Slack patches vulnerability in Windows client that could be used to hijack files

Enlarge / Strangers in your Slack channel could have messed with Slack for Windows’ download settings, redirecting files to a malicious shared folder. It’s fixed now. (credit: NOAH BERGER/AFP/Getty Images)

On May 17, researchers at Tenable revealed that they had discovered a vulnerability in the Windows version of the desktop application for Slack, the widely used collaboration service. The vulnerability, in Slack Desktop version 3.3.7 for Windows, could have been used to change the destination of a file download from a Slack conversation to a remote file share owned by an attacker. This would allow the attacker to not only steal the files that were downloaded by a targeted user, but also allow the attacker to alter the files and add malware to them. When victims opened the files, they would get a potentially nasty surprise.

Tenable reported the vulnerability to Slack via HackerOne. Slack has issued an update to the Windows desktop client that closes the vulnerability.

The potential attack used a weakness in the way the “slack://” protocol handler was implemented in the Windows application. By creating a crafted link posted in a Slack channel, the attacker could alter the default settings of the client—changing the download directory, for example, to a new location with a URL such as “slack://settings/?update={‘PrefSSBFileDownloadPath’:’’}”. That path could be directed to a Server Message Block (SMB) file-sharing location controlled by the attacker. Once clicked, all future downloads would be dropped onto the attacker’s SMB server. This link could be disguised as a Web link—in a proof-of-concept, the malicious Slack attack posed as a link to Google.