What Is a Cloud Access Security Broker (CASB)?

A cloud access security broker (CASB) is an enterprise security policy enforcement point situated between cloud service consumers and cloud service providers to combine and interject organizational security policies as cloud-based resources are accessed. CASBs consolidate multiple types of security policy enforcement — such as authentication, single sign-on, authorization, credential mapping, device profiling, data loss prevention, encryption, tokenization, and threat detection — across SaaS, PaaS, and IaaS environments.

As organizations accelerate cloudification and embrace remote or hybrid work environments, critical enterprise data moves beyond the traditional physical network perimeter. This decentralization exposes corporate networks to shadow IT, unmanaged devices, unauthorized access, and sophisticated cyberthreats like ransomware. A CASB provides security teams with deep visibility, threat protection, data security, and compliance monitoring across both sanctioned and unsanctioned cloud applications. By enforcing unified governance across distributed multicloud architectures, a CASB allows enterprises to maintain robust data protection without compromising employee agility or cloud adoption.

A cloud access security broker is critical because the rapid migration to cloud infrastructure and software as a service (SaaS) platforms has dismantled the traditional corporate security perimeter. Without a CASB, organizations lack central visibility and administrative control over sensitive corporate data flowing across third-party cloud environments and unmanaged personal devices.

Modern hybrid work practices rely on off-network connectivity, public internet access, and bring your own device (BYOD) policies. This shift introduces significant operational vulnerabilities, as employees frequently adopt unsanctioned SaaS applications — a phenomenon known as shadow IT — to streamline daily tasks. When sensitive intellectual property or personally identifiable information is uploaded to unvetted cloud platforms, security teams face severe blind spots, increasing the likelihood of data breaches, accidental exfiltration, and regulatory noncompliance.

Furthermore, traditional on-premises security controls, such as legacy network firewalls and local web proxies, cannot inspect direct-to-cloud traffic or evaluate SaaS application context. A CASB addresses these visibility gaps by enforcing granular security policy enforcement points directly at the cloud boundary. It monitors data in motion and data at rest, applies automated malware threat prevention, prevents unauthorized access from unmanaged endpoints, and ensures continuous regulatory compliance with global frameworks like GDPR, LGPD, PCI DSS, and HIPAA.

A cloud access security broker works by inspecting connection attempts, user traffic, and data exchanges occurring between corporate users and cloud service providers in real time or via API integrations. It acts as an inline proxy or out-of-band management engine to enforce security policies, evaluate user intent, and protect enterprise assets across public and private cloud environments.

CASB architecture operates through three primary deployment modes:

• Forward proxy: Installed on the client endpoint or deployed via a secure web gateway (SWG), a forward proxy redirects all user-initiated outbound traffic through the CASB before it reaches any cloud service. This model provides real-time inline inspection and threat prevention across both sanctioned and unsanctioned cloud applications.

• Reverse proxy: Positioned in front of sanctioned cloud applications, a reverse proxy proxies incoming traffic directed to corporate SaaS or IaaS tenants. It does not require endpoint agent installation, making it ideal for securing unmanaged devices and BYOD endpoints requesting access to corporate systems.

• API architecture: Utilizing native cloud service provider APIs, an API-based CASB connects directly to sanctioned cloud platforms out of band. This deployment allows deep inspection of data at rest, historical auditing, file configuration scanning, user behavior analytics, and cross-account policy enforcement without latency or routing overhead.

When a user attempts to interact with a cloud application, the CASB checks user credentials using single sign-on (SSO) and multi-factor authentication (MFA) integrations. Concurrently, machine learning algorithms and behavior analytics evaluate session context, device posture, and geolocation. If the request violates policy — such as an unmanaged endpoint trying to download sensitive financial records — the CASB intervenes in real time by blocking the action, redacting sensitive content, forcing tokenization, or routing the session through read-only controls.

A cloud access security broker architecture is built upon four foundational functional capabilities — commonly referred to as the four pillars of CASB — that collectively deliver comprehensive cloud governance and protection.

• Visibility: CASB platforms discover and catalog all cloud applications in use across the enterprise network, identifying shadow IT usage and assessing the security risk scores of thousands of SaaS, PaaS, and IaaS tools. This pillar provides security teams with detailed auditing of user identities, connected devices, locations, and data movement.

