It seems that Smith’s technique provides yet another way for cybercriminals to make their way into Windows. The regsvr32 component already has a history of providing backdoors it was the path used by the NeverQuest banking Trojan. The proof of concept can open a backdoor or a reverse shell over HTTP. Smith wrote a PowerShell server to handle execution and return output. It has been touted by Microsoft to be of better security than EMET, but Smith’s research revealed it seems to be rather vulnerable to exploits. It was introduced to specify which users can run apps within an organization. It is available for Windows 7 and Windows Server 2008 R2 or later. ocx).”ĪppLocker is one of the primary tools in Windows that enforces security. What Is AppLocker AppLocker is a security feature that prevents users from running unknown files on their computer. “These rules could be enforced for specific users or groups and could be used for executable files (.exe and. “When AppLocker was introduced in Windows 7 and Windows Server 2008 R2, Microsoft provided administrators with the ability to set rules to allow or deny applications from running,” SecurityWeek explains. In order to trigger the bypass, the code block, which can be either Visual Basic or JavaScript, is placed inside the element.Īdditionally, the COM object the script references never shows up in the Registry. sct file at an arbitrary but controlled location works just fine. He also discovered that regsvr32.exe can accept a URL for a script hosting the. He then unregistered the workstation using this code inside the Registration tag.įurthermore, he found regsvr32 is already proxy-aware, uses TLS, follows redirects and is a signed Microsoft binary. With some further research, he discovered that the code in the registration element executes on register and unregister. Smith found that if he placed the script block inside of the Registration tag and called regsvr32, the code would execute. Through his efforts, he found he could register his script to bypass AppLocker but still had to instantiate the object to trigger the code execution. He needed a reverse shell on a workstation that was locked down by the Windows AppLocker executable and the script rules that it enforced. Security researcher Casey Smith was trying to solve a particular problem and came up with a unique solution.
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