Windows 10 Sandbox has a Vulnerability inside

A researcher discovered a new zero-day vulnerability in most Windows 10 editions, which allows creating files in restricted areas of the operating system.

Exploiting the flaw is trivial and attackers can use it to further their attack after initial infection of the target host, albeit it works only on machines with Hyper-V feature enabled.

An unprivileged user can create an arbitrary file in ‘system32,’ a restricted folder holding vital files for Windows operating system and installed software.

However, this works only if Hyper-V is already active, something that limits the range of targets since the option is disabled by default and is present in Windows 10 Pro, Enterprise, and Education.

Hyper-V is Microsoft’s solution to creating virtual machines (VM) on Windows 10. Depending on the physical resources available on the host, it can run at least three virtual instances.

Given sufficient hardware resources, Hyper-V can run large VMs with 32 processors and 512GB of RAM. An average user user may not have a use for such a virtual machine but they may run Windows Sandbox, an isolated environment for executing programs or loading websites that are not trusted, without risking to infect the normal Windows operating system.

Microsoft introduced Windows Sandbox with the May 2019 Update, in Windows 10 version 1903. Turning on this feature automatically enables Hyper-V.

To demonstrate the vulnerability researchers created in \system32 an empty file named phoneinfo.dll. Making any changes in this location requires elevated privileges but these restrictions are irrelevant when Hyper-V is active.

The creator of the file is also the owner, an attacker can use this to place malicious code inside that would be execute with elevated privileges when needed.

CERT/CC vulnerability analyst Will Dormann confirmed that the vulnerability exists and that exploiting it requires literally no effort from an attacker on the host.

Although this vulnerability is easy to exploit there are more dangerous issues in Windows 10 that Microsoft should address. This is one reason he decided to make it public and not report it through Microsoft’s bug bounty program.