SSO: Verify that the userinfo subject matches the id_token subject

OpenID Connect Core 5.3.2 requires that the sub claim of the userinfo
response is verified to exactly match the sub claim of the id_token, and
that the userinfo claims are not used otherwise. The openidconnect crate
implements this check, but only when an expected subject is provided,
and the user_info call passed None, so the check never ran.

Thread the id_token subject through to the userinfo request made during
the initial code exchange, so that a mismatched response is rejected
before its email/email_verified/preferred_username claims are used. The
check_validity probe still passes None since no id_token is available at
that point.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/sso.rs

@@ -279,7 +279,8 @@ pub async fn exchange_code(
     let client = Client::cached().await?;
     let (token_response, id_claims) = client.exchange_code(code, client_verifier, &sso_auth).await?;
 
-    let user_info = client.user_info(token_response.access_token().to_owned()).await?;
+    let user_info =
+        client.user_info(token_response.access_token().to_owned(), Some(id_claims.subject().clone())).await?;
 
     let email = match id_claims.email().or(user_info.email()) {
         None => err!("Neither id token nor userinfo contained an email"),

src/sso_client.rs

@@ -5,7 +5,7 @@ use openidconnect::{
     AuthorizationRequest, ClientId, ClientSecret, CsrfToken, EmptyAdditionalClaims, EmptyExtraTokenFields,
     EndpointNotSet, EndpointSet, HttpClientError, HttpRequest, HttpResponse, IdTokenClaims, IdTokenFields, Nonce,
     OAuth2TokenResponse, PkceCodeChallenge, PkceCodeVerifier, RefreshToken, ResponseType, Scope, StandardErrorResponse,
-    StandardTokenResponse,
+    StandardTokenResponse, SubjectIdentifier,
     core::{
         CoreAuthDisplay, CoreAuthPrompt, CoreClient, CoreClientAuthMethod, CoreErrorResponseType, CoreGenderClaim,
         CoreIdTokenVerifier, CoreJsonWebKey, CoreJweContentEncryptionAlgorithm, CoreJwsSigningAlgorithm,
@@ -272,8 +272,12 @@ impl Client {
         }
     }
 
-    pub async fn user_info(&self, access_token: AccessToken) -> ApiResult<CoreUserInfoClaims> {
-        match self.core_client.user_info(access_token, None).request_async(&self.http_client).await {
+    pub async fn user_info(
+        &self,
+        access_token: AccessToken,
+        expected_subject: Option<SubjectIdentifier>,
+    ) -> ApiResult<CoreUserInfoClaims> {
+        match self.core_client.user_info(access_token, expected_subject).request_async(&self.http_client).await {
             Err(err) => err!(format!("Request to user_info endpoint failed: {err}")),
             Ok(user_info) => Ok(user_info),
         }
@@ -281,7 +285,8 @@ impl Client {
 
     pub async fn check_validity(access_token: String) -> EmptyResult {
         let client = Client::cached().await?;
-        match client.user_info(AccessToken::new(access_token)).await {
+        // No expected subject to verify against since only the access_token is kept after login
+        match client.user_info(AccessToken::new(access_token), None).await {
             Err(err) => {
                 err_silent!(format!("Failed to retrieve user info, token has probably been invalidated: {err}"))
             }