The developers of the work collaboration app Slack have issued a security update for its desktop client following the discovery of a medium-severity download hijack vulnerability that could let attackers modify the location where downloaded files are stored.

Malicious actors could exploit the flaw to steal and spy on users' documents by uploading them to a server they control. From there, the attackers could also manipulate the documents' contents, perhaps damaging data integrity by altering account numbers in financial documents, or injecting malware into an Office document in order to infect users who open them.

Attackers could potentially exploit the flaw – found in desktop version 3.3.7 for Windows by Tenable researcher David Wells – by crafting a malicious hyperlink that changes a document's download location path, and then posting that link into a Slack channel or private direct messaging conversation.

"An attacker can abuse the 'slack://' protocol handler, which has the capability to change sensitive settings in the Slack Desktop Application," explains a blog post published today by Tenable Research. "A crafted link like 'slack://settings/?update={‘PrefSSBFileDownloadPath’:’<pathHere>’}' will change the default download location."

For an extra element of deception, the attackers could also conceal the malicious hyperlink text by using Slack's attachment feature to replace the link's uniform resource identifier with custom text, Tenable notes.

For best chances of success, an attacker would have to be authenticated in order to post such a malicious link. Tenable says it is possible to pull off an unauthenticated attack "via external .rss feeds or other content pulled into a Slack channel from an external source." However, "there are variables that might reduce the chances of success, like knowing which .rss feeds the target Slack subscribes to."

"Tenable reported to Slack a vulnerability related to the Slack Desktop Application for Windows via the HackerOne bug bounty program," Slack said in a statement provided to SC Media. "We patched the bug as part of our latest update for Slack Desktop Application for Windows, v3.4.0. We investigated and found no indication that this vulnerability was ever utilized, nor reports that our users were impacted. As always, we encourage users to upgrade their apps and clients to the latest available version."