Windows Task Scheduler Vulnerabilities: Exploitation and Mitigation Strategies

Introduction: Recent vulnerabilities identified in the Windows Task Scheduler's

schtasks.exe

present significant security risks, allowing attackers to bypass User Account Control (UAC), manipulate metadata, alter event logs, and evade detection. This report provides a detailed analysis of these vulnerabilities, their exploitation in the wild, and strategic recommendations for mitigation.

Vulnerability Overview: The vulnerabilities primarily affect the Windows Task Scheduler's handling of scheduled tasks. Key issues include:

1. UAC Bypass via Credential-Based Exploitation:
2. Vulnerability allows attackers with local administrator credentials to execute commands with SYSTEM privileges without triggering a UAC prompt.
3. Exploitation involves creating a scheduled task using Batch Logon authentication (via

   /ru

   and

   /rp

   flags) instead of an Interactive Token.

4. Attackers can elevate privileges using known administrator passwords, NTLMv2 hash captures, or vulnerabilities like CVE-2023-21726, which stored cleartext credentials in the Windows registry.

5. Defense Evasion Techniques:

6. Scheduled Task Metadata Poisoning:
   • Exploits XML-based task registration to modify the

     Author

     tag in the metadata, misleading security tools by impersonating trusted entities such as "Administrator."
7. Task Event Log Poisoning and Overflow:
   • Manipulates the Windows Event Log (Event ID 4698) to overwrite critical task details, erasing evidence of malicious activity.
   • Security Logs Saturation technique achieves CWE-117 (Improper Output Neutralization) and CWE-400 (Uncontrolled Resource Consumption) by overflowing the Event Log's default size limit.

Exploitation in the Wild: - The vulnerabilities enable attackers to maintain persistence, escalate privileges, execute arbitrary code, and move laterally within networks. - Techniques align with several MITRE ATT&CK tactics: Persistence, Privilege Escalation (T1548.002), Execution (T1053.005), Lateral Movement (T1076), and Defense Evasion (T1070.001).

Mitigation Strategies: - Apply strict access controls and monitor Task Scheduler activities for anomalies. - Disable NTLM where possible and transition to Kerberos authentication. - Enforce least-privilege principles to limit potential exploitation. - Regularly update systems and apply patches to minimize the attack surface. - Utilize advanced monitoring solutions to detect and respond to sophisticated evasion tactics.

References: - GBHackers on Security - Windows Task Scheduler Vulnerabilities - MITRE ATT&CK Framework: T1548.002, T1053.005, T1076, T1070.001

Final Notes:

The Windows Task Scheduler vulnerabilities highlight the importance of robust cybersecurity measures. Organizations are encouraged to implement the recommended strategies to mitigate the risks associated with these vulnerabilities and enhance their overall security posture.

This detailed report is intended for Rescana's customers to provide insights into the specific vulnerabilities affecting Windows Task Scheduler and offer targeted mitigation advice based on current threat intelligence.