What Is CASB? Cloud Access Security Broker Explained
CASB provides visibility and control over cloud application usage, extending security policies to SaaS environments.
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.
Related Reading
What is a CASB?
A Cloud Access Security Broker is a security control point deployed between users and cloud service providers that provides visibility, compliance enforcement, data security, and threat protection for cloud application usage. CASB addresses the security gap created by cloud adoption: when organizational data and workflows move from on-premises systems into SaaS applications, traditional security controls — firewalls, DLP, network monitoring — lose visibility into what users are doing with data in those cloud environments.
What are the four core CASB capabilities?
Visibility — discovering all cloud services in use, including shadow IT applications IT does not know about, and inventorying cloud data flows. Compliance — evaluating cloud service configurations and data handling against regulatory requirements. Data Security — extending DLP policies to cloud services, preventing sensitive data from being uploaded to unauthorized applications or shared externally. Threat Protection — identifying anomalous usage patterns that indicate compromised accounts or insider threats.
What is the difference between API-based and proxy-based CASB?
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. Easier to deploy 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 — stronger enforcement at the cost of deployment complexity and latency.
What are the leading CASB platforms?
Microsoft Defender for Cloud Apps is the dominant CASB platform in Microsoft-centric environments, with deep native integration into Microsoft 365 services (SharePoint, OneDrive, Exchange, Teams). Netskope, Zscaler Internet Access, Palo Alto Prisma SaaS, and Cisco Umbrella are the leading independent enterprise CASB platforms. Many organizations deploy Defender for Cloud Apps for Microsoft 365 coverage alongside a third-party CASB for broader SaaS visibility.
How does CASB address shadow IT?
Shadow IT — cloud applications deployed by business units without IT knowledge — is endemic in enterprise environments, with the average organization having 61% of cloud services in use outside IT visibility. CASB platforms discover these applications through analysis of network traffic from corporate networks and endpoints, producing an inventory of every cloud service in use, the volume of data flowing to each, and risk ratings for unsanctioned applications. CASB-driven shadow IT discovery is the standard mechanism for converting shadow inventory into managed application portfolio.
What is the difference between CASB and SASE?
CASB is one component of the broader Secure Access Service Edge (SASE) architecture. SASE integrates CASB with secure web gateway (SWG), Zero Trust Network Access (ZTNA), firewall-as-a-service, and other cloud-delivered security controls into a unified service edge. CASB addresses cloud application security specifically; SASE provides the broader network and security architecture that delivers CASB capability alongside other controls. Many CASB vendors (Netskope, Zscaler, Palo Alto, Cisco) now position their CASB offerings as part of SASE platforms.
Do organizations using Microsoft 365 need a separate CASB?
Microsoft Defender for Cloud Apps — included in Microsoft 365 E5 licensing — provides CASB capability for Microsoft 365 services with deep native integration that third-party CASBs cannot match. For organizations where the majority of cloud data flows are Microsoft 365 (SharePoint, OneDrive, Exchange, Teams), Defender for Cloud Apps addresses the highest-priority data flows. Third-party CASB adds value where significant data flows exist outside the Microsoft ecosystem (Salesforce, Google Workspace, ServiceNow, third-party SaaS).
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.