A tool called TeamsPhisher, developed by a member of the U.S. Navy’s red team, exploits an unresolved security issue in Microsoft Teams, allowing bypassing of file restrictions for external users.
Recently highlighted by security services company Jumpsec, the problem enables attackers to deliver malware from external accounts by tricking the client-side protections of Microsoft Teams.
Furthermore, TeamsPhisher streamlines the attack process by providing a fully automated approach, integrating the ideas of Jumpsec researchers and leveraging techniques from other tools.
TeamsPhisher begins by verifying the existence and external message reception capabilities of the target user. It then creates a new thread, sending them a message with a Sharepoint attachment link, and appears in the sender’s Teams interface for potential manual interaction. Users need a Microsoft Business account with valid Teams and Sharepoint licenses, which are common for many major companies.
Additionally, the tool includes features like a “preview mode” for verifying target lists and checking message appearances, secure file links, delayed message transmissions, and logging outputs.
The unresolved issue exploited by TeamsPhisher remains present, with Microsoft deeming it not immediately requiring servicing. Despite BleepingComputer’s efforts to obtain a response regarding plans to fix the problem, no reply was received. Although TeamsPhisher was initially created for authorized red team operations, threat actors could also misuse it to deliver malware without detection.
Organizations are advised to disable communication with external tenants if unnecessary and establish allow-lists of trusted domains to reduce the risk of exploitation until Microsoft takes action.
