CVE-1999-0839 in Internet Explorer
Summary
by MITRE
Windows NT Task Scheduler installed with Internet Explorer 5 allows a user to gain privileges by modifying the job after it has been scheduled.
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Analysis
by VulDB Data Team • 04/19/2026
The vulnerability described in CVE-1999-0839 represents a critical privilege escalation flaw within the Windows NT Task Scheduler component that was bundled with Internet Explorer 5. This issue stems from inadequate access control mechanisms within the task scheduling system, allowing unauthorized users to modify scheduled tasks and subsequently execute malicious code with elevated privileges. The vulnerability specifically affects systems running Windows NT 4.0 and earlier versions where the Task Scheduler service operates with insufficient security boundaries between user and system contexts.
The technical root cause of this vulnerability lies in the improper implementation of access controls within the Windows NT Task Scheduler. When Internet Explorer 5 was installed, it included a version of the Task Scheduler that failed to properly validate user permissions before allowing modifications to scheduled jobs. This design flaw creates a scenario where a local user can manipulate task configurations after they have been scheduled, potentially enabling them to replace legitimate executable files with malicious payloads or modify task parameters to execute code with higher privileges than originally intended. The vulnerability operates at the system level and specifically targets the Windows NT Task Scheduler service which runs with elevated privileges.
The operational impact of this vulnerability is severe as it provides attackers with a straightforward path to privilege escalation without requiring additional exploitation techniques. An attacker who gains access to a low-privilege user account can leverage this vulnerability to modify scheduled tasks and execute malicious code with system-level privileges. This capability significantly undermines the security model of Windows NT systems and can lead to complete system compromise. The vulnerability is particularly dangerous because it can be exploited silently without requiring user interaction or additional attack vectors, making it an attractive target for malicious actors seeking persistent system access.
Mitigation strategies for CVE-1999-0839 should focus on implementing proper access control mechanisms and restricting user permissions within the Task Scheduler. System administrators should disable unnecessary scheduled tasks and ensure that only authorized personnel have the ability to modify task configurations. The recommended approach includes applying the latest security patches from Microsoft, which address the underlying access control issues in the Task Scheduler component. Additionally, implementing the principle of least privilege and regularly auditing scheduled tasks can help prevent unauthorized modifications. Organizations should also consider implementing application whitelisting policies to restrict which executables can be scheduled or modified within the task scheduler environment. This vulnerability aligns with CWE-276, which addresses incorrect permissions for critical resources, and represents a classic example of privilege escalation through inadequate access controls that maps to ATT&CK technique T1068, which covers privilege escalation through local exploits.
The vulnerability demonstrates the importance of proper security design principles in system components and highlights how seemingly benign features can become security risks when access controls are not properly implemented. This issue serves as a historical example of how bundled software components can introduce security weaknesses that persist across multiple versions of operating systems. The impact extends beyond immediate privilege escalation to include potential data compromise and system availability issues, as attackers can modify scheduled tasks to maintain persistence or execute destructive operations. Security professionals should consider this vulnerability when assessing legacy Windows NT systems and ensure that proper access controls are in place to prevent similar issues in modern systems.