The latest version of Microsoft Teams had a security flaw uncovered recently by Max Corbridge (@CorbridgeMax) and Tom Ellson (@tde_sec), JUMPSEC’s Red Team members.
Due to this flaw, there is a possibility for malware to be injected into organizations that rely on the default configuration of Microsoft Teams.
Microsoft Teams is used by over 280 million active users every month and is a popular way for organizations to talk and work together usin Microsoft 365.
Successful exploitation of this vulnerability enables the threat actors to evade the client-side security controls. This security feature prohibits users outside the organization from sending any file to the organization’s internal users.
Corbridge asserted in a report that the communication bridge they discovered is more vital because it can send harmful stuff straight to someone’s email, which is more potent than just tricking them.
Apart from this, two Jumpsec’s Red Team members uncovered a solution to circumvent the existing limitation.
They did this by altering the recipient ID in the POST request of a message for internal and external recipients, thereby tricking the system into recognizing an external user as an internal user.
In pragmatic trials, the researchers applied the technique. They successfully infiltrated a command and control payload into the inbox of a target organization, all while operating covertly as part of their red team exercise.
Attackers easily infect organizations using Microsoft Teams by bypassing security measures and anti-phishing training, exploiting the default configuration of it.
By registering a domain similar to the target’s Microsoft 365, the attacker can create messages that appear internal rather than external, increasing the chance of the target downloading the file without suspicion.
Response From Microsoft
Researchers notified Microsoft of their findings, expecting an immediate response due to the considerable impact observed.
Despite Microsoft acknowledging the flaw’s existence, its response indicated that it does not meet the threshold for immediate action, implying a lack of urgency to address the issue.
To minimize risk, organizations utilizing Microsoft Teams without requiring regular communication with external users should disable this feature. And to do this, you have to follow the simple steps that we have mentioned below:-
- First of all, go to Microsoft Teams Admin Center.
- Then access the External Access option.
- After that, you must disable the chat with external unmanaged Teams users.
Organizations can establish an allow-list for specific domains to mitigate exploitation risks when maintaining external communication channels.