CVE-2019-0995 in Internet Explorer
Summary
by MITRE
A security feature bypass vulnerability exists when urlmon.dll improperly handles certain Mark of the Web queries, aka 'Internet Explorer Security Feature Bypass Vulnerability'.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 06/10/2020
The vulnerability identified as CVE-2019-0995 represents a critical security feature bypass in Microsoft Internet Explorer's urlmon.dll component that allows attackers to circumvent important web security protections. This flaw specifically manifests when the system processes certain Mark of the Web queries, which are mechanisms designed to identify and restrict content downloaded from the internet to prevent unauthorized access and execution. The issue stems from improper handling of these security markers that should normally enforce strict access controls and sandboxing behaviors for internet-based content.
The technical implementation of this vulnerability resides within the urlmon.dll library which serves as a core component for handling internet protocol operations in Windows systems. When urlmon.dll encounters specific Mark of the Web queries, it fails to properly validate or enforce security boundaries that would normally prevent local file system access or elevated privilege operations. This improper handling creates a pathway for malicious actors to bypass security restrictions that should normally prevent downloaded content from executing with elevated privileges or accessing restricted system resources. The flaw essentially allows attackers to manipulate the security context in which downloaded content operates, potentially enabling privilege escalation or unauthorized system access.
From an operational impact perspective, this vulnerability poses significant risks to enterprise environments and individual users alike. Attackers can exploit this bypass to execute malicious code with higher privileges than intended, potentially leading to complete system compromise. The vulnerability affects systems running vulnerable versions of Internet Explorer and Windows, with particular concern for environments where users frequently download and execute content from untrusted sources. Security researchers have noted that this flaw can be leveraged to bypass modern security controls such as Data Execution Prevention, Application Control policies, and other sandboxing mechanisms that should normally protect against malicious downloaded content.
The vulnerability aligns with CWE-284, which addresses improper access control issues, and can be mapped to ATT&CK technique T1059.001 for command and scripting interpreter usage, as well as T1068 for exploit for privilege escalation. Organizations should implement immediate mitigations including disabling Internet Explorer's automatic activation of ActiveX controls, implementing strict group policy controls to restrict Mark of the Web behavior, and ensuring all systems are updated with the latest security patches. Microsoft released security updates addressing this vulnerability through the Windows Update mechanism, and organizations should prioritize deployment of these patches. Additional protective measures include network segmentation, implementing strict web content filtering policies, and educating users about the risks of downloading and executing content from untrusted sources. The vulnerability demonstrates the critical importance of proper security boundary enforcement in system components that handle internet-based content and highlights the need for comprehensive security testing of core system libraries that interface with external content sources.