diff --git a/app/Jobs/InboxPipeline/DeleteWorker.php b/app/Jobs/InboxPipeline/DeleteWorker.php
index bb9c027a6..830eb1254 100644
--- a/app/Jobs/InboxPipeline/DeleteWorker.php
+++ b/app/Jobs/InboxPipeline/DeleteWorker.php
@@ -185,6 +185,13 @@ class DeleteWorker implements ShouldQueue
         if (! $actor) {
             return false;
         }
+        // Rebind: the profile resolved by keyId must belong to the keyId host.
+        // This rejects a poisoned or stale row whose remote_url host differs
+        // from the request's keyId host, so a planted key_id -> attacker key
+        // binding cannot authenticate.
+        if (parse_url($actor->remote_url, PHP_URL_HOST) !== $keyDomain) {
+            return false;
+        }
         $pkey = openssl_pkey_get_public($actor->public_key);
         if (! $pkey) {
             return false;
diff --git a/app/Jobs/InboxPipeline/InboxValidator.php b/app/Jobs/InboxPipeline/InboxValidator.php
index 4b09fd04f..d77520e1e 100644
--- a/app/Jobs/InboxPipeline/InboxValidator.php
+++ b/app/Jobs/InboxPipeline/InboxValidator.php
@@ -152,6 +152,13 @@ class InboxValidator implements ShouldQueue
         if (! $actor) {
             return false;
         }
+        // Rebind: the profile resolved by keyId must belong to the keyId host.
+        // This rejects a poisoned or stale row whose remote_url host differs
+        // from the request's keyId host, so a planted key_id -> attacker key
+        // binding cannot authenticate.
+        if (parse_url($actor->remote_url, PHP_URL_HOST) !== $keyDomain) {
+            return false;
+        }
         $pkey = openssl_pkey_get_public($actor->public_key);
         if (! $pkey) {
             return false;
diff --git a/app/Jobs/InboxPipeline/InboxWorker.php b/app/Jobs/InboxPipeline/InboxWorker.php
index 48c313916..0c17c1512 100644
--- a/app/Jobs/InboxPipeline/InboxWorker.php
+++ b/app/Jobs/InboxPipeline/InboxWorker.php
@@ -133,6 +133,13 @@ class InboxWorker implements ShouldQueue
         if (! $actor) {
             return false;
         }
+        // Rebind: the profile resolved by keyId must belong to the keyId host.
+        // This rejects a poisoned or stale row whose remote_url host differs
+        // from the request's keyId host, so a planted key_id -> attacker key
+        // binding cannot authenticate.
+        if (parse_url($actor->remote_url, PHP_URL_HOST) !== $keyDomain) {
+            return false;
+        }
         $pkey = openssl_pkey_get_public($actor->public_key);
         if (! $pkey) {
             return false;
diff --git a/app/Util/ActivityPub/Helpers.php b/app/Util/ActivityPub/Helpers.php
index da1101cb7..67cf1ec8a 100644
--- a/app/Util/ActivityPub/Helpers.php
+++ b/app/Util/ActivityPub/Helpers.php
@@ -1500,7 +1500,29 @@ class Helpers
         $urlDomain = parse_url($url, PHP_URL_HOST);
         $domain = parse_url($res['id'], PHP_URL_HOST);
 
-        return strtolower($urlDomain) === strtolower($domain);
+        if (strtolower($urlDomain) !== strtolower($domain)) {
+            return false;
+        }
+
+        // The actor's key_id (publicKey.id) must live on the same host as the
+        // actor id. Without this, a remote actor could advertise a publicKey.id
+        // pointing at a victim's keyId URI, planting a poisoned
+        // key_id -> attacker-public-key binding in the unique profiles.key_id
+        // column. This mirrors the same-host check UpdatePersonValidator already
+        // enforces on the Update pipeline.
