Microsoft Releases High Priority Fixes in August Patch Tuesday

A remote code execution vulnerability exists when the Windows Print Spooler service improperly performs privileged file operations. An attacker who successfully exploited this vulnerability could run arbitrary code with SYSTEM privileges. An attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. UPDATE August 10, 2021: Microsoft has completed the investigation and has released security updates to address this vulnerability. Please see the Security Updates table for the applicable update for your system. We recommend that you install these updates immediately. This security update changes the Point and Print default behavior; please see KB5005652.

CVE-2021-34527, widely known as 'PrintNightmare', is a remote code execution flaw in the Microsoft Windows Print Spooler service, which improperly performs privileged file operations such as loading printer driver DLLs. An attacker with low-level access who can reach a machine's spooler, for example a domain user able to add a printer connection via Point and Print, can induce the SYSTEM-privileged service to load an attacker-controlled DLL with no user interaction required (CVSS:3.1 vector AV:N/AC:L/PR:L/UI:N). Successful exploitation yields arbitrary code execution as SYSTEM, letting the attacker install programs, view, change or delete data, and create new accounts with full user rights, effectively achieving complete host compromise. The flaw affects all supported Windows client and server releases in the CISA data, Windows 10 from 1507 through 22H2, Windows 11, Windows RT 8.1, and Windows Server 2008, 2012 and 2016, wherever the Print Spooler service is running. Exploitation is confirmed in the wild: the bug was added to CISA's Known Exploited Vulnerabilities catalog on 2021-11-03 with known ransomware use, the FBI and CISA have warned of Russian actors exploiting it, and EPSS places the 30-day exploitation probability at 99.8%.

Do: Install the July 2021 security updates immediately, released July 6, 2021 with additional updates on July 7 for Windows Server 2012, Windows Server 2016 and Windows 10 version 1607, and review KB5005010 for restricting installation of new printer drivers after applying the July 6 updates. Where patching is delayed, disable the Print Spooler service on hosts that do not need printing or restrict Point and Print, and verify that NoWarningNoElevationOnInstall and UpdatePromptSettings under HKLM\SOFTWARE\Policies\Microsoft\Windows NT\Printers\PointAndPrint are set to 0 or not defined (these keys do not exist by default, which is the secure state; NoWarningNoElevationOnInstall = 1 makes the system vulnerable by design). Prioritize domain controllers and servers with exposed spoolers, and hunt…

LedgerSMB does not check the origin of HTML fragments merged into the browser's DOM. By sending a specially crafted URL to an authenticated user, this flaw can be abused for remote code execution and information disclosure.

CVE-2021-36934 is an elevation of privilege vulnerability in Windows caused by overly permissive Access Control Lists (ACLs) on multiple system files, including the Security Accounts Manager (SAM) database. A local attacker who already has the ability to execute low-privileged code on a victim system can leverage the misconfigured ACLs to gain arbitrary code execution with SYSTEM privileges, then install programs, view, change or delete data, or create new accounts with full user rights. Affected products per the CPE data are Windows 10 versions 1809, 1909, 2004, 20H2 and 21H1, with related coverage noting the bug impacts all Windows 10 versions released in the past 2.5 years and also references Windows 11. CISA added the flaw to its Known Exploited Vulnerabilities Catalog on 2022-02-10, confirming in-the-wild exploitation, and EPSS assigns a 67.3% probability of exploitation within 30 days (99th percentile). Mitigation is two-step: installing the security update alone is not sufficient — administrators must also manually delete all shadow copies of system files, including the SAM database, per KB5005357.

Do: Apply Microsoft's security update per vendor instructions, then follow KB5005357 to manually delete all shadow copies of system files (including the SAM database), because the update alone does not fully mitigate the vulnerability. Restrict or verify ACLs on the System32 config directory and shadow-copy access if shadow-copy deletion cannot be done immediately, and prioritize patching given the flaw's listing in CISA's Known Exploited Vulnerabilities Catalog.

CVE-2021-36942 is a spoofing flaw in the Windows Local Security Authority (LSA), widely known as "PetitPotam," that lets an unauthenticated network attacker trick a Windows host into authenticating with NTLM to a machine the attacker controls. It is triggered remotely with no privileges and no user interaction (CVSS 3.1 AV:N/AC:L/PR:N/UI:N) by sending crafted requests that coerce the target system to authenticate. By relaying that coerced authentication to other services, an attacker can impersonate the machine — most critically a domain controller — and escalate toward domain-administrator access, producing a high confidentiality impact. All listed Windows Server releases are affected, with domain controllers and certificate-services servers as the highest-value targets. The flaw is actively exploited in the wild: it was added to CISA's KEV on 2021-11-03 with known ransomware use (LockFile and Babuk campaigns chained it with Exchange flaws), and Microsoft has released Windows updates to address it.

Do: Apply Microsoft's Windows updates per vendor instructions, prioritizing domain controllers and servers running Active Directory Certificate Services. As interim hardening, require SMB signing and restrict NTLM authentication per Microsoft guidance, and review authentication logs for unexpected NTLM connections from domain controllers to certificate-services endpoints.