diff --git a/.gitignore b/.gitignore
index c021301..90ea2f6 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,5 +6,6 @@
 backend/target/
 build/
 e2e/node_modules/
+google-e2e/node_modules/
 frontend/node_modules/
 frontend/dist/
diff --git a/docs/first-broker-login-security.md b/docs/first-broker-login-security.md
new file mode 100644
index 0000000..429d677
--- /dev/null
+++ b/docs/first-broker-login-security.md
@@ -0,0 +1,27 @@
+# First Broker Login security
+
+Keycloak 26.7.0's built-in `first broker login` flow does **not** silently
+auto-link by email. It contains:
+
+- `Create User If Unique`
+- `Handle Existing Account`
+- `Confirm link existing account`
+- email verification or re-authentication ownership proof
+
+`Automatically set existing user` is an explicit, dangerous opt-in. The local
+acceptance harness copies the built-in flow, enables AutoLink, disables the
+ownership-proof branch, and signs in through a controllable OIDC account whose
+email collides with `regular-user`. It verifies that the external identity is
+attached without proof. The harness then assigns the original built-in flow,
+repeats the login, observes the existing-account confirmation page, and verifies
+that no federated identity was attached.
+
+Run after the stack is healthy:
+
+```bash
+./scripts/verify-first-broker-login.sh
+```
+
+The vulnerable flow remains only as a disabled learning artifact. The
+`mock-google` provider is always returned to the secure built-in flow at the end
+of the verification.
diff --git a/google-e2e/first-broker-login.mjs b/google-e2e/first-broker-login.mjs
new file mode 100644
index 0000000..75dd7f5
--- /dev/null
+++ b/google-e2e/first-broker-login.mjs
@@ -0,0 +1,124 @@
+import assert from "node:assert/strict";
+import { chromium } from "playwright-core";
+
+const expectation = process.env.FIRST_BROKER_EXPECTATION;
+assert.ok(
+  expectation === "vulnerable" || expectation === "secure",
+  "FIRST_BROKER_EXPECTATION must be vulnerable or secure",
+);
+
+const keycloakUrl = process.env.KEYCLOAK_URL ?? "http://localhost:8080";
+const adminUsername = process.env.KC_BOOTSTRAP_ADMIN_USERNAME;
+const adminPassword = process.env.KC_BOOTSTRAP_ADMIN_PASSWORD;
+const mockPassword = process.env.MOCK_GOOGLE_USER_PASSWORD;
+assert.ok(adminUsername && adminPassword && mockPassword);
+
+async function adminToken() {
+  const body = new URLSearchParams({
+    client_id: "admin-cli",
+    grant_type: "password",
+    username: adminUsername,
+    password: adminPassword,
+  });
+  const response = await fetch(
+    `${keycloakUrl}/realms/master/protocol/openid-connect/token`,
+    { method: "POST", body },
+  );
+  assert.equal(response.status, 200);
+  return (await response.json()).access_token;
+}
+
+async function regularUser(token) {
+  const response = await fetch(
+    `${keycloakUrl}/admin/realms/keycloak-patterns/users?username=regular-user&exact=true`,
+    { headers: { Authorization: `Bearer ${token}` } },
+  );
+  assert.equal(response.status, 200);
+  const users = await response.json();
+  assert.equal(users.length, 1);
+  return users[0];
+}
+
+async function federatedIdentities(token, userId) {
+  const response = await fetch(
+    `${keycloakUrl}/admin/realms/keycloak-patterns/users/${userId}/federated-identity`,
+    { headers: { Authorization: `Bearer ${token}` } },
+  );
+  assert.equal(response.status, 200);
+  return response.json();
+}
+
