August 2018 Patch Tuesday: Microsoft fixes two actively exploited zero-days

CVE-2018-8174 is an out-of-bounds write (CWE-787) in the Microsoft Windows VBScript engine, caused by the way it handles objects in memory. An attacker triggers it by convincing a user to visit a specially crafted website or open crafted content that invokes the VBScript engine (for example via Internet Explorer or a document preview), requiring user interaction. Successful exploitation yields remote code execution with the privileges of the logged-on user, enabling program installation, data theft and account takeover. All listed Windows client and server releases are affected: Windows 7, 8.1, RT 8.1, Windows 10 (1607-1803), and Windows Server 2008/2008 R2, 2012/2012 R2, 2016. Exploitation is in the wild: the flaw was fixed in the May 2018 Patch Tuesday, is listed in CISA KEV with known ransomware use, and public PoCs (0patch, ExploitDB 44741) and exploit kit usage have been documented; EPSS puts its 30-day exploitation probability at 88.5%.

Do: Apply Microsoft's May 2018 security updates (and any later cumulative or Extended Security Updates) to every listed Windows client and server release, as required by the CISA KEV listing, prioritizing internet-reachable and user-facing systems given known ransomware use. Upgrade out-of-support platforms (Windows 7/8.1/RT 8.1, Server 2008/2008 R2, 2012/2012 R2) to supported builds or ensure ESU coverage. As interim mitigation, block VBScript execution in Internet Explorer web zones using Microsoft's documented Group Policy/registry settings, and hunt for prior exploitation on legacy systems.

A security feature bypass vulnerability exists when Active Directory Federation Services (AD FS) improperly handles multi-factor authentication requests, aka "AD FS Security Feature Bypass Vulnerability." This affects Windows Server 2016, Windows Server 2012 R2, Windows 10 Servers.

A remote code execution vulnerability exists in Microsoft Windows that could allow remote code execution if a .LNK file is processed, aka "LNK Remote Code Execution Vulnerability." This affects Windows 7, Windows Server 2012 R2, Windows RT 8.1, Windows Server 2008, Windows Server 2012, Windows 8.1, Windows Server 2016, Windows Server 2008 R2, Windows 10, Windows 10 Servers. This CVE ID is unique from CVE-2018-8346.

CVE-2018-8373 is an out-of-bounds write (CWE-787) in the Microsoft scripting engine's handling of objects in memory, which can corrupt memory and enable remote code execution in Internet Explorer 9, 10, and 11. It is triggered when a user is lured to an attacker-crafted web page or script in IE, with no privileges required but user interaction and relatively high attack complexity per the CVSS vector (AV:N/AC:H/UI:R). A successful attacker gains arbitrary code execution in the context of the current user, compromising that workstation's data and credentials. Any Windows system whose users browse with Internet Explorer 9, 10, or 11 was affected, and Microsoft fixed the flaw in its August 2018 Patch Tuesday release. The flaw was exploited as a zero-day in the wild at the time of patching — press coverage describes an in-the-wild VBScript zero-day blocked by endpoint protection — and it was added to CISA's Known Exploited Vulnerabilities catalog on 2022-03-25; EPSS places the 30-day exploitation probability at 61.9% (99th percentile).

Do: Apply Microsoft's August 2018 security updates for Internet Explorer on all supported Windows versions — the required action listed in the CISA KEV — and verify patch deployment via WSUS/SCCM/Intune across end-user workstations and RDS/browsing hosts. As interim mitigation, consider Microsoft's documented workaround of disabling VBScript execution in IE via feature-control keys and steer users away from IE for web browsing. Finally, migrate any remaining IE9/10/11 usage to Microsoft Edge (using IE mode for legacy sites), since IE11 is retired and this flaw is confirmed exploited in the wild.

Microsoft's Windows Shell improperly validates file paths in certain Windows 10 and Windows Server releases, a remote code execution flaw rooted in improper input validation (CWE-20). The flaw is triggered when the shell processes a specially crafted file path, typically requiring the victim to interact with a malicious file, as reflected in the CVSS user-interaction (UI:R) requirement. Successful exploitation lets an attacker execute arbitrary code in the context of the current user, with high impact on confidentiality, integrity, and availability. Affected products are Windows 10 versions 1703, 1709, and 1803 and Windows Server (Semi-Annual Channel) versions 1709 and 1803. The bug was patched in Microsoft's August 2018 Patch Tuesday after being reported as one of two zero-days actively exploited in attacks in the wild, was later added to CISA's Known Exploited Vulnerabilities catalog on 2022-03-25, and EPSS currently estimates a 74% probability of exploitation within 30 days.

Do: Apply Microsoft's August 2018 security updates or later cumulative updates for Windows 10 1703/1709/1803 and Windows Server 1709/1803, consistent with Microsoft guidance and CISA's KEV required action; because these builds are now legacy, upgrading to a currently supported Windows release is the durable fix. Until patched, treat untrusted files and shortcuts with caution since exploitation requires user interaction. Inventory endpoints for the affected builds to confirm they are fully remediated.