diff --git a/backend/src/app/http/auth_request.clj b/backend/src/app/http/auth_request.clj
index 867b9c3c4bb..7e3dd1e10ac 100644
--- a/backend/src/app/http/auth_request.clj
+++ b/backend/src/app/http/auth_request.clj
@@ -150,17 +150,18 @@
         (handler request)
 
         :else
-        (let [local-part (first (str/split email-claim #"@"))
-              email      (resolve-email email-claim)
-              fullname   (or (not-empty (yreq/get-header request "x-auth-request-user"))
-                             local-part)
-              profile    (try
-                           (get-or-register-profile cfg email fullname)
-                           (catch Throwable cause
-                             (l/err :hint "x-auth-request: error resolving profile"
-                                    :email email
-                                    :cause cause)
-                             nil))]
+        ;; Pass nil for the display name: get-or-register-profile is the single
+        ;; place that derives it from the resolved email local-part. We never
+        ;; trust an upstream-supplied name (oauth2-proxy put the Cognito sub
+        ;; UUID in x-auth-request-user), so there is nothing to forward here.
+        (let [email   (resolve-email email-claim)
+              profile (try
+                        (get-or-register-profile cfg email nil)
+                        (catch Throwable cause
+                          (l/err :hint "x-auth-request: error resolving profile"
+                                 :email email
+                                 :cause cause)
+                          nil))]
           (cond
             (nil? profile)
             ;; Header email doesn't resolve to a profile (and auto-register
diff --git a/backend/test/backend_tests/http_middleware_test.clj b/backend/test/backend_tests/http_middleware_test.clj
index 9b498c7437f..76177635553 100644
--- a/backend/test/backend_tests/http_middleware_test.clj
+++ b/backend/test/backend_tests/http_middleware_test.clj
@@ -379,14 +379,17 @@
 (t/deftest x-auth-request-auto-register-creates-active-profile
   (binding [cf/flags (conj cf/flags :x-auth-request-auto-register)]
     (let [email    "newuser@example.com"
-          fullname "New User"
           captured (volatile! nil)
           cfg      (make-xauth-cfg)
           handler  (#'app.http.auth-request/wrap-authz
                     (fn [req] (vreset! captured req) {::yres/status 200})
                     cfg)
+          ;; Regression guard: oauth2-proxy set x-auth-request-user to the
+          ;; Cognito sub UUID. The middleware must IGNORE that header and derive
+          ;; :fullname from the email local-part — sending it here proves it is
+          ;; not trusted for the display name.
           response (handler (->DummyRequest {"x-auth-request-email" email
-                                             "x-auth-request-user"  fullname} {}))]
+                                             "x-auth-request-user" "892ae5ac-0021-7000-8000-000000000000"} {}))]
       ;; Profile must be injected into the downstream request
       (t/is (uuid? (::session/profile-id @captured)))
       ;; A session cookie must be set so the browser is authenticated
@@ -397,6 +400,8 @@
                                   (profile/get-profile-by-email conn email)))]
         (t/is (some? profile))
         (t/is (true? (:is-active profile)))
+        ;; Derived from the email local-part, NOT the x-auth-request-user UUID.
+        (t/is (= "newuser" (:fullname profile)))
         (t/is (= (::session/profile-id @captured) (:id profile)))))))
 
 (t/deftest x-auth-request-auto-register-joins-named-smb-team
@@ -418,8 +423,7 @@
             handler  (#'app.http.auth-request/wrap-authz
                       (fn [req] (vreset! captured req) {::yres/status 200})
                       cfg)
-            _        (handler (->DummyRequest {"x-auth-request-email" email
-                                               "x-auth-request-user"  "Shared Team Join"} {}))
+            _        (handler (->DummyRequest {"x-auth-request-email" email} {}))
             profile  (db/tx-run! cfg
                                  (fn [{:keys [::db/conn]}]
                                    (profile/get-profile-by-email conn email)))