Organizations now run core operations on dozens of SaaS applications, IaaS platforms, and PaaS services simultaneously. Every new cloud service introduces another entry point for data to leave corporate control and another authentication surface for attackers to probe. Traditional network perimeter security cannot see traffic that never touches the corporate network, let alone enforce policy on it. A cloud access security broker (CASB) fills that gap by acting as a policy enforcement point between users and cloud providers. This guide explains what a CASB is, how its architecture works, the deployment models available, and where it fits in a modern cloud security stack.
What Is a Cloud Access Security Broker and Why Organizations Need One
A cloud access security broker (CASB) is a software tool or cloud-delivered service that sits between an organization’s users and its cloud service providers to enforce security policies. Gartner coined the term in 2012 as organizations began adopting SaaS applications faster than security teams could audit them. The original CASB category addressed a growing disconnect: corporate data was flowing to cloud services that existing security tools could not inspect or control.
The core problem a CASB solves is lack of visibility. When employees sign up for a file-sharing service, a collaboration tool, or an AI assistant without IT approval, that is shadow IT. The security team does not know which data is being uploaded, who has access, or whether the service meets compliance requirements. A CASB discovers every cloud application in use, classifies each one by risk, and enforces policies that govern how data moves between users and those applications.
The Four Pillars of CASB Functionality
Gartner defined CASB capabilities through four pillars that remain the standard framework for evaluating CASB solutions today.
Visibility. A CASB discovers all cloud services accessed from the corporate network or managed devices. It catalogs sanctioned applications, tolerated services, and unsanctioned shadow IT. This telemetry tells security teams which cloud apps employees use, how much data each one holds, and which users access each service.
Compliance. Regulated industries must demonstrate that data stored in cloud services meets requirements such as HIPAA, GDPR, PCI DSS, or SOC 2. A CASB maps cloud usage against these frameworks, generates compliance reports, and enforces access policies that prevent data handling practices that would violate regulations.
Data security. CASBs extend data loss prevention (DLP) controls into cloud environments where traditional DLP tools do not reach. They inspect data in transit and at rest, apply encryption or tokenization, and block uploads or shares that violate policy. This includes preventing sensitive data from being pasted into unsanctioned AI tools or shared externally through cloud storage.
Threat protection. CASBs detect anomalous user behavior, compromised accounts, and malware in cloud-stored files. Behavioral analytics profiles normal activity for each user and flags deviations such as a sudden mass download or a login from an unfamiliar location. When the CASB detects a threat, it can quarantine files, block access, or trigger automated response workflows.
How Cloud Access Security Broker Architecture Works
A CASB enforces policy through a three-phase process that runs continuously across the organization’s cloud footprint: discovery, classification, and remediation.
In the discovery phase, the CASB identifies every cloud application in use. It monitors network traffic to detect connections to cloud services, integrates with the organization’s firewall or proxy logs, and can use API integrations with major SaaS platforms to enumerate connected third-party apps. The result is a complete inventory of cloud services that includes applications the security team did not know existed.
In the classification phase, the CASB evaluates each discovered application against risk criteria. Factors include the type of data the application handles, its compliance certifications, its security track record, and community trust ratings from threat intelligence feeds. Each application receives a risk score that determines what policies apply.
In the remediation phase, the CASB enforces policies based on the risk classification. Actions range from blocking access entirely to applying conditional controls such as read-only access on unmanaged devices, encryption of sensitive files, or alerts when a user shares data externally.
How Inline Proxy Architecture Enforces Policies in Real Time
An inline proxy CASB sits directly in the data path between the user and the cloud service. Every request passes through the CASB, which inspects the traffic, applies policy, and either allows, blocks, or modifies the transaction in real time. This architecture provides the strongest enforcement because the CASB can intercept and change traffic before it reaches the cloud provider.
The primary trade-off is latency. Every request must round-trip through the CASB before reaching its destination. Inline proxies also require traffic redirection, which can be complex to configure for all cloud services, especially mobile users operating outside the corporate network. Organizations typically deploy inline proxy CASBs alongside a secure web gateway or as part of a SASE platform that handles traffic steering.