+        if (isset($res['publicKey']['id'])) {
+            if (! self::validateUrl($res['publicKey']['id'])) {
+                return false;
+            }
+
+            $keyDomain = parse_url($res['publicKey']['id'], PHP_URL_HOST);
+
+            if (strtolower($keyDomain) !== strtolower($domain)) {
+                return false;
+            }
+        }
+
+        return true;
     }
 
     /**
diff --git a/tests/Feature/Federation/ProfileKeyIdHostValidationTest.php b/tests/Feature/Federation/ProfileKeyIdHostValidationTest.php
new file mode 100644
index 000000000..e2d0d47c1
--- /dev/null
+++ b/tests/Feature/Federation/ProfileKeyIdHostValidationTest.php
@@ -0,0 +1,82 @@
+<?php
+
+use App\Util\ActivityPub\Helpers;
+use Illuminate\Foundation\Testing\LazilyRefreshDatabase;
+use Illuminate\Support\Facades\Cache;
+
+uses(LazilyRefreshDatabase::class);
+
+/*
+|--------------------------------------------------------------------------
+| Profile key_id host validation
+|--------------------------------------------------------------------------
+|
+| isValidProfileData() gates first-contact remote actor ingestion. Besides
+| requiring the actor `id` host to match the fetched URL host, it must also
+| require the actor `publicKey.id` host to match the actor `id` host.
+|
+| Without this guard a remote actor could advertise a publicKey.id pointing at
+| a victim's keyId URI (e.g. https://victim-host.test/users/alice#main-key)
+| with an attacker-controlled publicKeyPem, planting a poisoned
+| key_id -> attacker key binding in the unique profiles.key_id column and
+| silently black-holing the victim's federation.
+|
+*/
+
+/**
+ * validateUrl() (called internally by isValidProfileData) resolves the host to
+ * a public IP. Pre-seed the DNS cache so these tests stay deterministic and
+ * network-free while still exercising the host-comparison logic.
+ */
+function seedResolvableHost(string $host): void
+{
+    $key = 'helpers:url:public-ips:'.hash('xxh128', $host);
+    Cache::put($key, ['203.0.113.10'], 3600);
+}
+
+function keyIdActorDoc(array $overrides = []): array
+{
+    return array_replace_recursive([
+        'id' => 'https://attacker-host.test/users/attacker',
+        'inbox' => 'https://attacker-host.test/users/attacker/inbox',
+        'outbox' => 'https://attacker-host.test/users/attacker/outbox',
+        'preferredUsername' => 'attacker',
+        'publicKey' => [
+            'id' => 'https://attacker-host.test/users/attacker#main-key',
+            'owner' => 'https://attacker-host.test/users/attacker',
+            'publicKeyPem' => "-----BEGIN PUBLIC KEY-----\nattacker\n-----END PUBLIC KEY-----",
+        ],
+    ], $overrides);
+}
+
+beforeEach(function () {
+    seedResolvableHost('attacker-host.test');
+    seedResolvableHost('victim-host.test');
+});
+
+it('accepts an actor whose publicKey.id host matches its id host', function () {
+    $res = keyIdActorDoc();
+
+    expect(Helpers::isValidProfileData($res, $res['id']))->toBeTrue();
+});
+
+it('rejects an actor whose publicKey.id host points at another host', function () {
+    // The plant attempt: attacker actor advertises the victim's keyId URI.
+    $res = keyIdActorDoc([
+        'publicKey' => [
+            'id' => 'https://victim-host.test/users/alice#main-key',
+        ],
+    ]);
+
+    expect(Helpers::isValidProfileData($res, $res['id']))->toBeFalse();
+});
+
+it('rejects an actor whose publicKey.id is not a valid url', function () {
+    $res = keyIdActorDoc([
+        'publicKey' => [
+            'id' => 'not-a-url',
+        ],
+    ]);
+
+    expect(Helpers::isValidProfileData($res, $res['id']))->toBeFalse();
+});