An attacker can supply a malicious hyperlink in order to secretly alter the download path for files shared in a Slack channel.
A remotely exploitable vulnerability in the Windows desktop app version of the Slack collaboration platform has been uncovered, which allows attackers to alter where files from Slack are downloaded. Nefarious types could redirect the files to their own SMB server; and, they could manipulate the contents of those documents, altering information or injecting malware.
According to Tenable Research’s David Wells, who discovered the bug and reported it via the HackerOne bug-bounty platform, a download hijack vulnerability in Slack Desktop version 3.3.7 for Windows would allow an attacker to post a specially crafted hyperlink into a Slack channel that changes the document download location path when clicked. Victims can still open the downloaded document through the application, however, that will be done from the attacker’s Server Message Block (SMB) share.
Wells said in a posting on Friday.
The reason it has to be an SMB share is because of a security check built into the platform. The Slack application filters certain characters out – including colons – so an attacker can’t supply a path with a drive root.
Wells explained.
Remote Exploitation
An attack can be carried out by both authenticated and unauthenticated users, Wells said. In the first scenario, an insider could exploit the vulnerability for corporate espionage, manipulation or to gain access to documents outside of their role or privilege level.
In the second scenario, an outsider could place crafted hyperlinks into pieces of content that could be pulled into a Slack channel via external RSS feeds.
Wells said.
Success here would require knowing which RSS feeds the target Slack user subscribes to, of course.
Malware and More
In addition to being an information-disclosure concern (attackers could access sensitive company documents, financial data, patient records and anything else someone shares via the platform), the vulnerability could be used as a jumping-off point for broader attacks.
Wells explained. He added,
Because it does require user interaction to exploit, the vulnerability carries a medium-level CVSSv2 rating of 5.5. However, the researcher said that attackers can use a spoofing technique to mask the malicious URL behind a fake address, say “http://google.com,” to give it more legitimacy and convince a Slack user to click on the link.
More specifically, it’s possible to link to words within Slack by adding an “attachment” field to a Slack POST request with appropriate fields, Wells said.
The attack surface is potentially large. Slack said in January that it has 10 million active daily users, and 85,000 organizations use the paid version (it’s unclear how many are Windows users). Fortunately, Slack patched the bug as part of its latest update for Slack Desktop Application for Windows, v3.4.0, so users should upgrade their apps and clients.
Via threatpost.com