diff --git a/server/src/app.ts b/server/src/app.ts
index ceb85c2..c350bab 100644
--- a/server/src/app.ts
+++ b/server/src/app.ts
@@ -41,7 +41,7 @@ import {
   revokeAppPassword,
 } from "./account.js";
 import { imageProxyHandler } from "./imageproxy.js";
-import { SignInError, finish as finishSignIn, needsRefresh, oauthEnabled, refreshTokens, singleServer, start as startSignIn, type TokenSet } from "./oauth.js";
+import { SignInError, finish as finishSignIn, needsRefresh, oauthEnabled, passwordConfirms, refreshTokens, singleServer, start as startSignIn, type TokenSet } from "./oauth.js";
 import { icsProxyHandler } from "./icsproxy.js";
 import { staticHandler } from "./static.js";
 import { mailNode, webmailNode } from "./nodes.js";
@@ -1127,11 +1127,10 @@ async function readJson<T>(c: Context): Promise<T | null> {
  */
 async function confirmsPassword(session: LiveSession, candidate: string): Promise<boolean> {
   if (session.tokens) {
-    // Holding no password, the only judge is the server.
+    // Holding no password, the only judge is the server, asked on its sign-in
+    // endpoint since it takes no password over JMAP.
     try {
-      const authorization = `Basic ${Buffer.from(`${session.username}:${candidate}`, "utf8").toString("base64")}`;
-      await fetchUpstreamSession(authorization, upstreamFor(session.username));
-      return true;
+      return await passwordConfirms({ base: upstreamFor(session.username), username: session.username, password: candidate });
     } catch {
       return false;
     }
diff --git a/server/src/mock/index.ts b/server/src/mock/index.ts
index 729ebb1..491e2de 100644
--- a/server/src/mock/index.ts
+++ b/server/src/mock/index.ts
@@ -14,7 +14,7 @@ import { MAX_OBJECTS, MethodError, directory, enforceLimits, resolveRefs } from
 import { handlers } from "./handlers.js";
 export { account } from "./config.js";
 import { checkOtp } from "./auth.js";
-import { checkBearer, handleOAuth } from "./oauth.js";
+import { basicRefused, checkBearer, handleOAuth } from "./oauth.js";
 import { sseClients, broadcast } from "./events.js";
 
 /* ---------- http ---------- */
@@ -26,7 +26,7 @@ function unauthorized(res: ServerResponse) {
 function checkAuth(req: IncomingMessage): boolean {
   const h = req.headers.authorization ?? "";
   if (checkBearer(h)) return true;
-  if (!h.startsWith("Basic ")) return false;
+  if (!h.startsWith("Basic ") || basicRefused) return false;
   const raw = Buffer.from(h.slice(6), "base64").toString();
   const sep = raw.indexOf(":");
   if (sep < 0) return false;
diff --git a/server/src/mock/oauth.ts b/server/src/mock/oauth.ts
index 2f66115..c1b9957 100644
--- a/server/src/mock/oauth.ts
+++ b/server/src/mock/oauth.ts
@@ -23,11 +23,18 @@ const refreshTokens = new Map<string, Grant>();
 
 const base = () => `http://127.0.0.1:${PORT}`;
 
+/**
+ * Whether JMAP refuses Basic, as INBUXA's server does outside DAV (contract
+ * C-23). Off by default, since the mock also serves password sign-in.
+ */
+export let basicRefused = false;
+
 /** For tests: how long new access tokens last, and a way to end every token. */
 export const oauthMock = {
   setAccessTokenTtl(seconds: number) { accessTokenTtl = seconds; },
   expireAccessTokens() { for (const t of accessTokens.values()) t.expiresAt = 0; },
-  reset() { codes.clear(); accessTokens.clear(); refreshTokens.clear(); accessTokenTtl = 3600; },
+  refuseBasic(on: boolean) { basicRefused = on; },
+  reset() { codes.clear(); accessTokens.clear(); refreshTokens.clear(); accessTokenTtl = 3600; basicRefused = false; },
 };
 
 /** A bearer token the mock issued, still valid under the current password. */
@@ -50,6 +57,14 @@ function readForm(req: IncomingMessage): Promise<URLSearchParams> {
   });
 }
 
