diff --git a/website/app/auth/AuthCallback.tsx b/website/app/auth/AuthCallback.tsx
index 7c157d5..663be60 100644
--- a/website/app/auth/AuthCallback.tsx
+++ b/website/app/auth/AuthCallback.tsx
@@ -2,7 +2,7 @@
 
 import Link from "next/link";
 import { useEffect, useRef, useState } from "react";
-import { completeMystiraIdentityFlow } from "./oidc";
+import { completeMystiraIdentityFlow, consumeMystiraIdentityRequest } from "./oidc";
 
 type CallbackState = "loading" | "success" | "error";
 
@@ -25,19 +25,37 @@ export default function AuthCallback() {
       const returnedState = params.get("state");
 
       if (error) {
-        setState("error");
-        setMessage(params.get("error_description") ?? error);
+        try {
+          consumeMystiraIdentityRequest(returnedState);
+          window.history.replaceState({}, document.title, "/auth/callback");
+          setState("error");
+          setMessage(
+            error === "access_denied"
+              ? "Sign-in was cancelled."
+              : "Mystira Identity did not complete sign-in.",
+          );
+        } catch (ex) {
+          setState("error");
+          setMessage(ex instanceof Error ? ex.message : "Unable to validate sign-in.");
+        }
         return;
       }
 
-      if (!code || !returnedState) {
-        setState("error");
-        setMessage("Mystira Identity did not return an authorization code.");
+      if (!code) {
+        try {
+          consumeMystiraIdentityRequest(returnedState);
+          window.history.replaceState({}, document.title, "/auth/callback");
+          setState("error");
+          setMessage("Mystira Identity did not return an authorization code.");
+        } catch (ex) {
+          setState("error");
+          setMessage(ex instanceof Error ? ex.message : "Unable to validate sign-in.");
+        }
         return;
       }
 
       try {
-        await completeMystiraIdentityFlow(code, returnedState);
+        await completeMystiraIdentityFlow(code, returnedState ?? "");
         window.history.replaceState({}, document.title, "/auth/callback");
         setState("success");
         setMessage("Signed in with Mystira Identity.");
diff --git a/website/app/auth/__tests__/AuthCallback.test.tsx b/website/app/auth/__tests__/AuthCallback.test.tsx
new file mode 100644
index 0000000..707c70e
--- /dev/null
+++ b/website/app/auth/__tests__/AuthCallback.test.tsx
@@ -0,0 +1,48 @@
+import { render, screen } from "@testing-library/react";
+import { beforeEach, describe, expect, it } from "vitest";
+import AuthCallback from "../AuthCallback";
+import { CODEFLOW_AUTH_STATE_KEY } from "../oidc";
+
+const storedRequest = {
+  intent: "login",
+  state: "expected-state",
+  nonce: "nonce",
+  codeVerifier: "verifier",
+  redirectUri: "https://codeflow.phoenixvc.tech/auth/callback",
+  createdAt: Date.now(),
+};
+
+describe("AuthCallback", () => {
+  beforeEach(() => {
+    sessionStorage.clear();
+    sessionStorage.setItem(CODEFLOW_AUTH_STATE_KEY, JSON.stringify(storedRequest));
+  });
+
+  it("consumes a matched cancellation without rendering the provider description", async () => {
+    window.history.replaceState(
+      {},
+      "",
+      "/auth/callback?error=access_denied&error_description=Attacker-controlled&state=expected-state",
+    );
+
+    render(<AuthCallback />);
+
+    expect(await screen.findByText("Sign-in was cancelled.")).toBeInTheDocument();
+    expect(screen.queryByText("Attacker-controlled")).not.toBeInTheDocument();
+    expect(sessionStorage.getItem(CODEFLOW_AUTH_STATE_KEY)).toBeNull();
+  });
+
+  it("rejects a provider error when the callback state does not match", async () => {
+    window.history.replaceState(
+      {},
+      "",
+      "/auth/callback?error=access_denied&error_description=User+cancelled&state=wrong-state",
+    );
+
+    render(<AuthCallback />);
+
+    expect(await screen.findByText("Sign-in state did not match. Start login again.")).toBeInTheDocument();
+    expect(screen.queryByText("User cancelled")).not.toBeInTheDocument();
+    expect(sessionStorage.getItem(CODEFLOW_AUTH_STATE_KEY)).not.toBeNull();
+  });
+});
diff --git a/website/app/auth/oidc.ts b/website/app/auth/oidc.ts
index 13e049f..dc25e56 100644
--- a/website/app/auth/oidc.ts
+++ b/website/app/auth/oidc.ts
@@ -126,15 +126,7 @@ export async function startMystiraIdentityFlow(intent: CodeflowAuthIntent): Prom
 }
 
 export async function completeMystiraIdentityFlow(code: string, state: string): Promise<TokenResponse> {
-  const rawRequest = sessionStorage.getItem(CODEFLOW_AUTH_STATE_KEY);
-  if (!rawRequest) {
-    throw new Error("Missing saved sign-in request. Start login again.");
-  }
-
-  const request = JSON.parse(rawRequest) as StoredAuthRequest;
-  if (request.state !== state) {
-    throw new Error("Sign-in state did not match. Start login again.");
-  }
+  const request = getStoredAuthRequest(state);
 
   const body = new URLSearchParams();
   body.set("grant_type", "authorization_code");
@@ -162,6 +154,25 @@ export async function completeMystiraIdentityFlow(code: string, state: string):
   return tokenResponse;
 }
 
+export function consumeMystiraIdentityRequest(state: string | null): void {
+  getStoredAuthRequest(state);
+  sessionStorage.removeItem(CODEFLOW_AUTH_STATE_KEY);
+}
+
+function getStoredAuthRequest(state: string | null): StoredAuthRequest {
+  const rawRequest = sessionStorage.getItem(CODEFLOW_AUTH_STATE_KEY);
+  if (!rawRequest) {
+    throw new Error("Missing saved sign-in request. Start login again.");
+  }
+
+  const request = JSON.parse(rawRequest) as StoredAuthRequest;
+  if (request.state !== state) {
+    throw new Error("Sign-in state did not match. Start login again.");
+  }
+
+  return request;
+}
+
 async function validateIdToken(idToken: string | undefined, request: StoredAuthRequest): Promise<void> {
   if (!idToken) {
     throw new Error("Mystira Identity did not return an ID token.");