• Compliance: As data migrates to the cloud, organizations must maintain adherence to international data privacy frameworks and industry mandates. CASBs identify regulatory noncompliance, monitor configuration drifts in cloud infrastructure, enforce continuous data residency rules, and simplify audit reporting for international standards like GDPR, LGPD, PCI DSS, and regional regulations.

• Data security: Leveraging cloud native data loss prevention (DLP) engines, CASBs prevent unauthorized data exfiltration and accidental public exposure. They classify sensitive data in real time, apply automated encryption or tokenization to data in transit and data at rest, and restrict risky file-sharing behaviors.

• Threat protection: CASBs defend enterprise systems against cloud-centric cyberthreats, including credential harvesting, account takeover, malware distribution, and ransomware attacks. They integrate static and dynamic malware detection, leverage threat intelligence feeds, and employ user and entity behavior analytics (UEBA) to detect anomalous activities indicative of insider threats or compromised accounts.

Enterprise security leaders deploy CASB platforms to solve specific risk management challenges associated with cloud adoption, decentralized users, and sensitive data handling.

• Mitigating shadow IT and SaaS risk: CASBs automatically identify unsanctioned cloud-based applications accessed by employees, score their security posture, and allow administrators to block high-risk services or apply granular functional restrictions.

• Enforcing cloud data loss prevention (DLP): Organizations use CASBs to extend existing on-premises DLP rules into cloud environments, scanning file uploads, downloads, and real-time shares to block the exfiltration of intellectual property, healthcare data, or payment credentials.

• Securing unmanaged devices and BYOD: Through reverse proxy or enterprise browser controls, CASBs ensure contractors and employees using personal endpoints can access sanctioned SaaS applications safely without permitting local file downloads or copy/paste operations.

• Detecting compromised accounts and insider threats: By establishing behavioral baselines using machine learning, CASBs trigger real-time alerting and automated remediation when detecting suspicious activities, such as impossible travel logins, mass file deletions, or abnormal data transfers.

• Ensuring cloud infrastructure and storage compliance: CASBs continuously audit IaaS and PaaS configurations (such as AWS S3 buckets or Microsoft Azure storage containers) to detect misconfigurations, unencrypted data at rest, and open permissions before exploitation occurs.

A cloud access security broker is a core constituent component of modern security service edge (SSE) frameworks, which consolidate cloud security services into a unified, cloud-delivered platform. SSE represents the security-focused pillar of the broader secure access service edge (SASE) architecture, which merges software-defined wide area networking (SD-WAN) with edge-delivered security controls.

Within an SSE framework, a CASB operates alongside secure web gateways (SWG) and Zero Trust Network Access (ZTNA):

• CASB specializes in deep visibility, data security, and policy enforcement across SaaS, PaaS, and IaaS applications.

• SWG manages and filters outbound web traffic, protecting users from malicious web domains, drive-by downloads, and web-based malware.

• ZTNA provides secure, direct access to specific private enterprise applications hosted on-premises or in cloud infrastructure, replacing legacy virtual private networks (VPNs) by removing broad network access.

Integrating these capabilities into a single converged cloud edge allows organizations to eliminate fragmented point solutions, reduce operational management overhead, enforce unified data classification policies, and deliver consistent security enforcement for distributed workforces regardless of user location.

While stand-alone CASBs deliver value for cloud governance, reliance solely on traditional CASB deployments introduces operational friction, technical limitations, and security blind spots across modern enterprise environments.

• API dependencies and granularity variations: API-based CASB functionality is entirely dependent on the third-party application’s public APIs. Differences in API maturity across software vendors lead to inconsistent policy enforcement, variable scanning speeds, and delayed threat detection between different SaaS platforms.

• Complex network routing and performance latency: Forward proxy CASB architectures require altering network routing or deploying local agents, which can introduce latency, break complex web application scripts, and diminish the overall user experience.

• Inability to protect unsanctioned app interaction granularly: A forward-proxy CASB can block access to unsanctioned websites. However, it cannot manage individual in-session actions, such as preventing a user from pasting text into a generative AI tool.

• Agent deployment overhead for unmanaged devices: Forward proxy CASBs require endpoint software management, making deployment across third-party contractor devices, vendor endpoints, or unmanaged personal hardware operationally impractical.