https://t.co/1Of8EsOW8z Here's a low quality bug that is a pain to exploit.. still unpatched. I'm done with all this anyway. Probably going to get into problems because of being broke now.. but whatever. — SandboxEscaper (@SandboxEscaper) October 23, 2018

SandboxEscaper, a researcher who back in August tweeted out a Windows privilege escalation bug, has published another unpatched Windows flaw on Twitter.

The new bug has some similarities to the previous bug. Windows services usually run with elevated privileges. Sometimes they perform actions on behalf of a user, and to do this they use a feature called impersonation. These services act as if they were using a particular user's set of privileges. After they've finished that action, they revert to their normal, privileged identity.

Both this bug and SandboxEscaper's previous bug depend on improper use of impersonation—specifically, the services in question (last time it was Task Scheduler, this time it's the "Data Sharing Service") revert their impersonation too quickly and end up performing some actions with elevated privileges when they should in fact have been impersonated. The last bug allowed one file to be written over another. In this case, it's a call to delete a file that is improperly impersonated, ultimately giving regular unprivileged users the ability to delete any file on the system, even those that they should have no access to.

The new bug appears to have an important timing aspect to it; two actions must happen simultaneously to make the impersonation end prematurely. SandboxEscaper says that because of this, exploitation on a single core machine seems unlikely, but multicore machines are vulnerable. SandboxEscaper's proof of concept, published on GitHub, will attempt to delete Windows' PCI driver. As such, we wouldn't recommend running it on any system that you care about because it's not going to be able to boot once that file has been removed.

Data Sharing Service was only introduced with Windows 10, so the bug only affects Windows 10, Windows Server 2016, and Windows Server 2019.

The previous bug was later used by malicious parties in their malware. The new bug will be harder to exploit in that way, as the ability to delete files is less useful than the ability to overwrite files.