![]() “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. Thank you.Security researcher Casey Smith was trying to solve a particular problem and came up with a unique solution. I’ll monitor this issue and post here should further information become available. Denying this application from running would likely lead to unexpected behavior. While blocking Regsvr32.exe using AppLocker may seem like an obvious solution, this is a legitimate application that is often used by Windows especially during program installation and updates. How Can I Protect Myself From This Issue?Īs mentioned above, at this time there is no known workaround for this bypass. blocking a script from being manually entered at the command prompt is a good example of defence in-depth ( defined)(PDF) security. The enhancements it received in Windows 10 e.g. Since it runs with kernel level privileges ( defined) it isn’t easy for an attacker to shut it down and can be configured to block code that is run by an administrator ( defined) (unless that code is already whitelisted). An introductory post to configuring AppLocker would be this Malwarebytes blog post. I’m a particular fan of AppLocker since it provides a strong defence against zero-day malware ( defined) and ransomware ( defined). I’m hoping that for this new bypass a similar solution can be found. With a known bypass of AppLocker now being disclosed the effectiveness of AppLocker has been significantly reduced. In 2011 a bypass to AppLocker was discovered by Didier Stevens which was later addressed by Microsoft with a hotfix. However, it is uncertain if Microsoft will create a security update or mitigation to address this issue since AppLocker is functioning by design. Why Should This Issue Be Considered Important?Īccording to this ThreatPost article the researcher initially responsibly disclosed ( defined) this issue to Microsoft. I have written about this issue separately using Yammer but will provide more discussion below: ![]() Last week a security researcher made publically available proof of concept code that has the ability to bypass Windows AppLocker (application whitelisting). Re!) for his very useful insights on this topic. My thanks to a colleague (you know who you a Another well-known security researcher Alex Ionescu said that Device Guard (of Windows 10), fully enabled with script protection will block this bypass as well.įurther discussion and advice for this issue are available within this blog post. sct files may be found in the Temporary Internet Files folder. This bypass does not make changes to the Windows registry but. ![]() More information about the conhost.exe process is available in this article. The part of the string we are interested in detecting would be the text after the /i switch. Please refer to the support website of the manufacturer of your firewall or it’s user guide if you are using a 3rd party firewall.Īlternatively, you can create a YARA rule to detect the presence of the following string within the memory of the conhost.exe process that is spawned on Windows 7 and later when a script is executed: Example steps to create rules for the Windows Firewall are located here and here. You can use your installed firewall to do this or use the built-in Windows Firewall to create a rule to do this. These files are usually present in the following directories (folders):Ĭ:\Windows\System32 (32 bit systems only)Ĭ:\Windows\SysWOW64 (64 bit systems only)įor 64 bit systems you should block any regsvr32.exe or regsvr64.exe that you find in both of the above folders. After some further research, this bypass can be blocked by denying regsvr32.exe and regsvr64.exe (depending on your systems architecture) from accessing the internet.
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