Slack Technologies makes its debut on the New York Stock Exchange on Thursday through a direct listing of its shares. The company named Microsoft as its ‘primary competitor’ in its regulatory filings. (NYSE Photo)

As Slack makes its stock market debut, there’s a major company that won’t be allowing its employees to use the business collaboration and chat app as part of their daily work.

It’s Microsoft, and it’s not just because the Redmond giant is Slack’s biggest competitor through its own Microsoft Teams collaboration app. At least, that’s not the primary stated reason. And it turns out Slack is far from the only piece of popular technology to earn this distinction.

RELATED: Slack vs. Microsoft: Inside the competition to reshape how work gets done

GeekWire obtained an internal Microsoft list of prohibited and discouraged technology — software and online services that the company doesn’t want its employees using as part of their day-to-day work. We first picked up on rumblings of the prohibition from Microsoft employees who were surprised that they couldn’t use Slack at work, before tracking down the list and verifying its authenticity.

While the list references the competitive nature of these services in some situations, the primary criteria for landing in the “prohibited” category are related to IT security and safeguarding company secrets.

Slack is on the “prohibited” category of the internal Microsoft list, along with tools such as the Grammarly grammar checker and Kaspersky security software. Services in the “discouraged” category include Amazon Web Services, Google Docs, PagerDuty and even the cloud version of GitHub, the popular software development hub and community acquired by Microsoft last year for $7.5 billion.

Here’s the full description from the Slack entry on the list.

Slack Free, Slack Standard and Slack Plus versions do not provide required controls to properly protect Microsoft Intellectual Property (IP). Existing users of these solutions should migrate chat history and files related to Microsoft business to Microsoft Teams, which offers the same features and integrated Office 365 apps, calling and meeting functionality. Learn more about the additional features that Teams can provide your workgroup. Slack Enterprise Grid version complies with Microsoft security requirements; however, we encourage use of Microsoft Teams rather than a competitive software.

Slack and Microsoft declined to comment in response to our inquiries.

The security justifications distinguish the situation, to an extent, from the past era when Microsoft frowned upon employees using competing technologies, such as iPhones when Microsoft aspired to compete with Apple in smartphone hardware. At a company meeting during his tenure as CEO, Steve Ballmer once famously snatched an iPhone from an employee and pretended to stomp on it.

Since taking the reins five years ago, Microsoft CEO Satya Nadella has espoused a “learn it all” philosophy that encourages employees to understand and adapt to new viewpoints and information. Under his leadership, the company has struck a series of partnerships with longtime rivals.

Of course, Microsoft still competes energetically. In the competition with Slack, Microsoft has the benefit of decades of experience in enterprise software. It touts the security and compliance features of Microsoft Teams as a selling point for its big business customers. Slack launched its Enterprise Grid version in 2017 as a way of catering to many of these same customers.

The inclusion of Slack on Microsoft’s “prohibited” list is notable in part because Slack in April unveiled a series of integrations with Microsoft Office 365. However, Slack didn’t work directly with Microsoft on the initiative, instead relying on the tech giant’s APIs and other services for integrating products.

Here are some of the other notable services and technologies from Microsoft’s list of discouraged and prohibited software.

  • GitHub is perhaps the most surprising inclusion on the “discouraged” list, given Microsoft’s ownership of the coding repository. However, it’s not an outright ban. Microsoft cautions employees not to use the cloud version of GitHub “for Highly Confidential types of information, specs or code.” The on premises version of GitHub does not appear on the list.
  • Amazon Web Services, which competes with Microsoft’s Azure cloud platform, and Google Docs, which competes with Office 365, are both “discouraged for use” and “will require a business justification” to be used, according to the internal Microsoft summary. “It is highly recommended to start a migration plan to Azure prior to engaging the Governance team for new request or renewals,” the summary reads.
  • Grammarly, the writing and grammar-checking technology, is on the “prohibited” list. “The Grammarly Office add-in and browser extensions should not be used on the Microsoft network because they are able to access Information Rights Management (IRM) protected content within emails and documents,” Microsoft said, cautioning that this could lead to the exposure of sensitive data, and noting that Microsoft security is evaluating what can be done to make the technology secure for use within the company.

In many ways, the precautions make sense in an era of widespread security concerns, with tech companies at risk of sensitive data being intercepted.

“It’s not just the risk that Google will try to find trade secrets from data stored on their servers,” said Christopher Budd, who has worked in security technology for 20 years, including past roles in Microsoft security and privacy communications. “When you’re at Microsoft, you’re at risk of state sponsored industrial espionage. These days we generally think of hacking in criminal or traditional geopolitical espionage. But industrial espionage is still out there and brings the full force of nation-state hacking to bear.”

It makes sense that Microsoft would want to keep its people inside the ecosystem, but it’s also a risk. Slack is a hit among developers, engineers and other tech personnel. Putting too many restrictions on the tools they’re able to use risks rubbing those highly valuable workers the wrong way, which can have implications in a competitive recruiting environment

Microsoft Office vice president Kirk Koenigsbauer kicks off the Microsoft Teams launch event in Redmond in 2017. (GeekWire Photo / Todd Bishop)

The competition between Microsoft and Slack has been simmering for more than two years, dating back to when Microsoft launched Teams in 2017. Slack took the unusual step of placing a full-page ad in The New York Times both congratulating the tech giant and warning that “all this is harder than it looks.”

Slack listed Microsoft as its “primary competitor” in an IPO document in April. Slack CEO Stewart Butterfield has acknowledged the challenge it faces going up against Microsoft. It’s the most valuable company in the U.S., and “if they start channeling all their resources against you, that’s a lot to compete with,” Butterfield told Business Insider in 2017.

The competition is a two-way street. Last summer, Microsoft acknowledged the rivalry publicly, officially adding Slack to its list of competitors in its annual 10-K report.

Slack began its next chapter, and the rivalry with Microsoft, Thursday when it debuted on the New York Stock Exchange. It took the unusual route of listing its shares directly on the exchange, rather than going through intermediaries that buy stocks and sell them to the public.

Slack’s decision paid off out of the gate. The stock jumped more than 50 percent in early trading, pushing Slack’s valuation to more than $20 billion. On its second day of trading, Slack stock is down about 2.5 percent, but still well above its initial price.

Editor’s Note: This story has been updated to clarify that only the cloud version of GitHub is on the “discouraged” list of programs.

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