Continuous Access Evaluation: Maybe We Should Stop Trusting Yesterday’s Authentication

There is a basic assumption hiding inside many identity architectures in that if someone authenticated successfully, they should continue to have access.

That assumption is reasonable.

It is also increasingly wrong.

A user can authenticate at 9:00 AM and have their credentials compromised at 9:15.

A device can be compliant at 10:00 AM and become non-compliant at 10:01.

An administrator can disable an account while the user’s existing sessions are still active.

A user’s security posture can change without anything happening to the user’s password.

And yet, in many identity systems, the application knows none of this until something causes it to check again.

Usually, that something is token expiration.

That can leave a considerable gap between:

The OpenID Continuous Access Evaluation Profile (CAEP) is designed to reduce that gap.

CAEP provides standardized event types for communicating changes that can affect access to users, devices, sessions, and applications.

It is built on the OpenID Foundation’s Shared Signals Framework (SSF).

And, the basic idea is straightforward, access decisions should be able to change when the conditions behind those decisions change.

Authentication Is a Snapshot

Consider a typical OpenID Connect login.

A user authenticates to an identity provider:

The application now knows that the user successfully authenticated.

Depending on the architecture, it may also know things such as:

• User ID
• Authentication method
• Authentication time
• MFA status
• Organization
• Role
• Assurance level

Those facts become inputs to an access decision.

The problem is that they are essentially a snapshot.

The application knows what was true when the authentication occurred.

It doesn’t automatically know what becomes true afterward.

Suppose the user changes their password.

Or, the administrator changes their role.

Or, the security team determines that the account has been compromised.

Or, the user’s device is no longer compliant.

The application may continue operating under the assumption that the original authentication state remains valid.

That is the problem CAEP is intended to address.

What Does “Continuous” Actually Mean?

The name can be slightly misleading.

Continuous Access Evaluation does not mean that the application continuously contacts the identity provider every few seconds asking:

> “Is this user still okay?”

“How about now?”

“Now?”

“Now?”

That would be expensive, inefficient, and generally unpleasant for everyone involved.

Instead, CAEP provides an event-driven model.

Something changes.

The system that knows about that change sends a signal.

The receiving system can then reevaluate access.

Conceptually:

The important word is reevaluate.

CAEP does not necessarily tell the receiver, “Terminate this user.”

It tells the receiver, “Something relevant to the access decision has changed.”

The receiver decides what to do about it.

CAEP Is a Profile of SSF

CAEP isn’t a standalone transport protocol.

It is a profile of the OpenID Shared Signals Framework.

The relationship is roughly:

SSF provides the signaling infrastructure.

CAEP defines a standardized collection of events relevant to continuous access evaluation.

The OpenID Foundation describes CAEP as specifying event types that allow cooperating transmitters and receivers to communicate continuous updates concerning human or robotic users, devices, sessions, and applications.

This separation is useful.

SSF answers, how do systems exchange security signals.

CAEP answers what kinds of access-related changes should they communicate?

Transmitters and Receivers

CAEP uses the terminology established by SSF.

The system that knows something has changed is the Transmitter.

The system that needs to know about the change is the Receiver.

For example:

But, the transmitter doesn’t have to be an identity provider.

A device management platform might know that a device has become non-compliant.

A security system might know that an account has been compromised.

An application might know that a session has been revoked.

The architecture is deliberately broader than:

It is really:

That makes CAEP useful in distributed identity architectures.

Security Event Tokens

CAEP events are transported as Security Event Tokens, or SETs.

SETs are defined by the IETF in RFC-8417.

A SET is essentially a signed JWT carrying a security event.

Conceptually:

The resulting architecture is therefore layered:

Each layer has a different job.

The JWT provides a standard signed-token representation.

The SET defines the security-event token semantics.

SSF provides the signaling and stream infrastructure.

CAEP defines the access-related event types.

That layering is one of the more important things to understand about the architecture.

The CAEP Event Types

CAEP defines a number of standardized event types.

Among the most important are:

• Session Revoked
• Token Claims Change
• Credential Change
• Assurance Level Change
• Device Compliance Change

The OpenID Foundation identifies these as core CAEP events.

Each represents a different reason why an existing access decision might need to be reconsidered.

Let’s look at what that means.

Session Revoked

Suppose a user is logged into an application.

The identity system determines that the session should no longer be valid.

Without an event mechanism, the application may continue believing that the session is valid until its local session or token expires.