+async function removeMockLink(token, userId) {
+  const identities = await federatedIdentities(token, userId);
+  if (identities.some(({ identityProvider }) => identityProvider === "mock-google")) {
+    const response = await fetch(
+      `${keycloakUrl}/admin/realms/keycloak-patterns/users/${userId}/federated-identity/mock-google`,
+      {
+        method: "DELETE",
+        headers: { Authorization: `Bearer ${token}` },
+      },
+    );
+    assert.equal(response.status, 204);
+  }
+}
+
+const token = await adminToken();
+const user = await regularUser(token);
+await removeMockLink(token, user.id);
+
+const browser = await chromium.launch({
+  executablePath: process.env.CHROME_BIN ?? "/usr/bin/google-chrome",
+  headless: true,
+  args: ["--no-sandbox"],
+});
+
+try {
+  const context = await browser.newContext();
+  const page = await context.newPage();
+  const authorizationUrl = new URL(
+    `${keycloakUrl}/realms/keycloak-patterns/protocol/openid-connect/auth`,
+  );
+  authorizationUrl.search = new URLSearchParams({
+    client_id: "spa-public",
+    redirect_uri: "http://localhost:8088/",
+    response_type: "code",
+    scope: "openid profile email",
+    state: `first-broker-${expectation}`,
+    nonce: `nonce-${expectation}`,
+    code_challenge: "K2qUEfBl-nQvF2gB4dNxC2zYVwZc1CVnZb5CsX2L7fI",
+    code_challenge_method: "S256",
+    kc_idp_hint: "mock-google",
+  });
+
+  await page.goto(authorizationUrl.toString());
+  await page.waitForURL(/\/realms\/mock-google\//u);
+  await page.locator("#username").fill("mock-collision-user");
+  await page.locator("#password").fill(mockPassword);
+  await page.locator("#kc-login").click();
+  await page.waitForLoadState("domcontentloaded");
+
+  if (expectation === "vulnerable") {
+    await page.waitForURL(/localhost:8088\/\?.*code=/u);
+    const identities = await federatedIdentities(token, user.id);
+    assert.equal(
+      identities.some(({ identityProvider }) => identityProvider === "mock-google"),
+      true,
+      "unsafe AutoLink should attach the attacker-controlled identity",
+    );
+    await removeMockLink(token, user.id);
+  } else {
+    assert.match(page.url(), /\/realms\/keycloak-patterns\//u);
+    const body = (await page.locator("body").innerText()).toLowerCase();
+    assert.match(body, /account already exists|link existing account|existing account/u);
+    const identities = await federatedIdentities(token, user.id);
+    assert.equal(
+      identities.some(({ identityProvider }) => identityProvider === "mock-google"),
+      false,
+      "Confirm Link must not attach the identity without ownership proof",
+    );
+  }
+
+  console.log(`first broker login ${expectation} case verified`);
+} finally {
+  await browser.close();
+}
diff --git a/google-e2e/package-lock.json b/google-e2e/package-lock.json
new file mode 100644
index 0000000..6407ce5
--- /dev/null
+++ b/google-e2e/package-lock.json
@@ -0,0 +1,27 @@
+{
+  "name": "keycloak-google-broker-e2e",
+  "version": "1.0.0",
+  "lockfileVersion": 3,
+  "requires": true,
+  "packages": {
+    "": {
+      "name": "keycloak-google-broker-e2e",
+      "version": "1.0.0",
+      "dependencies": {
+        "playwright-core": "1.55.1"
+      }
+    },
+    "node_modules/playwright-core": {
+      "version": "1.55.1",
+      "resolved": "https://registry.npmjs.org/playwright-core/-/playwright-core-1.55.1.tgz",
+      "integrity": "sha512-Z6Mh9mkwX+zxSlHqdr5AOcJnfp+xUWLCt9uKV18fhzA8eyxUd8NUWzAjxUh55RZKSYwDGX0cfaySdhZJGMoJ+w==",
