Microsoft has taken steps to stop a China-based hacking group from using its TechNet website as part of its attack infrastructure, according to security vendor FireEye.

The group, which FireEye calls APT (advanced persistent threat) 17, is well-known for attacks against defense contractors, law firms, U.S. government agencies and technology and mining companies.

TechNet is highly trafficked website that has technical documentation for Microsoft products. It also has a large forum, where users can leave comments and ask questions.

APT17—nicknamed DeputyDog—created accounts on TechNet and then left comments on certain pages. Those comments contained the name of an encoded domain, which computers infected by the group’s malware were instructed to contact.

The encoded domain then referred the victim’s computer to a command-and-control server that was part of APT17’s infrastructure, said Bryce Boland, FireEye’s chief technology officer for Asia-Pacific.

The technique of requiring an infected computer to contact an intermediary domain is frequently used. Often, hackers want the infected machines to reach out to a domain that is unlikely to look suspicious before proceeding to another less-reputable one.

“It’s completely normal to see a lot of traffic going to TechNet,” Boland said.

Sometimes, the command-and-control domains are embedded in the malware itself, but that makes it easy for computer security researchers to figure out which ones it contacts. Other times, malware is coded with an algorithm that generates possible domains names it should contact, but that can also be reverse engineered by analysts, Boland said.

Security experts have seen attackers abuse other legitimate domains and services, such as Google Docs and Twitter, to accomplish the same goal as APT17, Boland said.

“This is a challenge for any open platform,” he said.

FireEye and Microsoft replaced the encoded domains on TechNet with ones the companies controlled, which gave them a glimpse of the problem when infected machines called out to those domains.

APT17 has been “targeting our customers for many years,” Boland said. Organizations are typically targeted through spear-phishing, which involves sending emails with malicious links or attachments, he said.

For the last couple of years, APT17 has infected computers with a malware program that FireEye calls BLACKCOFFEE. The malware can upload files, delete files and create a reverse shell on a computer, among other functions.