fix(hub): enforce registration captcha for every client

The server only checked the captcha when auth_key was absent — but
every real client (browser included, via the password split) sends
auth_key, so the check was off for everyone, and a bot skipped it by
including the field. The Register form still made humans solve a
widget whose token was never transmitted.

Gate is now unconditional on captcha.enabled. The web client
(registerUser in keyderive.js) forwards captcha.token; RegisterPage
resets the (single-use) token on a failed attempt. The desktop client
shares this UI source and is Chromium, so it renders the same widget
(see the paired meshbay-client commit for the CSP change that allows
it).

Tests: test_register_captcha.py.

Third security review, finding M1 (Option A).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pG75yGK3NthNfyjH74omG

3 files changed, 20 insertions, 4 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/api/users.py b/packages/meshbay-hub/src/meshbay_hub/api/users.py
index 559cfa6..9b70b59 100644
--- a/packages/meshbay-hub/src/meshbay_hub/api/users.py
+++ b/packages/meshbay-hub/src/meshbay_hub/api/users.py
@@ -150,8 +150,13 @@ async def register(
             return {"user_id": found.id, "email_verification_required": True}
         raise HTTPException(status_code=409, detail="Username already taken")
 
-    # Captcha gate — web path only (native clients send auth_key)
-    if _cfg and _cfg.captcha.enabled and not body.auth_key:
+    # Captcha gate — every fresh registration when a captcha is configured, with
+    # no client carve-out. The earlier `and not body.auth_key` exempted anything
+    # that sent an `auth_key`, which is *every* real client (the browser sends it
+    # too, from the password split) — so the check was off for everyone, and a
+    # bot skipped it by sending the field. The desktop client is Chromium and
+    # renders the same widget, so it has no need of an exemption either.
+    if _cfg and _cfg.captcha.enabled:
         await _verify_captcha_or_raise(body.captcha_token, request)
 
     # Email uniqueness (only active or pending accounts)
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/auth-page.js b/packages/meshbay-hub/src/meshbay_hub/static/auth-page.js
index df08bc4..4c00137 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/auth-page.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/auth-page.js
@@ -237,8 +237,11 @@ export function RegisterPage() {
         const rk = window.MeshBayKeys.generateRecoveryKey();
         // `name` (trimmed), not the raw field: the hub stores the trimmed
         // username and every key derivation must fold in the same string.
+        // `captcha.token` rides along — the submit button is already disabled
+        // until it is set when a captcha is configured (see the form below).
         await window.MeshBayKeys.registerUser(
-          name, email, password, emailRecovery ? rk.mnemonic : null);
+          name, email, password, emailRecovery ? rk.mnemonic : null,
+          captcha.token);
         setRecoveryMnemonic(rk.mnemonic);
         session.recoveryKey =
           await window.MeshBayKeys.deriveRecoveryKey(rk.mnemonic, name);
@@ -257,6 +260,10 @@ export function RegisterPage() {
       }
     } catch (err) {
       setError(err.message);
+      // A reCAPTCHA token is single-use: after a failed attempt (name taken,
+      // e-mail in use…) it is spent, so clear it and make the user solve a
+      // fresh one before the next try. No-op when no captcha is configured.
+      captcha.reset();
     } finally {
       setLoading(false);
     }
diff --git a/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js b/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js
index 0aaa6a5..a540a94 100644
--- a/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js
+++ b/packages/meshbay-hub/src/meshbay_hub/static/keyderive.js
@@ -276,7 +276,7 @@ async function decryptBundle(bundleB64, password, username) {
  *
  * Returns the raw private keys for immediate use after registration.
  */
-async function registerUser(username, email, password, recoveryMnemonic) {
+async function registerUser(username, email, password, recoveryMnemonic, captchaToken) {
   // No keypair here any more. Identity keys are per node: one is generated the
   // first time this account joins a given node, encrypted under the passphrase,
   // and left with that node. So an operator who cracks what sits on their own
@@ -291,6 +291,10 @@ async function registerUser(username, email, password, recoveryMnemonic) {
   // appends it to the verification e-mail and stores it nowhere
   // (docs/auth-confirm.md §4.4). Omitted when they chose to save it themselves.
   if (recoveryMnemonic) payload.recovery_key = recoveryMnemonic;
+  // reCAPTCHA response, when the hub has a captcha configured. The widget lives
+  // in RegisterPage (auth-page.js); this function just forwards its token. A
+  // hub with no captcha configured sends nothing and the server does not check.
+  if (captchaToken) payload.captcha_token = captchaToken;
 
   const resp = await hubCall('/v1/users/register', {
     method: 'POST',