API-Based Architecture for Data-at-Rest Protection and Visibility
An API-based CASB connects directly to cloud service providers through their published APIs. Rather than intercepting traffic, the CASB queries the cloud service to enumerate users, files, permissions, and activity logs. It enforces policy by calling the cloud service’s own API to apply labels, revoke sharing, quarantine files, or disable compromised accounts.
API-based CASBs introduce no latency because they do not sit in the data path. They provide visibility into data already stored in cloud services, including files that were uploaded before the CASB was deployed. The limitation is that API-based enforcement depends on what each cloud provider exposes through its API. Some actions, such as blocking a file upload in real time, are not possible through API-based controls because the transaction has already completed by the time the API call returns.
Hybrid Multimode CASB Deployment for Comprehensive Coverage
Most enterprise CASB deployments now use a hybrid multimode approach that combines inline proxy and API-based methods. The inline proxy handles real-time traffic inspection and blocking for web-based cloud access. The API-based integration provides deep visibility into data at rest, retrospective analysis of historical activity, and enforcement actions such as file quarantine and permission revocation. Multimode CASB deployments cover both data in motion through the proxy and data at rest through API connections.
Key CASB Use Cases in Enterprise Cloud Security
CASB solutions address several distinct security problems in organizations that use cloud services at scale. The most common deployments target shadow IT visibility, SaaS data protection, compliance automation, and threat detection.
Shadow IT Discovery and Governance
The average enterprise uses hundreds of cloud services, and security teams typically approve only a fraction of them. CASB auto-discovery surfaces every cloud application in use, regardless of whether IT sanctioned it. Once discovered, the CASB applies a governance model that categorizes each service as sanctioned, tolerated, or blocked.
This capability matters because unsanctioned cloud services store corporate data without any security controls enforced on them. A marketing team using an unapproved file-sharing tool may store customer lists and campaign data in an environment the security team cannot monitor. The CASB discovers that tool, assesses its risk, and applies an appropriate policy such as blocking access from managed devices or alerting the security team.
Data Loss Prevention for SaaS Applications
Enterprise DLP tools that inspect email and web traffic often cannot see data inside SaaS applications. A CASB extends DLP controls into those applications by monitoring what users upload, download, and share. When a user attempts to share a file containing payment card information or protected health information through a cloud storage service, the CASB can block the share, quarantine the file, or encrypt it before it leaves corporate control.
The rise of generative AI tools has made CASB DLP especially relevant. Employees paste proprietary code, customer data, and internal strategy documents into AI chat interfaces. A CASB that integrates with those services can detect sensitive data in prompts and block the transmission before it reaches the AI provider.
Compliance Monitoring and Reporting
Organizations subject to regulatory frameworks use CASBs to demonstrate continuous compliance across cloud services. The CASB scans cloud storage for data types covered by relevant regulations, verifies that access controls meet regulatory standards, and generates audit-ready reports. For healthcare organizations under HIPAA, the CASB can confirm that ePHI stored in cloud services is encrypted and that access logs are complete. For financial services firms under PCI DSS, the CASB can validate that cardholder data in cloud environments is properly segmented and access-controlled.
Threat Detection and Compromised Account Response
CASBs profile normal user behavior across cloud applications and detect anomalies that signal a compromised account. A user who typically accesses five files per day who suddenly downloads 500 files from a document management system triggers an alert. The CASB can automatically revoke that user’s access, force a password reset, or quarantine downloaded files for forensic analysis.
CASB Deployment Models and Evaluation Criteria
Choosing the right CASB deployment approach depends on the organization’s cloud architecture, user distribution, and security requirements. Three models dominate the market.
Forward proxy deployment routes all user traffic through the CASB before it reaches the internet. This model works best for organizations with centralized network traffic and controlled endpoints. The CASB inspects all outbound requests and enforces policy on every cloud service transaction. The main limitation is that traffic from mobile devices and remote users outside the corporate network must be tunneled back through the proxy, which increases complexity and can degrade performance.
Reverse proxy deployment places the CASB in front of specific sanctioned cloud applications. Users connect to the CASB instead of directly to the cloud service, and the CASB forwards authenticated requests to the provider. This model provides granular control over sanctioned applications but does not discover or control unsanctioned shadow IT.