+      "license": "Apache-2.0",
+      "bin": {
+        "playwright-core": "cli.js"
+      },
+      "engines": {
+        "node": ">=18"
+      }
+    }
+  }
+}
diff --git a/google-e2e/package.json b/google-e2e/package.json
new file mode 100644
index 0000000..5727e22
--- /dev/null
+++ b/google-e2e/package.json
@@ -0,0 +1,12 @@
+{
+  "name": "keycloak-google-broker-e2e",
+  "version": "1.0.0",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "test:first-broker": "node first-broker-login.mjs"
+  },
+  "dependencies": {
+    "playwright-core": "1.55.1"
+  }
+}
diff --git a/scripts/set-first-broker-login-mode.sh b/scripts/set-first-broker-login-mode.sh
new file mode 100755
index 0000000..fd92950
--- /dev/null
+++ b/scripts/set-first-broker-login-mode.sh
@@ -0,0 +1,178 @@
+#!/usr/bin/env sh
+set -eu
+
+mode="${1:-}"
+case "$mode" in
+  vulnerable|secure) ;;
+  *)
+    echo "usage: $0 vulnerable|secure" >&2
+    exit 1
+    ;;
+esac
+
+if [ ! -f .env ]; then
+  echo "missing .env" >&2
+  exit 1
+fi
+
+set -a
+. ./.env
+set +a
+
+keycloak_url="${KEYCLOAK_URL:-http://localhost:8080}"
+realm="${KEYCLOAK_REALM:-keycloak-patterns}"
+admin_base="$keycloak_url/admin/realms/$realm"
+vulnerable_flow="vulnerable first broker login"
+idp_url="$admin_base/identity-provider/instances/mock-google"
+
+admin_token="$(
+  curl -fsS \
+    -d client_id=admin-cli \
+    -d grant_type=password \
+    -d "username=$KC_BOOTSTRAP_ADMIN_USERNAME" \
+    -d "password=$KC_BOOTSTRAP_ADMIN_PASSWORD" \
+    "$keycloak_url/realms/master/protocol/openid-connect/token" |
+    jq -er .access_token
+)"
+
+auth_header="Authorization: Bearer $admin_token"
+encode() {
+  jq -rn --arg value "$1" '$value | @uri'
+}
+
+flows="$(curl -fsS -H "$auth_header" "$admin_base/authentication/flows")"
+flow_id="$(
+  printf '%s' "$flows" |
+    jq -r --arg alias "$vulnerable_flow" '
+      .[] | select(.alias == $alias) | .id
+    ' |
+    head -1
+)"
+
+if [ -n "$flow_id" ]; then
+  existing_executions="$(
+    curl -fsS -H "$auth_header" \
+      "$admin_base/authentication/flows/$(encode "$vulnerable_flow")/executions"
+  )"
+  if printf '%s' "$existing_executions" | jq -e '
+    any(.[]; .authenticationFlow == true)
+  ' >/dev/null; then
+    idp_before_delete="$(curl -fsS -H "$auth_header" "$idp_url")"
+    printf '%s' "$idp_before_delete" |
+      jq '.firstBrokerLoginFlowAlias = "first broker login"' |
+      curl -fsS -X PUT \
+        -H "$auth_header" \
+        -H "Content-Type: application/json" \
+        --data @- \
+        "$idp_url"
+    curl -fsS -X DELETE \
+      -H "$auth_header" \
+      "$admin_base/authentication/flows/$flow_id"
+    flow_id=""
+  fi
+fi
+
+if [ -z "$flow_id" ]; then
+  curl -fsS -X POST \
+    -H "$auth_header" \
+    -H "Content-Type: application/json" \
+    --data "$(
+      jq -n --arg alias "$vulnerable_flow" '{
+        alias: $alias,
+        description: "INSECURE LEARNING FLOW - automatic email linking",
+        providerId: "basic-flow",
+        topLevel: true,
+        builtIn: false
+      }'
+    )" \
+    "$admin_base/authentication/flows"
+fi
+
+executions_url="$admin_base/authentication/flows/$(encode "$vulnerable_flow")/executions"
+executions="$(curl -fsS -H "$auth_header" "$executions_url")"
+create_user_id="$(
+  printf '%s' "$executions" |
+    jq -r '.[] | select(.providerId == "idp-create-user-if-unique") | .id' |
+    head -1
