Windows Flaw Exploited to Deliver PowerShell Backdoor

A local privilege escalation vulnerability patched recently by Microsoft in its Windows operating system had been exploited by malicious actors to deliver a PowerShell backdoor.

The flaw, tracked as CVE-2019-0859, was fixed by Microsoft with its April 2019 Patch Tuesday updates. According to the company, the issue exists due to the way the Win32k component in Windows handles objects in memory. The weakness allows an authenticated attacker to execute arbitrary code in kernel mode.

Microsoft has credited researchers at Kaspersky Lab for reporting the flaw and the cybersecurity firm on Monday made details of the vulnerability public. The security hole was reported to the vendor on March 17.

Kaspersky says the vulnerability is caused by a use-after-free bug. It has been used by an unnamed threat actor to execute PowerShell, Microsoft’s task automation and configuration management framework, in an effort to download a second-stage script from Pastebin. This second-stage script runs another script that unpacks shellcode and executes it.

The shellcode creates an HTTP reverse shell that allows the attackers to take complete control of the targeted device.

“The exploit we found in the wild was targeting 64-bit versions of Windows (from Windows 7 to older builds of Windows 10) and exploited the vulnerability using the well-known HMValidateHandle technique to bypass ASLR,” the researchers said in a blog post.

Kaspersky experts have pointed out that the use of PowerShell to create a backdoor makes the attack stealthier and increases its chances of evading detection. Another benefit of using PowerShell is that it saves the attackers time as they have to write less code.

This was the fifth Windows zero-day identified by Kaspersky in recent months. The company has not shared any information about the threat actor exploiting CVE-2019-0859. The previously reported vulnerabilities had been exploited mainly in targeted attacks by several groups, including by ones tracked by Kaspersky as FruityArmor and SandCat.

Another Windows vulnerability (CVE-2019-0803), similar to the one identified by Kaspersky, was also patched this month by Microsoft. That flaw, reported to the company by Donghai Zhu of the Alibaba Cloud Intelligence Security Team, had also been exploited, but no details have been made available about the attacks.

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