With CAEP:

The application can then terminate the session or take whatever other action its policy requires.

This is particularly useful when the desired response needs to happen sooner than ordinary token expiration would allow.

Token Claims Change

Tokens often contain claims that influence authorization.

For example:

department = finance role = administrator clearance = high

But those attributes can change.

Suppose the user moves from Finance to Marketing.

If an existing access token contains the old information, simply possessing that token doesn’t make the old information true.

CAEP provides a mechanism for communicating that relevant token claims have changed.

The receiver can then reevaluate the user’s access.

This is an important distinction.

The token isn’t necessarily being treated as invalid because the cryptographic signature failed.

The token may still be perfectly valid cryptographically.

The problem is that the world changed after the token was issued.

Credential Change

Credentials can change independently of a user’s current session.

For example:

• Password changed
• Authenticator changed
• Credential reset
• Credential compromised

Depending on the event and receiving system, this can be a reason to reconsider existing access.

The important concept is that authentication isn’t necessarily a one-time event.

A credential’s security state can change after authentication.

CAEP gives cooperating systems a standardized way to communicate that change.Assurance Level Change

Not every authentication is equally strong.

A system may distinguish between:

• Password only
• Password + MFA
• Hardware-backed authentication
• Phishing-resistant authentication

An application’s policy might therefore depend on an authentication assurance level.

Imagine that a user authenticated with a strong authentication method and was granted access to a sensitive resource.

Later, the relevant assurance state changes.

The application may need to reevaluate whether that level of assurance is still sufficient.

CAEP provides an event specifically for communicating such changes.

This is particularly relevant in environments where authorization policies depend on authentication assurance rather than simply on the fact that authentication occurred.

Device Compliance Change

Modern identity systems increasingly make access decisions based on the device as well as the user.

For example:

A device might initially satisfy organizational requirements:

• Encrypted disk
• Up-to-date operating system
• Managed device
• Endpoint protection enabled

Then, something changes.

Perhaps, the device becomes unmanaged.

Perhaps, security software is disabled.

Perhaps, the device falls below the organization’s required patch level.

The identity provider or device-management system can communicate that change through a CAEP event.

This is a good example of why continuous access evaluation cannot be reduced to an identity-provider problem.

The identity provider may not be the system that knows whether a device is compliant.

CAEP Doesn’t Make the Access Decision

This deserves emphasis.

Suppose a receiver gets:

Device Compliance Change

CAEP doesn’t dictate:

DENY ACCESS

The receiver might decide:

Terminate session

or:

Require MFA

or:

Restrict access to sensitive resources

or:

Allow access but require remediation

or:

User’s Device: Initiate Self-Destruct

That is a policy decision.

CAEP communicates the fact that something changed.

This distinction makes CAEP much more flexible.

Different applications can react differently to the same event.

The Zero Trust Connection

CAEP is particularly relevant to Zero Trust architectures.

Zero Trust is often summarized with the phrase: Never trust, always verify.

But there is another concept hiding behind that statement: A previous verification shouldn’t necessarily remain sufficient forever.

Consider:

That is much closer to the spirit of continuous verification.

The OpenID Foundation specifically describes CAEP as a standardized mechanism for communicating changes to access properties and identifies it as a cornerstone of Zero Trust security.

Short-Lived Tokens Aren’t Enough

A natural response to this problem is to just make the access tokens expire faster.

That certainly helps.

Suppose, an access token lasts eight hours.

A compromised token might remain useful for almost eight hours.

So, someone changes the expiration time to:

15 minutes

That’s better.

But, now the system has a different problem.

The token still doesn’t know that something changed.

If an account is compromised one minute after a token is issued, the token may remain usable for another fourteen minutes.

Make the token lifetime one minute and the problem gets smaller, but now you’ve increased token issuance and validation overhead.

And, you’ve still built a fundamentally time-based solution to an event-based problem.

CAEP offers another approach:

These mechanisms can complement each other.

What Happens When a CAEP Event Arrives?

The exact response is implementation-specific, but the conceptual flow looks something like:

The receiver needs to verify that the event is trustworthy.

It needs to determine what subject the event concerns.

It may need to correlate the subject with local sessions.

Then, it can apply its own access policy.

This last step is critical.

A CAEP receiver isn’t simply a message consumer.

It is part of the security architecture.

The Subject Correlation Problem

There is a practical issue hiding underneath all of this.

