The company's vice-president of marketing, Andrew Fife, said while the lack of detail was not a big deal on its own, Microsoft had such a massive range of software that the difference between an attacker gaining access to the first-generation Zune source code and the source for Windows Server 2019 or GitHub, was massive.
"It's reasonable to guess that, if the breached codebase was inconsequential, it would have been mentioned to demonstrate the breach was immaterial," Fife argued.
"Furthermore, holidays are widely known in the PR community to be days to announce bad news. Both timing and lack of detail suggest Microsoft probably viewed this as bad news, which means we probably should too."
Five days later, FireEye published details about attacks using malware which it called SUNBURST; it said this malware had been used to hit both private and public entities, by corrupting the Orion network management software.
On Wednesday AEDT, the alleged attackers put out a notice offering source code from Microsoft, Cisco, the FireEye Red Team tools and multiple SolarWinds products for sale.
At the time of its announcement, Microsoft said: "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." Fife dismissed this statement as disingenuous.
Attackers could still benefit from viewing source code, he said, pointing out three ways by which they could gain.
The first, he said, was by finding hard-coded secrets. "Whether usernames/passwords, API Keys, or tokens, modern applications are built as a mesh network of microservices, libraries, APIs, and SDKs that often require authentication to deliver the core service. It's common for developers to write these secrets directly into the source code on the assumption that only insiders can view them."
And, he added, "While Microsoft claims their 'threat models assume that attackers have knowledge of source code', it would be far more reassuring if they directly addressed whether or not the breached code contained secrets."
Fife said viewing source code could also be useful for those looking to reverse engineer products. "In much the same way that the source code is a software company's IP, it is also the blueprint on how to reverse engineer and exploit an application.
"If I can see your source code, I can trace the routes of inputs to determine which routes have security transforms for appropriate sanitisation (validation, redaction, encryption, etc.) before they hit their intended sink/database. Hence, by viewing Microsoft's source code the attackers now may have an easier time crafting zero-day exploits to compromise Microsoft directly and/or its customers, depending on which codebase was exposed."
Finally, Fife pointed out, the SolarWinds attackers had ensured they would stay unnoticed for more than a year by observing and learning how the company's employees wrote their code and following similar practices.
"Hence, if SolarWinds' developers looked at the malicious code they would most likely assume this code came from another SolarWinds developer and give it far less scrutiny," he said.
"External auditors had even less chance of identifying the attacker's malicious code. The attackers even appear to have tested their malicious code with a benign implant via an empty class that was delivered six months before the real backdoor was distributed to SolarWinds customers."
Fife said his company was rooting for Microsoft. "Not only is it the right thing to do, but what other choice do any of us really have? The consequences of a widespread Microsoft supply chain attack could necessitate an Internet 'shelter in place order'. We hope this is the final chapter of Microsoft's breach, but we fear it may have been reconnaissance for the next bigger operation."