has confirmed that the July OoB security patches to be effective. Bypassing the patches is only possible where 'Point and Print Restrictions' are modified from the standard configuration. Configuration for Point and Print is stored in the following registry location:
HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows NT\Printers\PointAndPrint
NoWarningNoElevationOnInstall = 0
NoWarningNoElevationOnUpdate = 0
Note the following:
- Microsoft states these registry keys do not exist by default and are therefore not vulnerable to the patch bypass
- In our testing, exploit methods which claim to bypass the OoB July patches only succeeded when NoWarningNoElevationOnInstall was set to “1”. This is not the standard configuration in most environments
PrintNightmare was originally tracked as CVE-2021-1675. Microsoft has since stated the vulnerability is unique from CVE-2021-1675 and is officially tracked as CVE-2021-34527. The July 6th - 7th security patches for CVE-2021-34527 are cumulative updates and cover CVE-2021-1675.
CVE-2021-34527 (CVSS: 8.8) is classified as a Windows Print Spooler Remote Code Execution vulnerability. A threat actor with local access to a vulnerable asset may exploit this vulnerability to run arbitrary code with system privileges.
The newly released security patches only mitigate the Remote Code Execution (RCE) portion of PrintNightmare. The local privilege escalation portion of the vulnerability still exists after patching. A threat actor with previous access may exploit the vulnerability to escalate privileges on the victim device despite the security patches.
The PoC exploit code was originally planned to be released by a security firm at BlackHat USA but was released in error on June 29th. The code was publicly available for multiple hours before being taken down. The security company plans to officially release the code and additional details at BlackHat USA in July 2021. eSentire has confirmed that despite the code takedown, it was copied and remains publicly available.
References:
[1] https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-34527
[2] https://support.microsoft.com/en-us/topic/july-6-2021-kb5004945-os-builds-19041-1083-19042-1083-and-19043-1083-out-of-band-44b34928-0a71-4473-aa22-ecf3b83eed0e
[3] https://support.microsoft.com/en-us/topic/july-6-2021-kb5004946-os-build-18363-1646-out-of-band-18c5ffac-6015-4b3a-ba53-a73c3d3ed505
[4] https://support.microsoft.com/en-us/topic/july-6-2021-kb5004950-os-build-10240-18969-out-of-band-7f900b36-b3cb-4f5e-8eca-107cc0d91c50
[5] https://support.microsoft.com/en-us/topic/july-7-2021-kb5004948-os-build-14393-4470-out-of-band-fb676642-a3fe-4304-a79c-9d651d2f6550
https://support.microsoft.com/en-us/topic/july-6-2021-kb5004947-os-build-17763-2029-out-of-band-71994811-ff08-4abe-8986-8bd3a4201c5d
[6] https://support.microsoft.com/en-us/topic/july-6-2021-kb5004958-security-only-update-out-of-band-d439df52-8f5a-4cb8-9d0d-c2f1bb036a5e
[7] https://support.microsoft.com/en-us/topic/july-6-2021-kb5004951-security-only-update-out-of-band-e05a81cd-9b45-4622-b715-ddb2367bca47
[8] https://support.microsoft.com/en-us/topic/july-7-2021-kb5004948-os-build-14393-4470-out-of-band-fb676642-a3fe-4304-a79c-9d651d2f6550
[9] https://support.microsoft.com/en-us/topic/july-7-2021-kb5004960-security-only-update-out-of-band-b5068c71-3e11-4d30-98da-f630da1a1add
[10] https://support.microsoft.com/en-us/topic/kb5005010-restricting-installation-of-new-printer-drivers-after-applying-the-july-6-2021-updates-31b91c02-05bc-4ada-a7ea-183b129578a7
[11] https://docs.microsoft.com/en-us/defender-for-identity/cas-isp-print-spooler
[12] https://msrc.microsoft.com/update-guide/en-US/vulnerability/CVE-2021-1675
