Slack Pays Bounty for Critical Vulnerability in Desktop App


A security researcher was awarded a $1,750 bug bounty reward for discovering a remote code execution vulnerability in the Slack desktop applications. 

An attacker could exploit the vulnerability to execute arbitrary code within Slack’s desktop apps for macOS, Linux, and Windows. 

The issue was discovered by security engineer Oskars Vegeris of Evolution Gaming, who reported it in January 2020 via the company’s bug bounty program on HackerOne. 

“With any in-app redirect – logic/open redirect, HTML or JavaScript injection – it’s possible to execute arbitrary code within Slack desktop apps. This report demonstrates a specifically crafted exploit consisting of an HTML injection, security control bypass and a RCE JavaScript payload,” the researcher explained

Vegeris notes that an attacker looking to exploit the flaw would need to upload on their server a file containing the RCE payload, then create a Slack post that contains HTML injection code and post that to a channel or send it to a specific user, to achieve one-click remote code execution. 

Once the payload is triggered, the attacker gains access to private conversations and files on Slack, as well as to private files on the system, private keys, passwords, secrets, internal network access, and more.

Furthermore, the payload could be designed to be wormable, meaning that it would be automatically re-posted to all user workspaces after click.

In addition to this vulnerability, the researcher identified a Cross-Site Scripting (XSS) flaw in files.slack.com that could lead to arbitrary HTML content being displayed in *.slack.com and phishing attacks via fake HTML login pages, but which could also be abused to store the RCE exploit. 

Details on the security bug became public only last week, after the researcher discovered that Slack had addressed the bug (through the introduction of a sandbox) without crediting his work and complained about that on HackerOne. Slack has since rectified that. 

“My name is Larkin Ryder and I am currently serving as the interim Chief Security Officer here at Slack. @brandenjordan made me aware of this misstep and I am writing to convey very sincere apologies for any oversight in crediting your work. We very much appreciate the time and effort you’ve invested in making Slack safer,” Slack’s CSO said on HackerOne. 

Following the public disclosure last week, the infosec community has started mocking Slack on Twitter for awarding the researcher such a small bug bounty reward for his finding. However, the company typically offers $1,500 for critical vulnerabilities discovered in its products. 

Related: Slack Vulnerability Allowed Hackers to Hijack Accounts

Related: Slack Flaw Allows Hackers to Steal, Manipulate Downloads

Related: Slack Lists Cybersecurity Risks Ahead of Going Public

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Ionut Arghire is an international correspondent for SecurityWeek.

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