+function readBody(req: IncomingMessage): Promise<Buffer> {
+  return new Promise((resolve) => {
+    const chunks: Buffer[] = [];
+    req.on("data", (c) => chunks.push(c));
+    req.on("end", () => resolve(Buffer.concat(chunks)));
+  });
+}
+
 function issue(res: ServerResponse, refresh: string | null) {
   const access = `mock-at-${randomBytes(16).toString("hex")}`;
   accessTokens.set(access, { password: account.password, expiresAt: Date.now() + accessTokenTtl * 1000 });
@@ -93,6 +108,35 @@ export async function handleOAuth(req: IncomingMessage, res: ServerResponse, url
     res.end();
     return true;
   }
+  if (url.pathname === "/api/auth" && req.method === "POST") {
+    // The server's sign-in page posts here; the mock answers for the demo user.
+    let body: Record<string, unknown>;
+    try {
+      body = JSON.parse((await readBody(req)).toString()) as Record<string, unknown>;
+    } catch {
+      json(res, 400, { error: "invalid_request" });
+      return true;
+    }
+    const redirectUri = typeof body.redirectUri === "string" ? body.redirectUri : "";
+    if (body.type !== "authCode" || body.clientId !== OAUTH_CLIENT_ID || !redirectUri || body.codeChallengeMethod !== "S256") {
+      json(res, 400, { error: "invalid_request" });
+      return true;
+    }
+    const secret = typeof body.accountSecret === "string" ? body.accountSecret : "";
+    const passwordOk = body.accountName === USER && (secret === account.password || account.appPasswords.some((a) => a.secret === secret));
+    if (!passwordOk) {
+      json(res, 200, { type: "failure" });
+      return true;
+    }
+    if (account.otpUrl && secret === account.password && !body.mfaToken) {
+      json(res, 200, { type: "mfaRequired" });
+      return true;
+    }
+    const code = randomBytes(16).toString("hex");
+    codes.set(code, { challenge: String(body.codeChallenge ?? ""), redirectUri, issuedAt: Date.now() });
+    json(res, 200, { type: "authenticated", client_code: code, iss: base() });
+    return true;
+  }
   if (url.pathname === "/auth/token" && req.method === "POST") {
     const form = await readForm(req);
     if (form.get("client_id") !== OAUTH_CLIENT_ID || form.get("client_secret") !== OAUTH_CLIENT_SECRET) {
diff --git a/server/src/oauth.test.ts b/server/src/oauth.test.ts
index d18e647..236111c 100644
--- a/server/src/oauth.test.ts
+++ b/server/src/oauth.test.ts
@@ -198,6 +198,8 @@ test("a password change signs the session out, since the server revokes its toke
 
 test("creating an app password checks the typed password with the server", async () => {
   await signIn();
+  // As INBUXA's server does: no password over JMAP (contract C-23).
+  oauthMock.refuseBasic(true);
   const wrong = await call("/api/account/app-passwords", { method: "POST", body: JSON.stringify({ description: "Phone", current: "nope" }) });
   assert.equal(wrong.status, 403);
   const right = await call("/api/account/app-passwords", { method: "POST", body: JSON.stringify({ description: "Phone", current: "demo-password" }) });
diff --git a/server/src/oauth.ts b/server/src/oauth.ts
index 5490ed1..e9c81d0 100644
--- a/server/src/oauth.ts
+++ b/server/src/oauth.ts
@@ -134,6 +134,39 @@ export async function start(params: { username: string; base: string; remember:
   return { location: url.toString(), state };
 }
 
+/**
+ * Whether `password` is the account's password, for a session that holds a
+ * token and so has no password to compare with.
+ *
+ * Asked of the server's sign-in endpoint, the one its own sign-in page posts
+ * to, because the server takes no password over JMAP (contract C-23). The
+ * request is this client's, to its registered redirect URI, so it passes the
+ * same checks a real sign-in does. A code it issues can never be exchanged:
+ * the PKCE verifier behind its challenge is thrown away here.
+ *
+ * "Two-factor code needed" counts as confirmed: the server says so only once
+ * the password has matched.
+ */
+export async function passwordConfirms(params: { base: string; username: string; password: string }): Promise<boolean> {
+  const res = await fetch(absoluteUpstream("/api/auth", params.base), {
+    method: "POST",
+    headers: { "content-type": "application/json", accept: "application/json" },
+    body: JSON.stringify({
+      type: "authCode",
+      accountName: params.username,
+      accountSecret: params.password,
+      clientId: config.oauthClientId,
+      redirectUri: redirectUri(),
+      codeChallenge: challengeOf(randomToken(48)),
+      codeChallengeMethod: "S256",
+    }),
+    signal: AbortSignal.timeout(config.upstreamTimeout),
+  });
+  if (!res.ok) throw new UpstreamError(`Password check failed (${res.status})`, 502);
+  const answer = (await res.json()) as { type?: string };
+  return answer.type === "authenticated" || answer.type === "mfaRequired";
+}
+
 export class SignInError extends Error {
   constructor(readonly code: "state_mismatch" | "expired" | "denied" | "exchange_failed", message: string) {
     super(message);