What Is A Cloud Access Security Broker?

How CASBs Work: The Enforcement Layer Between Users and Cloud Apps

CASBs sit inline or operate via API integration to inspect user-to-cloud traffic, enforce security policies, and provide visibility into sanctioned and unsanctioned cloud application usage. The architecture determines what the CASB can see, when it can act, and how much latency it introduces.

There are two primary deployment modes: forward proxy (inline) and API-based (out-of-band). Inline modes intercept traffic in real time, enabling policy enforcement before data reaches the cloud.

Forward Proxy (Inline) Mode

Inline CASBs intercept traffic between users and cloud services in real time, enabling policy enforcement before data reaches the cloud. Traffic is routed through the CASB, either via explicit proxy configuration, transparent redirection, or reverse proxy, so every session passes through the enforcement point.

API-Based (Out-of-Band) Mode

API-based CASBs connect directly to cloud service APIs to pull logs, user activity, and configuration data after the fact. They don't sit in the traffic path; instead, they query SaaS platforms (Office 365, Salesforce, Google Workspace) via OAuth or service accounts to retrieve event logs, file metadata, user permissions, and security configurations.

The Four Pillars of CASB

CASBs are built around four core functions: Visibility, Compliance, Data Security (DLP), and Threat Protection. These pillars define what a cloud access security broker delivers and where it fits in your security stack.

Visibility: Discovering Shadow IT

CASBs identify all cloud applications in use across the organization, including unsanctioned apps that IT teams may not know about. They do this by analyzing network traffic (inline mode) or querying cloud service logs (API mode) to build an inventory of every SaaS platform your users access.

Each discovered app is assigned a risk score based on security posture, compliance certifications, vendor reputation, and data handling practices. High-risk apps (personal file-sharing services, unapproved collaboration tools, or platforms with weak encryption) are flagged for review or blocked outright.

Compliance: Enforcing Regulatory and Policy Controls

CASBs help organizations meet compliance requirements (GDPR, HIPAA, PCI-DSS) by enforcing data residency rules, access controls, and audit logging for cloud services. They apply policies that restrict where data can be stored, who can access it, and how it must be protected.

For example, a CASB can block uploads of personally identifiable information (PII) to cloud storage services hosted outside the EU, enforce multi-factor authentication (MFA) for access to financial data, or log every file download for audit trails. These controls are applied at the cloud layer, independent of what the SaaS vendor offers natively.

Data Security: Data Loss Prevention (DLP) in the Cloud

CASBs apply DLP policies to prevent sensitive data (PII, PHI, intellectual property) from being uploaded to or shared via cloud applications. They inspect file metadata, content patterns, and contextual signals to identify sensitive information and enforce protective actions: block the upload, quarantine the file, encrypt it, or alert the security team.

DLP rules are typically based on regular expressions (regex), keyword matching, or machine learning classifiers trained to recognize sensitive data types. For example, a CASB might block any file containing credit card numbers from being uploaded to Dropbox, or flag any document labeled "Confidential" that's shared externally via Google Drive.

Threat Protection: Detecting Malware and Anomalous Behavior

CASBs scan files for known malware signatures and use behavioral analytics (UEBA) to flag compromised accounts or insider threats. They integrate with threat intelligence feeds to identify malicious file hashes, suspicious IP addresses, and known command-and-control (C2) domains.

Behavioral analytics track user activity patterns (login locations, access times, data download volumes) and flag anomalies that suggest account compromise. For example, if a user who normally logs in from New York suddenly accesses Salesforce from Eastern Europe and downloads 10 GB of customer data, the CASB triggers an alert or blocks the session.

Common CASB Use Cases: Where Organizations Deploy CASBs

CASBs deliver value in specific scenarios where cloud visibility, policy enforcement, and compliance are the primary concerns. Here's where enterprises deploy them in practice.

