Hackers were able to exploit a serious vulnerability in Microsoft Teams desktop apps to execute arbitrary code remotely and spread infection across a company network by simply sending a specially-crafted message.

The zero-click flaw, which is wormable, can be triggered by cross-site scripting (XSS) injection in Teams, with hackers able to transmit a malicious message which will execute code without user interaction.

This remote code execution (RCE) flaw was first reported to Microsoft in August, with the company fixing the bugs in October 2020. However, security researcher Oskars Vegaris, who discovered the flaw,  has complained that the firm didn’t take his report as seriously as it should have, with Microsoft not even assigning the bug a CVE tag.

Microsoft considered the Teams vulnerability as ‘important’ although described its impact as ‘spoofing’ in its bug bounty programme. As for the CVE element, Microsoft doesn’t issue CVE tags on products that automatically update without user interaction.

“This report contains a new XSS vector and a novel RCE payload which are used together,” Vegaris wrote on GitHub. “It affects the chatting system within Microsoft Teams and can be used in e.g. direct messages, channels.”

In a technical breakdown of the vulnerability, the researcher highlighted how RCE can be achieved by chaining two flaws, including stored XSS in Teams chat functionality and a cross-platform JavaScript exploit for the Teams desktop client. 

The impact is seemingly alarming, with its wormable nature meaning the exploit payload can be spread across other users, channels and companies without any interaction. The execution of malicious code could also happen without any user interaction, given users need to only view the specially-crafted message. 

The consequences of infection range from complete loss of confidentiality and integrity for victims, to access to private communications, internal networks, private keys as well as personal data outside of Microsoft Teams.

Hackers can also gain access to single sign-on (SSO) tokens for other services, including Microsoft services such as Outlook or Microsoft 365. This will expose them to possible phishing attacks too, as well as keylogging with specially-crafted payloads, according to Vegaris.

IT Pro approached Microsoft for comment.