Suppose the identity provider calls a user:

user-12345

while the SaaS application calls that same user:

00u7abcxyz

The device-management system might have another identifier entirely.

How does the receiver know which local account the event concerns?

SSF provides standardized subject identification mechanisms to help solve this problem.

This is more important than it may initially appear.

Security signaling is only useful if the receiver can reliably determine who or what the signal is about.

A false positive can lock out the wrong person.

A false negative can leave the actual compromised account untouched.

Subject correlation is therefore a security-critical part of the architecture.

CAEP and Session Management

One of the most interesting consequences of CAEP is that it changes how we think about sessions.

Traditionally:

With continuous evaluation:

The session becomes something that exists subject to continuing security conditions.

That is a significant architectural shift.

This Doesn’t Mean Constantly Interrupting Users

There is sometimes a concern that continuous access evaluation means users will constantly be thrown back to a login screen.

That isn’t the goal.

In fact, the point is to make security responses more contextual.

A low-risk change might require no visible action.

A change affecting authentication assurance might require reauthentication.

A severe compromise might terminate the session immediately.

The receiver’s policy determines the appropriate response.

The signaling mechanism simply provides the information required to make that decision.

CAEP Is Useful Beyond Human Users

The specification isn’t limited to traditional human-user sessions.

The SSF/CAEP architecture can also apply to:

• Human users
• Robotic users
• Devices
• Sessions
• Applications

This matters because modern enterprises contain enormous numbers of non-human identities.

• Service accounts
• Workload identities
• Automation
• APIs
• Agents
• Devices

The security state of these entities can change too.

A continuous signaling architecture therefore has applicability well beyond browser-based SSO.

CAEP and RISC Are Related, But Different

CAEP and RISC are both profiles of SSF, but they address different classes of events.

A useful simplification is:

CAEP is concerned with changes that affect access posture.

RISC is focused more heavily on security incidents and account-compromise scenarios.

The distinction is useful because not every security event means: “This account has been compromised.”

Sometimes, the message is simply: “Something about the conditions under which access was granted has changed.”

That is where CAEP fits particularly well.

CAEP Is Not a Replacement for OAuth or OpenID Connect

It is also important to understand what CAEP does not do.

CAEP doesn’t replace OAuth2, OIDC, and SAML2.

Those protocols continue to handle authentication, authorization, and token issuance in their respective roles.

CAEP adds another capability:

It is therefore better thought of as a complementary layer.

The protocols establish and use access.

CAEP provides a mechanism for communicating when the conditions around that access change.

A More Complete Architecture

Putting the pieces together:

Meanwhile:

The two paths are complementary.

The first establishes access.

The second communicates changes that may require that access to be reconsidered.

The Real Value of CAEP

The most important thing CAEP provides isn’t another token format.

It isn’t another login protocol.

It isn’t another mechanism for putting claims into a JWT.

The important change is conceptual.

Traditional identity systems tend to think in terms of:

CAEP encourages a different model:

That is a much better match for modern distributed systems.

Identity isn’t static.

Devices aren’t static.

Credentials aren’t static.

Risk isn’t static.

Sessions aren’t static.

And therefore, access decisions shouldn’t necessarily be static either.

The Bigger Picture

There is a subtle limitation in the phrase, “The user authenticated successfully.”

It tells us something important, but it tells us something about a particular point in time.

The security properties surrounding that authentication can change immediately afterward.

That is the fundamental problem CAEP addresses.

It provides a standardized way for systems to communicate, “The thing you relied upon when granting access has changed.”

The receiving system can then decide what that means.

Perhaps, nothing.

Perhaps, reauthentication.

Perhaps, reduced privileges.

Perhaps, session termination.

Perhaps, complete denial.

That separation between signal and policy is important.

CAEP doesn’t try to become the world’s universal authorization engine.

It provides a standardized mechanism for communicating changes that matter to authorization and session decisions.

And, that is ultimately what makes the architecture powerful.

The old model was authenticate once and trust the result until it expires.

The newer model is authenticate, establish access, and continuously reevaluate that access as relevant security conditions change.

That doesn’t mean every request needs to trigger another login.

It means that when the security facts change, the systems making access decisions can find out about it.

In a world of cloud services, federated identity, managed devices, APIs, workload identities, and increasingly distributed security controls, that capability is becoming less of a luxury and more of a requirement.

Authentication tells you what was true when you authenticated.

CAEP gives you a standardized way to hear when something important has changed.

Notes

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