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A security researcher has uncovered a flaw in Slack that could've been exploited to steal files over the business messaging app and potentially spread malware.

The flaw involves Slack's Windows desktop app, and how it can automatically send downloaded files to a certain destination—whether it be on your PC or to an online storage server. You can set a download location in the app's preferences section. However, David Wells, a researcher at the security firm Tenable, noticed there's another way to configure the option: Via a special link.

"Crafting a link like 'slack://settings/?update={ 'PrefSSBFileDownloadPath':'}' would change the default download location if clicked," Wells wrote in a blog post on the vulnerability.

Wells realized the same function could be abused. Imagine a hacker using the links to secretly reconfigure a Slack desktop app to send all downloaded files to an outside server. "Using this attack vector, an insider could exploit this vulnerability for corporate espionage, manipulation, or to gain access to documents outside of their purview," Well's security firm Tenable said in a separate report.

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Credit: david wells / medium / screenshot

The vulnerability can also pave the way for potential malware infections. Any downloaded files sent to the hacker-controller server can be altered and booby-trapped to include malicious code. The attack will commence once the victim opens the file on the Slack desktop app.

The main obstacle of carrying out this attack is circulating the hacker-created links to people on Slack, which keeps its channels private to paying clients and their companies. To pull this off, Wells noticed how Slack channels can be configured to subscribe to RSS feeds, including threads on Reddit.

"I could make a post to a very popular Reddit community that Slack users around the world are subscribed to," Wells said. The hacker-created link will then populate inside the Slack channel and possibly attract some clicks.

"This technique could be unmasked by savvy Slack users, however if decades of phishing campaigns have taught us anything, it's that users click links, and when leveraged through an untrusted RSS feed, the impact can get much more interesting," he added.

Slack has patched the flaw in version 3.4.0 of the Windows desktop app. "We investigated and found no indication that this vulnerability was ever utilized, nor reports that our users were impacted," the company said in an email.

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