Security Experts:

VMware Tools Flaw Allowed Code Execution via DLL Hijacking

An advisory published by VMware on Thursday describes two important vulnerabilities that affected several of the company’s products.

The first security hole, tracked as CVE-2016-5330, is a DLL hijacking issue in the Windows version of VMware Tools. The flaw can be exploited to execute arbitrary code on the targeted system.

The vulnerability was reported to VMware late last year by Yorick Koster, researcher and co-founder of Dutch security firm Securify. Koster told SecurityWeek that the issue was addressed by the vendor in April, but it was not disclosed until now to give users enough time to patch.

According to Koster, the flaw is related to the VMware Host Guest Client Redirector component of VMware Tools. The component is used for the Shared Folders feature, which allows users to share files between the guest and the host operating system.

The researcher noticed that when a document is opened from a uniform naming convention (UNC) path, the Client Redirector injects a DLL named “vmhgfs.dll” into the application that is used to open the file. Since the DLL was loaded from a relative path, Windows searched for it using the dynamic-link library search order.

This allowed an attacker to place a malicious DLL in a location from where it would likely be loaded before the legitimate file. By getting the Client Redirector to load the malicious DLL into the application, an attacker could have executed arbitrary code with the privileges of the targeted user. An attack could have resulted in the system getting completely compromised.

For the attack to work, the hacker needed to trick the victim into opening any document from the share containing the malicious DLL file. The researcher also believes the attack could have been launched over the Internet if the WebDAV Mini-Redirector was enabled.

Mini-Redirector is a Windows WebDAV client that allows users to access remote shares over the Internet as if they were on the local network. An attacker could have created their own malicious website with WebDAV enabled and use it to host bait documents and the malicious DLL. The vulnerability could have been exploited by luring the victim to the malicious website and getting them to open one of the documents.

Koster said VMware addressed the vulnerability by ensuring that the DLL is loaded from an absolute path. The flaw affects VMware vSphere Hypervisor (ESXi), Workstation Player and Pro, and Fusion.

Another vulnerability disclosed on Thursday by VMware is an HTTP header injection issue affecting vCenter Server and ESXi. The flaw, caused by lack of input validation, allows an attacker to set arbitrary HTTP response headers and cookies, and launch cross-site scripting (XSS) or malicious redirect attacks.

The security hole, tracked as CVE-2016-5331, was reported to VMware independently by several researchers.

Related: VMware Patches Critical Flaw in NSX, vCNS Products

Related: VMware Updates Products to Patch Critical, Important Flaws

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.