CASB (Cloud Access Security Broker)
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What Is a CASB? Your Complete Guide to Controlling Cloud App Risk in 2024
Overview: A Cloud Access Security Broker (CASB) is a security tool that sits between your users and the cloud services they access. It gives your security team visibility into cloud usage, enforces access policies, and protects sensitive data moving to and from cloud applications.
Your employees are using cloud apps your IT team never approved. Dropbox, personal Google Drive accounts, ChatGPT, unauthorized SaaS tools, the list keeps growing. Research suggests the average organization uses over 1,000 cloud services, and security teams are aware of only a fraction of them. A CASB closes that visibility gap and gives you control over cloud activity that would otherwise be completely invisible.
What Is a CASB?
A Cloud Access Security Broker is a software tool or service that acts as an intermediary between your organization’s users and the cloud applications they use. Every time a user accesses a cloud service, the CASB enforces your organization’s security policies, monitors behavior, and protects data.
The term was coined by Gartner in 2012, and the category has since become a core part of enterprise cloud security programs. CASBs were initially designed to handle shadow IT, the unauthorized use of cloud services that IT teams couldn’t see or control. Today they handle a much broader range of cloud security functions.
A CASB can be deployed as a proxy, sitting in line with cloud traffic, or as an API integration, connecting directly to cloud service APIs to monitor activity and enforce policies without redirecting traffic.
How Does a CASB Work?
A CASB works through four core functions: visibility, data security, compliance, and threat protection.
Visibility means discovering every cloud service your organization uses, not just the ones IT approved. The CASB analyzes traffic, identifies cloud apps, and builds a complete picture of your cloud usage. This includes the risk profile of each app based on factors like encryption standards, data residency, and the provider’s security certifications.
Data security means applying policies to control how sensitive information moves between your organization and cloud services. A CASB can block uploads of confidential files to unauthorized apps, apply encryption to data before it reaches a cloud service, or watermark documents to track where they go.
Compliance means monitoring cloud activity against regulatory requirements like GDPR, HIPAA, or PCI-DSS. A CASB can generate audit trails, enforce data residency requirements, and flag activity that could create compliance exposure.
Threat protection means detecting and responding to abnormal user behavior that could indicate a compromised account or an insider threat. If a user suddenly downloads thousands of files at 2 a.m. from an unusual location, a CASB can flag or block that activity automatically.
Types of CASB Deployments
Forward Proxy: The CASB intercepts all outbound cloud traffic from your managed devices before it reaches the cloud service. This works well for managed devices but misses unmanaged ones.
Reverse Proxy: The CASB intercepts traffic coming into cloud applications, which allows it to enforce policies even on unmanaged devices like personal phones or contractor laptops.
API-Based Integration: The CASB connects directly to cloud service APIs to monitor activity and enforce policies after the fact. This mode offers deeper integration with specific apps and works well for sanctioned services like Microsoft 365 or Salesforce.
Most enterprise deployments use a combination of these modes to maximize coverage.
Key CASB Use Cases
Discovering and assessing shadow IT applications your employees use without authorization is one of the most immediate uses. A CASB can generate a complete inventory of cloud services in use, along with a risk score for each one.
Preventing data exfiltration is another critical use. A CASB can block users from uploading sensitive files to personal cloud storage or unauthorized apps, even if they try from a device outside your corporate network.
Enforcing conditional access policies lets you allow access to cloud apps only from managed devices, specific locations, or when users meet certain risk conditions.
Detecting compromised accounts involves monitoring for behavioral anomalies that suggest a credential has been stolen and is being used by an attacker.
CASB Best Practices
Start by inventorying your cloud usage before configuring policies. You need to understand what your organization actually uses before you can make informed decisions about what to allow, restrict, or monitor.
Prioritize sanctioned applications first. Deploy API-based integrations for your most critical cloud services and build out shadow IT monitoring from there.
Combine proxy and API modes where possible. Each mode has coverage gaps that the other fills.
Align your CASB policies with your broader data classification framework. Policies are most effective when they reflect how your organization actually categorizes and handles sensitive data.
Review your CASB reports regularly. Threat detection is only useful if someone acts on the alerts it generates.
Frequently Asked Questions
CASB stands for Cloud Access Security Broker. It refers to a security tool that monitors and controls cloud application usage within an organization, sitting between users and cloud services to enforce security and compliance policies.
Organizations need a CASB because employees routinely use cloud applications that IT teams haven't vetted or approved. Without a CASB, sensitive data could flow to unsecured cloud services without your knowledge. A CASB provides visibility into that activity and lets you enforce consistent policies across all cloud usage.
Traditional firewalls and web proxies operate at the network level and apply broad rules based on IP addresses, ports, and protocols. A CASB operates at the application and user level, understanding context like which cloud app is being used, what data is being moved, and whether the behavior matches normal patterns for that user.
A CASB and a VPN serve different purposes. A VPN encrypts and routes network traffic through your organization's network. A CASB monitors and controls access to cloud applications specifically. Organizations often use both, but modern zero trust architectures are increasingly replacing VPNs with CASB and ZTNA solutions for cloud access.
Yes, through reverse proxy deployment. A reverse proxy CASB can enforce policies on traffic reaching cloud applications from any device, including personal phones and contractor laptops that aren't enrolled in your mobile device management system.
