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Microsoft Teams Vulnerability Exposed Organizations to Attacks

Microsoft has addressed a vulnerability that could have been exploited by hackers to hijack Microsoft Teams accounts by sending specially crafted links or GIF images to the targeted organization’s users.

Sending the malicious link or image was simple, but preparing the attack involved multiple steps that would be difficult to achieve for unsophisticated attackers.

“We addressed the issue discussed in this blog and worked with the researcher under Coordinated Vulnerability Disclosure. While we have not seen any use of this technique in the wild, we have taken steps to keep our customers safe,” a Microsoft spokesperson told SecurityWeek.

Microsoft Teams is a communication and collaboration platform that includes chat, video conferencing, file storage, and application integration capabilities. The solution can be highly useful to organizations, particularly during this COVID-19 coronavirus outbreak, when many employees are forced to work from home.

Researchers at CyberArk, a company that specializes in privileged access security solutions, discovered a vulnerability related to how Teams passes authentication access tokens to image resources.

An attacker can exploit this weakness to create a link or GIF file that, when processed by Teams, sends an authentication token to a server they control. In the case of links, the victim needs to click on the link, but in attacks involving GIF images, the victim simply needs to view the GIF in the Teams chat and their token is sent to the hacker.

GIF image used to hijack Microsoft Teams accounts

Once they have obtained the token, the attacker can use it to hijack the victim’s account through the Teams API interfaces. The attacker can use this method to read the user’s Teams messages, send messages on their behalf, create groups, add or remove users from a group, and change group permissions.

The entire attack can be automated, allowing malicious actors to spread through an organization like a worm by using compromised accounts to send the malicious GIF to other Teams users. This enables them to obtain potentially sensitive information from Teams accounts, including confidential data, passwords, meeting and calendar data, and business plans.

“Maybe even more disturbing, they could also exploit this vulnerability to send false information to employees – impersonating a company’s most trusted leadership – leading to financial damage, confusion, direct data leakage, and more,” CyberArk researchers explained.

In an interview with SecurityWeek, CyberArk representatives described several attack scenarios involving this vulnerability, including one where the attacker uses a hijacked account to request a password reset from a member of the organization’s IT team. The attacker can also contact the CEO using the hijacked account of another executive and ask for sensitive financial information. The hacker can also use calendar data to learn about meetings between executives, and impersonate one of the parties in an effort to trick the other party into installing a piece of malware by requesting the use of a different app for the meeting.

Hijacking Microsoft Teams accounts

However, there are certain prerequisites for the attack to work. First of all, when the victim sees the malicious GIF in Teams, their access token can only be sent to a subdomain of, so the attacker somehow needs to hijack such a subdomain.

During their tests, CyberArk researchers discovered two subdomains that allowed them to conduct an attack: and

Recently conducted research shows that there are hundreds of legitimate Microsoft subdomains that can be hijacked and abused for phishing, malware delivery and scams. However, an attacker would need to find subdomains to conduct an attack.

CyberArk has admitted that finding a suitable domain is not an easy task, but it believes that an attacker with the right method and the right resources will likely find more subdomains.

Microsoft has ensured that the domains identified by CyberArk cannot be abused any longer and says it has taken other measures to prevent exploitation. However, the cybersecurity firm told SecurityWeek that it believes the attack still works if someone is able to find Teams subdomains that can be hijacked.

In order to launch an attack such as the one described by CyberArk, the attacker also somehow needs to find a way to obtain access to a Teams account from which they can start sending malicious links or GIFs in order to spread within an organization. However, the researchers noted that an attack can also be launched from outside the targeted organization. For example, if an attacker can convince the target to invite them to a job interview on Teams.

CyberArk told SecurityWeek that the tokens obtained by the attacker are only valid for one hour, but a new token that is valid for another hour is created every time the victim views the malicious GIF.

CyberArk has published a blog post containing a technical description of the attack and a video showing the attack.

Related: Facebook Awards $55,000 for Flaw That Could Lead to Account Hijacking

Related: Slack Vulnerability Allowed Hackers to Hijack Accounts

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.