API-based deployment connects the CASB to cloud services through their management APIs. It provides visibility and control without traffic redirection, making it suitable for organizations with distributed workforces and mobile users. API-based CASBs cannot block threats in real time because they operate on data after the transaction completes, but they excel at data classification, compliance scanning, and retrospective threat hunting.
Most organizations at scale choose a multimode approach that deploys an API-based CASB for data-at-rest protection and a proxy-based CASB for inline threat blocking. The proxy component handles user-facing web traffic, and the API component provides deep integration with critical SaaS applications such as Microsoft 365, Google Workspace, and Salesforce.
How CASB Differs from Adjacent Cloud Security Tools
CASB is one component of a complete cloud security program, and organizations often evaluate it alongside or in combination with complementary tools. Understanding the boundaries between these tools prevents both overlap and blind spots.
CASB vs. Secure Web Gateway
A secure web gateway (SWG) protects users from web-based threats by filtering internet traffic, blocking malicious URLs, and enforcing acceptable use policies. An SWG operates at the network layer and inspects all web traffic, not just traffic to cloud applications. A CASB operates at the application layer and enforces policies specific to cloud services, such as who can share a file in a SaaS application or whether sensitive data can be uploaded. The two tools are complementary and increasingly converge in SASE platforms.
CASB vs. Cloud Security Posture Management
Cloud security posture management (CSPM) focuses on misconfigurations in IaaS and PaaS environments. A CSPM scans cloud infrastructure for settings that create risk, such as publicly accessible storage buckets, overly permissive IAM roles, or unencrypted databases. A CASB focuses on user behavior and data protection across SaaS applications. An organization running workloads in AWS or Azure needs both: a CSPM to prevent infrastructure misconfigurations and a CASB to monitor how users interact with SaaS services and protect data moving to and from those services.
CASB vs. Zero Trust Network Access
Zero trust network access (ZTNA) replaces traditional VPNs by granting users access to specific applications based on identity and device posture rather than network location. ZTNA controls access to private applications hosted in data centers or cloud environments. A CASB controls access to SaaS applications and enforces data-level policies. In a SASE framework, ZTNA and CASB work together: ZTNA secures access to internal applications, and CASB secures access to external cloud services.
The Role of CASB in SASE and Zero Trust Architectures
CASB has evolved from a standalone security tool into a core component of secure access service edge (SASE) and security service edge (SSE) architectures. Gartner identified CASB as one of the foundational elements of SSE, alongside secure web gateway and zero trust network access.
In a SASE architecture, the CASB function is delivered as part of a unified cloud platform that bundles networking and security services. The platform handles traffic steering, applies policy based on user identity and device posture, and enforces data protection rules across all cloud services. This convergence eliminates the operational overhead of managing separate proxy configurations, API integrations, and policy consoles for each security tool.
The MarketsandMarkets 2026 SASE market report projects the global SASE market will grow from USD 19.19 billion in 2026 to USD 68.06 billion by 2032 at a CAGR of 28.8 percent, with CASB cited as a key SSE component driving adoption. Organizations transitioning from legacy VPN and web gateway architectures to SASE platforms are effectively adopting CASB capabilities as part of that migration.
Zero trust principles map directly to CASB functions. The zero trust model assumes no user or device is trusted by default, even if connected to the corporate network. A CASB enforces this by verifying user identity, assessing device posture, and applying least-privilege access to cloud resources on every request, regardless of where the request originates.
Conclusion
A cloud access security broker provides the visibility, data protection, and threat detection capabilities that organizations need as their operations shift to cloud services. The four pillars of CASB functionality — visibility, compliance, data security, and threat protection — address specific security gaps that traditional tools cannot close. Deployment via inline proxy, API integration, or a hybrid multimode approach gives organizations flexibility to match the CASB architecture to their environment. As CASB capabilities converge with secure web gateways and zero trust network access in SASE platforms, the distinction between standalone CASB tools and integrated cloud security platforms will continue to blur. Organizations that have not yet deployed CASB capabilities should prioritize it as part of their cloud security strategy, starting with shadow IT discovery and progressing toward full data protection and threat detection across all cloud services.