+)"
+if [ -z "$create_user_id" ]; then
+  curl -fsS -X POST \
+    -H "$auth_header" \
+    -H "Content-Type: application/json" \
+    --data '{"provider":"idp-create-user-if-unique"}' \
+    "$admin_base/authentication/flows/$(encode "$vulnerable_flow")/executions/execution"
+  executions="$(curl -fsS -H "$auth_header" "$executions_url")"
+  create_user_id="$(
+    printf '%s' "$executions" |
+      jq -r '.[] | select(.providerId == "idp-create-user-if-unique") | .id' |
+      head -1
+  )"
+fi
+
+auto_link_id="$(
+  printf '%s' "$executions" |
+    jq -r '.[] | select(.providerId == "idp-auto-link") | .id' |
+    head -1
+)"
+
+if [ -z "$auto_link_id" ]; then
+  curl -fsS -X POST \
+    -H "$auth_header" \
+    -H "Content-Type: application/json" \
+    --data '{"provider":"idp-auto-link"}' \
+    "$admin_base/authentication/flows/$(encode "$vulnerable_flow")/executions/execution"
+  executions="$(curl -fsS -H "$auth_header" "$executions_url")"
+  auto_link_id="$(
+    printf '%s' "$executions" |
+      jq -r '.[] | select(.providerId == "idp-auto-link") | .id' |
+      head -1
+  )"
+fi
+
+if [ "$mode" = "vulnerable" ]; then
+  curl -fsS -X PUT \
+    -H "$auth_header" \
+    -H "Content-Type: application/json" \
+    --data "$(jq -n --arg id "$create_user_id" '{id: $id, requirement: "ALTERNATIVE"}')" \
+    "$executions_url"
+  curl -fsS -X PUT \
+    -H "$auth_header" \
+    -H "Content-Type: application/json" \
+    --data "$(jq -n --arg id "$auto_link_id" '{id: $id, requirement: "ALTERNATIVE"}')" \
+    "$executions_url"
+  selected_flow="$vulnerable_flow"
+else
+  selected_flow="first broker login"
+fi
+
+idp="$(curl -fsS -H "$auth_header" "$idp_url")"
+printf '%s' "$idp" |
+  jq --arg flow "$selected_flow" '.firstBrokerLoginFlowAlias = $flow' |
+  curl -fsS -X PUT \
+    -H "$auth_header" \
+    -H "Content-Type: application/json" \
+    --data @- \
+    "$idp_url"
+
+assigned="$(
+  curl -fsS -H "$auth_header" "$idp_url" |
+    jq -r .firstBrokerLoginFlowAlias
+)"
+test "$assigned" = "$selected_flow"
+
+if [ "$mode" = "secure" ]; then
+  secure_executions="$(
+    curl -fsS -H "$auth_header" \
+      "$admin_base/authentication/flows/$(encode "first broker login")/executions"
+  )"
+  printf '%s' "$secure_executions" | jq -e '
+    any(.[];
+      .providerId == "idp-confirm-link" and .requirement == "REQUIRED"
+    ) and
+    (any(.[];
+      .providerId == "idp-auto-link" and .requirement != "DISABLED"
+    ) | not)
+  ' >/dev/null
+fi
+
+echo "mock-google First Broker Login mode: $mode ($selected_flow)"
diff --git a/scripts/verify-first-broker-login.sh b/scripts/verify-first-broker-login.sh
new file mode 100755
index 0000000..28f5ce3
--- /dev/null
+++ b/scripts/verify-first-broker-login.sh
@@ -0,0 +1,29 @@
+#!/usr/bin/env sh
+set -eu
+
+if [ ! -f .env ]; then
+  echo "missing .env" >&2
+  exit 1
+fi
+
+set -a
+. ./.env
+set +a
+
+restore_secure_flow() {
+  ./scripts/set-first-broker-login-mode.sh secure >/dev/null 2>&1 || true
+}
+trap restore_secure_flow 0 1 2 15
+
+npm --prefix google-e2e ci
+
+./scripts/set-first-broker-login-mode.sh vulnerable
+FIRST_BROKER_EXPECTATION=vulnerable \
+  npm --prefix google-e2e run test:first-broker
+
+./scripts/set-first-broker-login-mode.sh secure
+FIRST_BROKER_EXPECTATION=secure \
+  npm --prefix google-e2e run test:first-broker
+
+trap - 0 1 2 15
+echo "First Broker Login verified: unsafe AutoLink reproduced, Confirm Link restored"