What Is CASB? Cloud Access Security Broker

CASB, or Cloud Access Security Broker, is a security control layer that helps organizations monitor, govern, and protect access to cloud apps and SaaS data. In practical terms, a CASB gives security teams visibility into cloud usage, applies policy to that usage, and helps reduce risks like shadow IT, data exposure, and account compromise.

If your business relies on Microsoft 365, Google Workspace, Salesforce, Slack, or other SaaS platforms, CASB is one of the main technologies used to enforce cloud security policy.

CASB definition

A Cloud Access Security Broker is a control point between users and cloud services, or an integration layer connected directly to those services, that enforces security and compliance policies.

CASB platforms are commonly used to:

discover cloud applications in use
identify unsanctioned SaaS usage
apply access controls
protect sensitive data
detect risky behavior in cloud apps
support compliance and audit requirements

The exact feature set varies by vendor, but the main purpose is consistent: bring visibility and policy enforcement to cloud usage that would otherwise be hard to control.

How CASB works

CASB works by gathering visibility into cloud usage and then applying policy to users, devices, data, and actions. Some products operate inline, some connect by API, and many use a combination of both.

Discover cloud app usage

One of the first CASB use cases is discovering shadow IT, meaning cloud services employees use without formal IT approval.

A CASB can analyze sources such as:

firewall logs
proxy traffic
endpoint telemetry
identity provider activity
SaaS audit logs
direct API data from cloud platforms

This helps answer questions like:

Which cloud apps are employees using?
Which ones are approved?
Which departments are using them?
What data is being uploaded, downloaded, or shared?

For many organizations, this visibility alone is a major improvement because unmanaged cloud use is often much broader than expected.

Apply access and session policies

Once usage is visible, the CASB can enforce rules around who can access which applications and under what conditions.

Examples include:

blocking unsanctioned SaaS apps
allowing view-only access from unmanaged devices
preventing downloads of sensitive files
requiring stronger authentication for high-risk actions
restricting external sharing
limiting access by user role, geography, or device posture

This makes CASB especially useful in environments where employees work remotely, bring their own devices, or access cloud apps from different locations.

Protect sensitive data

Data protection is a core CASB function. Many platforms inspect activity and content to identify sensitive information in cloud services.

This often includes:

DLP policy enforcement
detection of regulated data types
alerts for public or external sharing
blocking high-risk uploads
monitoring mass downloads
flagging misconfigured sharing settings

For example, a CASB may detect that a user is sharing files containing customer records outside the company and either block the action or alert security.

Detect risky behavior and threats

CASB tools can also help identify suspicious behavior in cloud environments, including:

impossible travel logins
suspicious file access
abnormal download volume
malicious OAuth app grants
privilege misuse in SaaS platforms
access from unmanaged devices
compromised account behavior

This is where CASB often overlaps with broader identity and cloud monitoring. If you are comparing those wider detection and response models, see what is mdr.

Integrate with other security controls

CASB is rarely a standalone solution. It usually works with other security layers such as:

identity providers
MFA
endpoint management
SIEM or XDR
DLP tooling
secure web gateways
Zero Trust access controls

That integration is important because CASB is not a replacement for strong identity, endpoint security, or cloud configuration management. It is a policy and visibility layer focused on cloud application use.

CASB deployment models

There are two common ways CASB platforms operate.

API-based CASB

API-based CASB connects directly to supported SaaS applications. This gives visibility into stored data, sharing settings, user actions, and configuration issues.

Benefits usually include:

broad visibility into sanctioned SaaS apps
easier deployment for supported platforms
inspection of data already in the cloud
governance of sharing and permissions

A limitation is that API-based CASB may not always stop an action in real time before it happens.

Inline or proxy-based CASB

Inline CASB sits in the traffic path and can make real-time decisions about sessions, downloads, uploads, and access.

Benefits usually include:

real-time policy enforcement
session control
stronger unmanaged-device restrictions
immediate blocking of risky actions

The tradeoff is that inline architectures can be more complex to deploy and may depend on routing traffic through specific paths or gateways.

Many organizations use both models because they solve different parts of the problem.

What CASB is good at

CASB is especially useful for:

discovering shadow IT
enforcing SaaS usage policy
extending DLP into cloud apps
controlling access from unmanaged devices
monitoring risky sharing behavior
improving visibility across sanctioned SaaS platforms

It is particularly valuable when a business has moved quickly into cloud services and needs governance without shutting down productivity.

What CASB does not do by itself

CASB does not automatically solve:

poor identity hygiene
weak MFA enforcement
insecure endpoint devices
cloud workload misconfigurations outside supported SaaS apps
every insider threat or data theft scenario
all cloud infrastructure security problems

In practice, CASB works best as part of a broader cloud and identity strategy. For the access-control side of that model, our guide to what is zero trust is a useful companion.

When organizations use CASB

You will usually encounter CASB when a company wants more control over cloud usage without trying to ban SaaS outright.

SaaS governance and shadow IT reviews

CASB often appears when leadership realizes employees are using many more cloud services than IT approved. The first step is usually discovery, followed by sanctioning or blocking decisions.

Microsoft 365, Google Workspace, Box, Dropbox, and similar platforms create constant sharing and download decisions. CASB helps enforce policy around those workflows.

Remote work and BYOD

When employees access business apps from personal laptops or mobile devices, CASB can help allow limited access while still restricting risky actions.

Data loss prevention in the cloud

Organizations extending DLP beyond email and endpoints often use CASB to monitor and control sensitive data in SaaS applications.

Incident response and insider risk reviews

CASB telemetry is useful when investigating suspicious SaaS logins, mass downloads, risky OAuth grants, or unusual sharing activity.

Final takeaway

CASB is a cloud security control that gives organizations visibility and policy enforcement across SaaS and cloud app usage. It helps discover shadow IT, protect sensitive data, control risky access, and monitor suspicious behavior in the cloud.

If your environment depends heavily on SaaS and remote access, CASB is one of the key technologies you will encounter in any serious cloud security program.