The "TeamsPhisher" cyberattack tool gives pen testers — and adversaries — a way to deliver malicious files directly to a Teams user from an external account, or tenant.

4 Min Read
Laptop computer displaying logo of Microsoft Teams
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A new tool is available on GitHub that gives attackers a way to leverage a recently disclosed vulnerability in Microsoft Teams and automatically deliver malicious files to targeted Teams users in an organization.

The tool, dubbed "TeamsPhisher," works in environments where an organization allows communications between its internal Teams users and external Teams users — or tenants. It allows attackers to deliver payloads directly into a victim's inbox without relying on a traditional phishing or social engineering scams to get it there.

"Give TeamsPhisher an attachment, a message, and a list of target Teams users," said the tool's developer Alex Reid, a member of the US Navy's Red Team, in a description of the tool on GitHub. "It will upload the attachment to the sender's Sharepoint and then iterate through the list of targets."

Fully Automated Cyberattack Flows

TeamsPhisher incorporates a technique that two researchers at JUMPSEC Labs recently disclosed for getting around a security feature in Microsoft Teams. While the collaboration app allows communications between Teams users from different organizations, it blocks the sharing of files between them.

JUMPSEC researchers Max Corbridge and Tom Ellson found a relatively easy way to bypass this restriction, using what is known as the Insecure Direct Object Reference (IDOR) technique. As security vendor Varonis noted in a recent blog post, "IDOR bugs allow an attacker to maliciously interact with a Web application by manipulating a 'direct object reference' such as a database key, query parameter, or filename."

Corbridge and Ellson found they could exploit an IDOR issue in Teams simply by switching the ID of the internal and external recipient when submitting a POST request. The two researchers discovered that when a payload is sent in this manner, the payload is hosted on the sender's SharePoint domain and arrives in the victim's Team's inbox. Corbridge and Ellson identified the vulnerability as affecting every organization running Teams in a default configuration and described it as something an attacker could use to bypass anti-phishing mechanisms and other security controls. Microsoft acknowledged the issue but assessed it as something not deserving of an immediate fix.

TeamsPhisher Incorporates Multiple Attack Techniques

Reid described his TeamsPhisher tool as incorporating JUMPSEC's techniques as well as some earlier research on how to leverage Microsoft Teams for initial access by independent researcher Andrea Santese. It also incorporates techniques of TeamsEnum, a tool for enumerating Teams users, that a researcher from Secure Systems Engineering GmbH had previously released to GitHub.

According to Reid, the way TeamsPhisher works is to first enumerate a target Teams user and verify that the user can receive external messages. TeamsPhisher then creates a new thread with the target user. It uses a technique that allows the message to arrive in the target's inbox without the usual "Someone outside your organization messaged you, are you sure you want to view it" splash screen, Reid said. 

"With the new thread created between our sender and the target, the specified message will be sent to the user along with a link to the attachment in Sharepoint," he noted. "Once this initial message has been sent, the created thread will be visible in the sender's Teams GUI and can be interacted with manually, if need be, on a case-by-case basis."

Microsoft said it is aware of TeamsPhiser and has determined that the tool relies on social engineering to be successful. "We encourage customers to practice good computing habits online, including exercising caution when clicking on links to web pages, opening unknown files, or accepting file transfers," the company said in an emailed statement.

Microsoft did not directly respond to a Dark Reading question on whether the release of TeamsPhiser had changed its stance on releasing a patch and/or guidance for affected users. Instead, the company pointed to its Microsoft Security Servicing Criteria webpage . The page describes the criteria the Microsoft Security Response Center (MSRC) uses to determine whether a reported vulnerability affecting currently supported versions of affected software might be updated/patched or addressed in the next version of the affected software.

JUMPSEC itself has urged organizations using Microsoft Teams to review whether there is any business need for enabling communications between internal Teams users and external tenants. 

"If you are not currently using Teams for regular communication with external tenants, tighten up your security controls and remove the option altogether," the company has advised.

About the Author(s)

Jai Vijayan, Contributing Writer

Jai Vijayan is a seasoned technology reporter with over 20 years of experience in IT trade journalism. He was most recently a Senior Editor at Computerworld, where he covered information security and data privacy issues for the publication. Over the course of his 20-year career at Computerworld, Jai also covered a variety of other technology topics, including big data, Hadoop, Internet of Things, e-voting, and data analytics. Prior to Computerworld, Jai covered technology issues for The Economic Times in Bangalore, India. Jai has a Master's degree in Statistics and lives in Naperville, Ill.

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