Microsoft Teams has established itself as a central communication platform in organizations worldwide -- with over 300 million monthly active users. But this very widespread adoption makes Teams an attractive target for cybercriminals. Security researchers have found that a vulnerability in the handling of external messages is being actively exploited for sophisticated phishing campaigns. In this article, we analyze the threat and present concrete protective measures.
The Vulnerability in Detail
Microsoft Teams allows communication between different organizations by default -- a feature intended for collaboration with external partners, customers, and suppliers. The vulnerability lies in how Teams handles messages from external users. Although external messages are flagged with a notice, this is easy to overlook. Attackers exploit this by impersonating trusted partners, IT support, or even internal employees.
Particularly problematic is that attackers can distribute malicious files, links, and even complete phishing pages through Teams messages. Since employees perceive Teams as a "safe" internal communication channel -- unlike emails where they are typically more vigilant -- the success rate of such attacks is disturbingly high.
Typical Attack Scenarios
Scenario 1: Fake IT Support Messages
Attackers create Microsoft 365 accounts resembling the target organization's IT support and send messages through Teams with urgent security warnings. The message contains a link to a fake Microsoft login page designed to harvest credentials. Since the message comes through Teams, many employees assume it is legitimate.
Scenario 2: Malware via Teams Chats
In another variant, attackers send files through Teams disguised as harmless documents -- such as project plans, contracts, or meeting notes. These files contain malware that executes upon opening. Since Teams files are frequently classified as trustworthy, they often bypass the precautions employees would apply to email attachments.
Scenario 3: Abuse of Teams Tabs and Connectors
Advanced attackers exploit the ability to create custom tabs in Teams channels to embed phishing pages directly within the Teams interface. This is particularly dangerous because the phishing page is displayed within the trusted Teams environment and the browser's URL bar is not visible.
Concrete Protective Measures
Administrative Measures
- Restrict External Communication: Review whether communication with external Teams users is actually necessary. In the Teams admin settings, you can restrict external access to specific domains or disable it entirely.
- Conditional Access Policies: Implement conditional access policies that control Teams access based on device compliance, location, and risk assessment.
- Microsoft Defender for Office 365: Enable Safe Links and Safe Attachments for Teams messages to automatically detect and block malicious links and files.
- Audit Logging: Ensure comprehensive logging for Teams activities is enabled, particularly for external communication, file sharing, and guest activities.
Awareness Measures
- Expand Employee Training: Integrate Teams-specific phishing scenarios into your awareness training. Employees must understand that Teams messages can also be phishing attacks.
- Establish Reporting Processes: Ensure employees know how to report suspicious Teams messages. A simple, clearly communicated reporting process increases the likelihood that attacks are detected early.
- Pay Attention to External Message Banners: Sensitize employees to the external message indicator in Teams and the significance of this labeling.
Conclusion
The exploitation of Microsoft Teams for phishing attacks is an example of how attackers deliberately abuse users' trust in established platforms. Organizations must adapt their security strategy and consider Teams as a potential attack vector. The combination of technical protective measures, administrative configurations, and targeted awareness training provides the best protection. SecTepe supports organizations in securing their Microsoft 365 environment and conducts specialized phishing simulations that also cover Teams-based attack vectors.