Microsoft PowerShell Improper Input Validation Vulnerability (CVE-2026-26143)

CVE-2026-26143 describes a vulnerability in Microsoft PowerShell stemming from improper input validation. This flaw could allow a local, unauthorized attacker to bypass security features implemented within PowerShell. The vulnerability has a CVSS v3.1 score of 7.8, indicating a high severity. Successful exploitation could lead to significant compromise of the affected system. The vulnerability was reported to Microsoft and assigned CVE-2026-26143. Defenders should prioritize patching affected systems to mitigate the risk. The affected versions of PowerShell are not explicitly stated in the source material, therefore all installations of PowerShell on Windows should be considered potentially vulnerable.

Attack Chain

1. Attacker gains local access to a Windows system. This could be through existing malware, physical access, or other initial access vectors.
2. Attacker crafts a malicious PowerShell command or script designed to exploit the input validation vulnerability (CVE-2026-26143).
3. The attacker executes the malicious PowerShell command, bypassing intended security controls due to the input validation flaw.
4. PowerShell processes the crafted input, failing to properly sanitize or validate it.
5. The bypassed security feature allows the attacker to perform actions that would normally be restricted, such as elevated privileges.
6. Attacker leverages the bypassed security feature to execute unauthorized code or modify system configurations.
7. The attacker can now maintain persistence via registry keys (T1547.001) or scheduled tasks (T1053.005).
8. The attacker achieves their objective, which could include data exfiltration, system compromise, or further lateral movement within the network.

Impact

Successful exploitation of CVE-2026-26143 can allow a local attacker to bypass security features within Microsoft PowerShell, potentially leading to arbitrary code execution with elevated privileges. This vulnerability could lead to a full system compromise. The number of potential victims is substantial, as PowerShell is a standard component of Windows operating systems. Systems lacking the security patch are vulnerable.

Recommendation

• Apply the security update provided by Microsoft for CVE-2026-26143 to remediate the improper input validation vulnerability.
• Implement the Sigma rule “Detect Suspicious PowerShell Input Validation Bypass” to identify potential exploitation attempts in your environment.
• Monitor PowerShell execution logs for suspicious command-line arguments and script content, which could indicate an attempt to exploit this vulnerability.
• Restrict local user access to reduce the attack surface and limit the potential for local exploitation.
• Enable PowerShell logging and auditing to capture detailed information about PowerShell activity, which can aid in detecting and investigating suspicious behavior.