Researchers unearth Windows backdoor that’s unusually stealthy

Researchers have discovered a clever piece of malware that stealthily exfiltrates data and executes malicious code from Windows systems by abusing a feature in Microsoft Internet Information Services (IIS).

IIS is a general-purpose web server that runs on Windows devices. As a web server, it accepts requests from remote clients and returns the appropriate response. In July 2021, network intelligence company Netcraft said there were 51.6 million instances of IIS spread across 13.5 million unique domains.

IIS offers a feature called Failed Request Event Buffering that collects metrics and other data about web requests received from remote clients. Client IP addresses and port and HTTP headers with cookies are two examples of the data that can be collected. FREB helps administrators troubleshoot failed web requests by retrieving ones meeting certain criteria from a buffer and writing them to disk. The mechanism can help determine the cause of 401 or 404 errors or isolate the cause of stalled or aborted requests.