Windows ALPC Local Elevation of Privilege Vulnerability Technical Advisory (CVE-2018-8440)

1. Vulnerability Overview & Root Cause

This vulnerability exists in the way the Windows operating system handles Advanced Local Procedure Call (ALPC) requests. Specifically, a flaw in the Task Scheduler service's SchRpcRegisterTask API allows a local user to trigger a DACL (Discretionary Access Control List) modification on arbitrary files. Because the Task Scheduler runs with SYSTEM privileges, it improperly processes file operations without verifying the caller's impersonation level, leading to an arbitrary file write/permission modification vulnerability.

2. Exploitation Mechanics & Attack Scenario

An attacker with local access executes the exploit using the following steps:

• The attacker calls the Task Scheduler's ALPC interface to register a temporary task.
• The Task Scheduler service attempts to write a temporary file inside the c:\windows asks directory.
• Before the write operation completes, the attacker replaces the target file path with a hard link pointing to a critical system file (e.g., a system DLL or configuration file).
• The high-privileged Task Scheduler service follows the hard link and modifies the DACL of the target system file, granting write permissions to the low-privilege user.
• The attacker overwrites the target system file with a malicious payload, which is subsequently executed by a high-privileged process, resulting in arbitrary code execution as SYSTEM.

3. Infrastructure & Business Impact

As a vulnerability listed in the CISA KEV catalog, CVE-2018-8440 poses a severe threat to enterprise environments. If an attacker gains initial access to an endpoint or server via phishing or web exploitation, they can immediately leverage this vulnerability to escalate privileges to SYSTEM. This bypasses local security controls, facilitates credential dumping (e.g., LSASS harvesting), and enables lateral movement across the entire corporate network.