Microsoft has revealed that hackers viewed some of its source code as part of the SolarWinds attack that government agencies are still investigating.

The SolarWinds attack is one of the most devastating cyberattacks perpetrated against US companies and government agencies. Believed to be the work of Russian hackers, the attack was a supply chain attack, compromising SolarWind’s Orion IT monitoring and management software.

As one of the organizations impacted, Microsoft has now revealed the hackers viewed some of its source code, but did not make any modifications.

We detected unusual activity with a small number of internal accounts and upon review, we discovered one account had been used to view source code in a number of source code repositories. The account did not have permissions to modify any code or engineering systems and our investigation further confirmed no changes were made. These accounts were investigated and remediated.

Microsoft is not concerned about the source code being viewed, since the company’s security protocols assume its source is being viewed by outside elements.

At Microsoft, we have an inner source approach – the use of open source software development best practices and an open source-like culture – to making source code viewable within Microsoft. This means we do not rely on the secrecy of source code for the security of products, and our threat models assume that attackers have knowledge of source code. So viewing source code isn’t tied to elevation of risk.

As with many companies, we plan our security with an “assume breach” philosophy and layer in defense-in-depth protections and controls to stop attackers sooner when they do gain access.

Although Microsoft seems to be containing any damage adequately, the degree to which the attackers compromised one of the biggest tech companies in the world is further evidence just how successful the SolarWinds attack was.