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What is CASB? Cloud Access Security Broker Explained
CASB provides visibility and control over cloud application usage, extending security policies to SaaS environments. Learn what CASB does, its deployment models, and why it matters for Microsoft 365 security.
What CASB Does
CASB platforms provide four core capabilities across cloud services. Visibility: discovering all cloud services in use, including shadow IT applications that IT does not know about, and providing a comprehensive inventory of cloud data flows. Compliance: evaluating cloud service configurations and data handling practices against regulatory requirements and security policies, identifying gaps. Data Security: extending DLP policies to cloud services, preventing sensitive data from being uploaded to unauthorized cloud applications or shared externally in policy-violating ways. Threat Protection: identifying anomalous usage patterns in cloud services that may indicate compromised accounts or insider threats.
CASB Deployment Models
API-based CASB connects directly to sanctioned cloud services through their management APIs, providing comprehensive visibility into historical and current activity without routing live traffic through a proxy. API-based deployment is easier to implement but cannot inspect real-time traffic or provide inline blocking. Proxy-based CASB routes cloud application traffic through the CASB platform, enabling real-time inspection, DLP enforcement, and blocking of policy-violating actions. Proxy-based deployment provides stronger enforcement but requires routing traffic through the CASB, creating latency and deployment complexity.
CASB and Microsoft 365 Security
Microsoft Defender for Cloud Apps is the dominant CASB platform in Microsoft-centric environments, providing native visibility into Microsoft 365 services — SharePoint, OneDrive, Exchange, Teams — with deep integration into the Microsoft security ecosystem. For organizations that have standardized on Microsoft 365, Defender for Cloud Apps extends security visibility into the cloud services where the majority of organizational data resides, making it a high-priority deployment for any Microsoft-centric security program.
Real-World Example: Shadow IT Discovery Uncovers Data Exposure
A Cloudskope CASB implementation for a PE-backed financial services company discovered that employees were using 847 cloud applications that IT was unaware of. Among them: a document sharing application that had been configured to allow public link sharing by default, which had been used to share documents containing non-public financial information. The exposure had occurred over 14 months without detection. CASB deployment identified the shadow application, identified the exposed documents, and enabled policy enforcement that blocked future public sharing.
Of cloud services in use at the average enterprise are shadow IT — cloud applications deployed by business units without IT knowledge or approval, completely outside security monitoring, DLP controls, and access management.