ams-spring-boot-shopping: `/products` always returns 403

Summary

In ams-spring-boot-shopping, calling GET /products (and GET /privileges) with a valid IAS token is rejected with HTTP 403, even though the app is bound, token is valid, policies are pushed.

Steps to reproduce

1. Deploy the sample as single-tenant IAS app on CF. Bind the app to a IAS app, create app2app trust, create client credentials for the consumer app.
2. Obtain a valid IAS token for the consumer app. Make sure to hit the right dependency. Should look like [1]
3. curl -H "Authorization: Bearer <token>" https://<app>/products

Expected: 200 with the product list (the GetProducts API maps to internal.GetProducts → shopping.ReadProducts → GRANT read ON products).
Actual: 403.

Root cause

The request is authenticated but no AMS principal is ever established, so the AMS authorization check runs against empty authorizations and denies.

SecurityConfiguration#filterChain wires the resource server with:

https://github.com/SAP-samples/ams-samples-java/blob/10e80968de39c73cafe70f83fc30ced2b0c72b31/ams-spring-boot-shopping/src/main/java/com/sap/cloud/security/ams/samples/config/SecurityConfiguration.java#L62

With the default converter, Spring produces a plain org.springframework.security.oauth2.server.resource.authentication.JwtAuthenticationToken whose principal is a plain org.springframework.security.oauth2.jwt.Jwt, not a SAP com.sap.cloud.security.spring.token.authentication.AuthenticationToken.

The cloud-security JavaSecurityContextHolderStrategy only copies the SAP Token into com.sap.cloud.security.token.SecurityContext when the Spring Authentication's principal is a SAP Token. Because it isn't, SecurityContext.getToken() stays null, so com.sap.cloud.security.ams.api.Principal.fromSecurityContext() returns null, and SciAuthorizationsProvider short-circuits to empty authorizations. Every @CheckPrivilege then evaluates granted=false → 403. Logs here [2].

Direction for a fix

The app needs a jwtAuthenticationConverter that turns the validated IAS Jwt into a SAP AuthenticationToken, so the AMS principal is established. Worked for me afterwards.

[1]

{
  "ias_apis": [
    "GetProducts"
  ],
  "sub": "consumer",
  "aud": "ams-secured-app",
  "app_tid": "app-tid",
  "sap_id_type": "app",
  "azp": "consumer",
  "iss": "https://iasid.ondemand.com",
  "azpacr": "1",
  "exp": 1787302610,
  "iat": 1787299010,
  "jti": "096f3ac5-adf2-4516-845b-dd295afbfab0"
}

[2]

IasJwtDecoder            : The token of service IAS was successfully validated.
BearerTokenAuthenticationFilter : Set SecurityContextHolder to JwtAuthenticationToken
    [Principal=org.springframework.security.oauth2.jwt.Jwt@..., Granted Authorities=[]]
SciAuthorizationsProvider : No principal provided. Using empty authorizations.
...
horizationManagerBeforeMethodInterceptor : Failed to authorize ... ProductsService.getProducts()
    ... result AuthorizationDecision [granted=false]

Start in ams-spring-boot-shopping/src/main/java/com/sap/cloud/security/ams/samples/config/SecurityConfiguration.java at SecurityConfiguration#filterChain. Reproduce the request with a valid IAS token, then inspect how the resource server converts the Jwt before AMS authorization runs. Done means the AMS principal is established and GET /products and GET /privileges return the expected authorized responses instead of 403.