diff --git a/coderd/workspaceapps/apptest/apptest.go b/coderd/workspaceapps/apptest/apptest.go
index d73336cedc..f0993e8f02 100644
--- a/coderd/workspaceapps/apptest/apptest.go
+++ b/coderd/workspaceapps/apptest/apptest.go
@@ -1247,6 +1247,34 @@ func Run(t *testing.T, appHostIsPrimary bool, factory DeploymentFactory) {
 			assertWorkspaceLastUsedAtNotUpdated(t, appDetails, testutil.WaitLong)
 		})
 
+		// Security (PLAT-260): must 404 when the URL username segment
+		// names a different owner than the resolved workspace.
+		t.Run("WorkspaceUUIDOwnerMismatchShould404", func(t *testing.T) {
+			t.Parallel()
+
+			appDetails := setupProxyTest(t, nil)
+			otherUserClient, otherUser := coderdtest.CreateAnotherUser(t, appDetails.SDKClient, appDetails.FirstUser.OrganizationID, rbac.RoleMember())
+			appClient := appDetails.AppClient(t)
+			appClient.SetSessionToken(otherUserClient.SessionToken())
+
+			ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
+			defer cancel()
+
+			forgedApp := appDetails.Apps.Public
+			forgedApp.Username = otherUser.Username
+			forgedApp.WorkspaceName = appDetails.Workspace.ID.String()
+
+			resp, err := requestWithRetries(ctx, t, appClient, http.MethodGet, appDetails.SubdomainAppURL(forgedApp).String(), nil)
+			require.NoError(t, err)
+			defer resp.Body.Close()
+			require.Equal(t, http.StatusNotFound, resp.StatusCode)
+
+			body, err := io.ReadAll(resp.Body)
+			require.NoError(t, err)
+			require.Contains(t, string(body), "404 - Application Not Found")
+			assertWorkspaceLastUsedAtNotUpdated(t, appDetails, testutil.WaitLong)
+		})
+
 		t.Run("RedirectsWithSlash", func(t *testing.T) {
 			t.Parallel()
 
diff --git a/coderd/workspaceapps/db_test.go b/coderd/workspaceapps/db_test.go
index 1631c7d403..5e341a7bc8 100644
--- a/coderd/workspaceapps/db_test.go
+++ b/coderd/workspaceapps/db_test.go
@@ -908,6 +908,50 @@ func Test_ResolveRequest(t *testing.T) {
 		require.Len(t, connLogger.ConnectionLogs(), 0)
 	})
 
+	// Security (PLAT-260): a UUID workspace lookup must reject when
+	// the URL's username segment names a different owner. Otherwise a
+	// same-owner origin can be spoofed for credentialed cross-origin
+	// reads.
+	t.Run("WorkspaceUUIDOwnerMismatch", func(t *testing.T) {
+		t.Parallel()
+
+		req := (workspaceapps.Request{
+			AccessMethod:      workspaceapps.AccessMethodPath,
+			BasePath:          "/app",
+			UsernameOrID:      secondUser.Username,
+			WorkspaceNameOrID: workspace.ID.String(),
+			AgentNameOrID:     agentName,
+			AppSlugOrPort:     appNamePublic,
+		}).Normalize()
+
+		connLogger := connectionlog.NewFake()
+		auditableIP := testutil.RandomIPv6(t)
+
+		rw := httptest.NewRecorder()
+		r := httptest.NewRequest("GET", "/app", nil)
+		r.Header.Set(codersdk.SessionTokenHeader, client.SessionToken())
+		r.RemoteAddr = auditableIP
+
+		token, ok := workspaceappsResolveRequest(t, connLogger, rw, r, workspaceapps.ResolveRequestOptions{
+			Logger:              api.Logger,
+			SignedTokenProvider: api.WorkspaceAppsProvider,
+			DashboardURL:        api.AccessURL,
+			PathAppBaseURL:      api.AccessURL,
+			AppHostname:         api.AppHostname,
+			AppRequest:          req,
+		})
+		require.False(t, ok)
+		require.Nil(t, token)
+
+		w := rw.Result()
+		defer w.Body.Close()
+		b, err := io.ReadAll(w.Body)
+		require.NoError(t, err)
+		require.Contains(t, string(b), "404 - Application Not Found")
+		require.Equal(t, http.StatusNotFound, w.StatusCode)
+		require.Len(t, connLogger.ConnectionLogs(), 0)
+	})
+
 	t.Run("RedirectSubdomainAuth", func(t *testing.T) {
 		t.Parallel()
 
diff --git a/coderd/workspaceapps/request.go b/coderd/workspaceapps/request.go
index 980ec7c3a6..c0c85e74f3 100644
--- a/coderd/workspaceapps/request.go
+++ b/coderd/workspaceapps/request.go
@@ -248,6 +248,9 @@ func (r Request) getDatabase(ctx context.Context, db database.Store) (*databaseR
 	)
 	if workspaceID, uuidErr := uuid.Parse(r.WorkspaceNameOrID); uuidErr == nil {
 		workspace, workspaceErr = db.GetWorkspaceByID(ctx, workspaceID)
+		if workspaceErr == nil && workspace.OwnerID != user.ID {
+			workspaceErr = sql.ErrNoRows
+		}
 	} else {
 		workspace, workspaceErr = db.GetWorkspaceByOwnerIDAndName(ctx, database.GetWorkspaceByOwnerIDAndNameParams{
 			OwnerID: user.ID,