Shadow IT Discovery and Risk Assessment

CASBs identify unsanctioned cloud apps (e.g., personal file-sharing services, unapproved collaboration tools) and assign risk scores based on security posture, compliance certifications, and vendor reputation. Security teams use this data to build an approved app catalog, block high-risk services, and educate users about safer alternatives.

For example, if your finance team is using a personal WeTransfer account to share budget files, the CASB flags it as high-risk (no encryption at rest, no audit logging, no compliance certifications) and recommends a sanctioned alternative like Box or OneDrive with enterprise controls enabled.

Securing SaaS Applications (Office 365, Salesforce, Google Workspace)

CASBs integrate with major SaaS platforms to enforce access controls, monitor user activity, and prevent data exfiltration. They apply conditional access policies, require MFA for external access, block downloads to unmanaged devices, restrict sharing to specific domains, and log every action for compliance auditing.

This use case is particularly valuable for organizations that rely heavily on SaaS but lack the in-house expertise to configure and monitor each platform's native security controls. The CASB provides a unified policy layer across all cloud services, reducing configuration drift and ensuring consistent enforcement.

Preventing Data Leakage from Cloud Storage

CASBs apply DLP rules to cloud storage services (Dropbox, Box, OneDrive) to block uploads of sensitive files or enforce encryption requirements. They inspect files in transit and at rest (via API), identify sensitive data patterns, and take protective actions before the data leaves your control.

For example, a CASB might block any file containing Social Security numbers from being uploaded to a personal Dropbox account, or automatically encrypt any file labeled "Confidential" before it's stored in OneDrive. These controls reduce the risk of accidental or malicious data leakage via cloud storage.

CASB Deployment Models: Cloud-Native vs. On-Premises

CASBs are delivered in two primary architectures: cloud-delivered (SaaS-based) and on-premises (appliance or virtual machine). The choice depends on your network architecture, latency tolerance, and operational preferences.

Cloud-Delivered CASB

Cloud-native CASBs are deployed as SaaS services, requiring no on-premises hardware and offering faster updates and scalability. You redirect traffic to the CASB's cloud infrastructure via proxy configuration, DNS redirection, or endpoint agents, and the vendor manages the platform, updates, and threat intelligence feeds.

This model is faster to deploy, easier to scale, and requires minimal operational overhead. It's well-suited for distributed workforces, remote users, and organizations that prefer to offload infrastructure management to the vendor.

On-Premises CASB

On-premises CASBs are deployed as physical or virtual appliances within your data center or network perimeter. Traffic is routed through the appliance before reaching the internet, giving you full control over data handling, encryption, and policy enforcement.

This model is preferred by organizations with strict data sovereignty requirements, regulatory constraints, or low tolerance for third-party data handling. It also reduces latency for users on-premises, since traffic doesn't need to hairpin to a cloud-based CASB.

What CASBs Can't Do: The Content Inspection Gap

CASBs cannot inspect encrypted content at the application layer. They enforce policies based on metadata, user identity, file attributes, and behavioral signals, but they can't see inside encrypted payloads to detect threats hidden in legitimate file formats.

This is the fundamental limitation of cloud access security brokers. If an attacker embeds malware in a PDF, hides a C2 beacon in an image file, or exfiltrates data via an encrypted channel, the CASB won't detect it unless the threat matches a known signature or triggers a behavioral anomaly.

When to Use a CASB (and When You Need More)

CASBs deliver clear value in scenarios where shadow IT discovery, SaaS security posture management, compliance enforcement, and basic DLP for cloud applications are the primary concerns. They're effective at surfacing what's in use, applying governance rules, and auditing user activity across cloud services.

But CASBs fall short in scenarios where advanced threats are delivered via encrypted traffic, zero-day exploits are embedded in legitimate file formats, or data exfiltration occurs via covert channels. They can't detect malware hidden inside encrypted payloads, can't prevent polymorphic threats that evade signatures, and can't stop attackers who use living-off-the-land techniques to blend in with normal user behavior.

Cloud Access Security Broker FAQ