A Cloud Access Security Broker (CASB) is a security layer that sits between users and cloud services, enforcing policy and providing visibility into cloud application usage.

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Main takeaways

• CASBs were built for a network-centric world. In a distributed, SaaS-first environment, their architectural assumptions no longer hold.
• CASBs are effective at governing sanctioned applications they're configured to monitor. They have limited visibility into shadow SaaS, shadow AI, and OAuth-based integrations.
• "Lock and block" enforcement drives employees toward unsanctioned workarounds—creating the shadow IT problem rather than solving it.
• Modern SaaS security requires discovery-first approaches that work with employee behavior, not against it.

What is a CASB?

The CASB category emerged around 2012, when organizations were rapidly adopting cloud services and needed a way to extend security policy beyond the corporate network. Gartner formalized the pattern, and the core architectural assumption seemed reasonable at the time: that traffic could be intercepted and inspected—that security teams still controlled a network through which users accessed cloud services. CASBs were built on that assumption, delivering four core capabilities: visibility into cloud app usage, data security (DLP), threat protection, and compliance enforcement.

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Where CASBs break down

That assumption no longer holds for most organizations.

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The modern workforce accesses SaaS from personal devices, home networks, and mobile connections that sit entirely outside the corporate perimeter. Remote and hybrid work has made the network-centric interception model structurally incomplete.

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More fundamentally, CASBs are configured around applications the security team already knows about. They have no mechanism to discover SaaS tools employees adopt independently, AI tools connected through OAuth, or third-party integrations that never touch the network proxy.

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The result is a "known apps" problem: excellent governance over what's already sanctioned and configured, a growing blind spot for everything else.

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Other structural limitations:

• Deployment complexity—Proxy-based CASBs require significant network configuration and ongoing maintenance. Many organizations never fully deploy them.
• Behavioral friction—Hard blocks on unapproved applications push employees toward personal accounts and workarounds, actually increasing shadow IT.
• AI tool gap—CASBs are not designed to discover or govern AI-specific risks: prompt data exposure, OAuth grants to AI platforms, or LLM integrations embedded in existing SaaS apps.
• OAuth blindness—CASBs generally don't map the web of SaaS-to-SaaS integrations authorized through OAuth, which represents a growing share of real-world access.

What a modern approach looks like

The limitations of CASBs reflect a broader shift in how SaaS security needs to work: from traffic inspection to identity-based discovery; from blocking to governance; from known apps to comprehensive visibility across the entire SaaS estate.

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Effective SaaS security today starts by discovering every application in use—including ones IT never approved—and understanding the full map of identities, integrations, and permissions connected to each. From there, governance can be applied in ways that work with employee behavior rather than against it.

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See how Nudge Security compares to CASBs—and what comprehensive SaaS visibility looks like in practice →