As the infosec community talked about potential cyber attacks leveraging vulnerabilities in antivirus products, Microsoft took notes and started to work on a solution. The company announced that its Windows Defender can run in a sandbox.
Antivirus software runs with the highest privileges on the operating system, a level of access coveted by any threat actor, so any exploitable vulnerabilities in these products add to the possibilities of taking over the system.
By making Windows Defender run in a sandbox, Microsoft makes sure that the security holes its product may have stay contained within the isolated environment; unless the attacker finds a way to escape the sandbox, which is among the toughest things to do, the system remains safe.
Remote code execution flaws
Windows Defender has seen its share of vulnerability reports. Last year, Google's experts Natalie Silvanovich and Tavis Ormandy announced a remote code execution (RCE) bug severe enough to make Microsoft release an out-of-band update to fix the problem.
In April this year, Microsoft patched another RCE in Windows Defender, which could be abused via a specially crafted RAR file. When the antivirus got to scanning it, as part of its protection routine, the would trigger, giving the attacker control over the system in the context of the local user.
Microsoft is not aware of any attacks in-the-wild actively targeting or exploiting its antivirus solution but acknowledges the potential risk hence its effort to sandbox Windows Defender.
Turn on sandboxing for Windows Defender
The new capability has been gradually rolling out for Windows Insider users for test runs, but it can also be enabled on Windows 10 starting version 1703.
Regular users can also run Windows Defender in a sandbox if they have the operating system version mentioned above. They can do this by enabling the following system-wide setting from the Command Prompt with admin privileges:
setx /M MP_FORCE_USE_SANDBOX 1
Restarting the computer is necessary for the setting to take effect. Reverting the setting is possible by changing the value for forcing sandboxing to 0 (zero) and rebooting the system.
Sandboxing Windows Defender
Forcing an antivirus product to work from an insulated context is no easy thing to do due to the app's need to check a large number of inputs in real time, so access to these resources is an absolute requirement. An impact on performance is a likely effect of this.
"It was a complex undertaking: we had to carefully study the implications of such an enhancement on performance and functionality. More importantly, we had to identify high-risk areas and make sure that sandboxing did not adversely affect the level of security we have been providing," the official announcement reads.
Despite the complexity of the task, Microsoft was not the first to sandbox Windows Defender. Last year, experts from security outfit Trail of Bits, who also specialize in virtualization, created a framework that could run Windows applications in their own containers. Windows Defender was one of the projects that Trail of Bits was able to containerize successfully and open-sourced it.
AVs are as susceptible to flaws as other software
Despite their role on the operating system, security products are susceptible to flaws just like other complex software. Windows Defender is definitely not the only one vulnerable.
In 2008, security researcher Feng Xue talked at BlackHat Europe about techniques for finding and exploiting vulnerabilities in antivirus software, referencing bugs as old as 2004.
Xue pointed out that the flaws in this type of software stem from the fact that it has to deal with hundreds of files types that need to be checked with components called content parsers. A bug in one parser could represent a potential path on the protected system.
Six years later, another researcher, Joxean Koret, took the matter further and showed just how vulnerable are the defenders of the computer systems, and let the world know that exploiting them "is not different to exploiting other client-side applications."
His analysis at the time on 14 antivirus solutions on the market revealed dozens of vulnerabilities that could be exploited remotely and locally, including denial of service, privilege escalation, and arbitrary code execution. His list included big names like Bitdefender and Kaspersky.
Antivirus developers do not leave their customers high and dry and audit their products constantly. The result is patching any of the bugs discovered during the code review and improving the quality assurance process for finer combing for potential flaws.