  Business Collaboration Platforms like Microsoft Teams and Slack enable
teamwork by supporting text chatting and third-party resource integration. A
user can access online file storage, make video calls, and manage a code
repository, all from within the platform, thus making them a hub for sensitive
communication and resources. The key enabler for these productivity features is
a third-party application model. We contribute an experimental security
analysis of this model and the third-party apps. Performing this analysis is
challenging because commercial platforms and their apps are closed-source
systems. Our analysis methodology is to systematically investigate different
types of interactions possible between apps and users. We discover that the
access control model in these systems violates two fundamental security
principles: least privilege and complete mediation. These violations enable a
malicious app to exploit the confidentiality and integrity of user messages and
third-party resources connected to the platform. We construct proof-of-concept
attacks that can: (1) eavesdrop on user messages without having permission to
read those messages; (2) launch fake video calls; (3) automatically merge code
into repositories without user approval or involvement. Finally, we provide an
analysis of countermeasures that systems like Slack and Microsoft Teams can
adopt today.
