use original prompt default value to avoid a breaking change

Signed-off-by: Morgan Martinet <morgan@mmm-experts.com>

app-config.yaml

@@ -312,7 +312,11 @@ auth:
         #
         # scope: saml-login-selector openid profile email
     oidc:
-      # Note that you must define a session secret (above) since the oidc provider requires session support
+      # Note that you must define a session secret (see above) since the oidc provider requires session support.
+      # Note that by default, this provider will use the 'none' prompt which assumes that your are already logged on in the IDP.
+      # You should set prompt to:
+      # - auto: will let the IDP decide if you need to log on or if you can skip login when you have an active SSO session
+      # - login: will force the IDP to always present a login form to the user
       development:
         metadataUrl: ${AUTH_OIDC_METADATA_URL}
         clientId: ${AUTH_OIDC_CLIENT_ID}
@@ -321,7 +325,7 @@ auth:
         tokenUrl: ${AUTH_OIDC_TOKEN_URL}
         tokenSignedResponseAlg: ${AUTH_OIDC_TOKEN_SIGNED_RESPONSE_ALG} # default='RS256'
         scope: ${AUTH_OIDC_SCOPE} # default='openid profile email'
-        prompt: ${AUTH_OIDC_PROMPT} # default='' (allowed values: '', 'none', 'consent', 'login')
+        prompt: ${AUTH_OIDC_PROMPT} # default=none (allowed values: auto, none, consent, login)
     auth0:
       development:
         clientId: ${AUTH_AUTH0_CLIENT_ID}

plugins/auth-backend/src/providers/oidc/provider.ts

@@ -73,12 +73,16 @@ export class OidcAuthProvider implements OAuthHandlers {
 
   async start(req: OAuthStartRequest): Promise<RedirectInfo> {
     const { strategy } = await this.implementation;
-    return await executeRedirectStrategy(req, strategy, {
+    const options: Record<string, string> = {
       accessType: 'offline',
-      prompt: this.prompt || '',
       scope: req.scope || this.scope || 'openid profile email',
       state: encodeState(req.state),
-    });
+    };
+    const prompt = this.prompt || 'none';
+    if (prompt !== 'auto') {
+      options.prompt = prompt;
+    }
+    return await executeRedirectStrategy(req, strategy, options);
   }
 
   async handler(