Security Experts:

Wormable, Zero-Click Vulnerability in Microsoft Teams

Security researcher Oskars Vegeris has published documentation on a wormable, cross-platform vulnerability in Microsoft Teams that could allow invisible malicious hacker attacks.

Vegeris, a security engineer at Evolution Gaming, warned that a novel cross-site scripting (XSS) vulnerability at the ‘teams.microsoft.com’ domain could be abused to trigger a remote code execution flaw in the Microsoft Teams desktop application.

Microsoft Teams, which competes in the enterprise space with the likes of Slack and Zoom, counts about 115 million daily active users and is widely deployed as part of Microsoft’s Office 365 family of products. Teams is a proprietary business communication platform that provides users with workspace chat, file storage and sharing, app integration, and videoconferencing capabilities. 

According to an advisory published by Vegeris, an attacker simply needs to send a specially crafted message to any Teams user or channel to launch a successful exploit that runs silently in the background, without the user noticing anything.

“Remote Code Execution has been achieved in desktop applications across all supported platforms (Windows, macOS, Linux). Code execution gives attackers full access to victim devices and company internal networks via those devices,” Vegeris warned

He said an attacker could abuse the XSS flaw to obtain SSO authorization tokens for Teams or other Microsoft services, or to access confidential conversations and files from the communications service. 

On top of that, the vulnerability is wormable, allowing a successful attacker to automatically send the exploit payload to other users/channels, also without interaction.

The successful exploitation of the bug could potentially give access to private keys and personal data outside Teams, possibly leaking internal network information and allowing adversaries to set up for phishing attacks. 

Remote code execution, Vegeris explains, can be achieved only if the XSS in teams.microsoft.com (in user 'mentions' functionality) is chained with the novel cross-platform exploit for the Teams desktop clients. 

The security researcher, who provides technical details on the flaw and a demonstration on how it can be exploited, claims Microsoft has downplayed the severity of the vulnerability, assigning an "important" rating with a "spoofing" risk.

He said Microsoft took the Teams desktop clients “out of scope” and told the researcher it wouldn’t issue a CVE number for the flaw, because vulnerabilities in Microsoft Teams are fixed via automatic updates.

Affected products include Microsoft Teams for macOS v 1.3.00.23764, Windows v 1.3.00.21759, and Linux v 1.3.00.16851. The company has already addressed the vulnerability. 

“We mitigated the issue with an update in October, which has automatically deployed and protected customers,” a Microsoft spokesperson said, responding to a SecurityWeek inquiry.

*updated with statement from Microsoft

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