NullPointerException during WebAuthn registration when authenticator transport is unknown (e.g. "cable")

Describe the bug

When invoking the WebAuthn registration endpoint (/webauthn/register), Spring Security throws a NullPointerException if the registration payload contains an authenticator transport value that is not recognized by Spring Security.

For example, when the transport list contains "cable" (Cloud-assisted Bluetooth Low Energy), the request processing fails with:

java.lang.NullPointerException: Cannot invoke
"org.springframework.security.web.webauthn.api.AuthenticatorTransport.getValue()"
because "transport" is null
    at org.springframework.security.web.webauthn.management.Webauthn4JRelyingPartyOperations.convertTransportsToString(Webauthn4JRelyingPartyOperations.java:309)

To Reproduce

1. Configure a Spring Security application with WebAuthn enabled.
2. Call the WebAuthn registration endpoint (/webauthn/register).
3. Include an authenticator transport value that is not mapped to Spring Security's AuthenticatorTransport enum, for example:

{
...
"transports": ["cable"]
...
}

Observe that request processing fails with a NullPointerException.

Expected behavior
The framework should not throw a NullPointerException when an unknown transport is received.
Possible expected behaviors:

• Reject the request with a meaningful validation exception.
• Ignore unsupported transports.
• Preserve unknown transports as strings if supported by the implementation.

In any case, the framework should fail gracefully instead of throwing an NPE.

Analysis

The issue appears to originate in:

org.springframework.security.web.webauthn.jackson.AuthenticatorTransportDeserializer

When an unknown authenticator transport value is encountered (for example "cable"), the deserializer's deserialize() method returns null.

The resulting collection is later processed by:

org.springframework.security.web.webauthn.management.Webauthn4JRelyingPartyOperations

Specifically, in:

convertTransportsToString(...)

which assumes all transport values are non-null and invokes:

transport.getValue()

As a result, the following exception is thrown:

java.lang.NullPointerException: Cannot invoke
"org.springframework.security.web.webauthn.api.AuthenticatorTransport.getValue()"
because "transport" is null

From my analysis, the root cause is that unsupported transport values are silently converted to null during deserialization and are subsequently dereferenced without validation.

Spring Security should either:

• Reject unknown transport values with a meaningful exception during deserialization, or
• Filter/ignore unsupported values before processing, or
• Preserve unknown values in a forward-compatible manner.

Any of these approaches would prevent the NullPointerException and provide a more predictable failure mode.

Start with AuthenticatorTransportDeserializer and trace the unknown "cable" value into Webauthn4JRelyingPartyOperations.convertTransportsToString(...). Reproduce registration with an unsupported transport, then verify the request no longer fails with a NullPointerException and follows the chosen graceful-handling behavior.