diff --git a/.github/changelog/fix-signature-bare-host b/.github/changelog/fix-signature-bare-host
new file mode 100644
index 0000000000..3bf4afa0ec
--- /dev/null
+++ b/.github/changelog/fix-signature-bare-host
@@ -0,0 +1,4 @@
+Significance: patch
+Type: fixed
+
+Signed requests from other servers are now checked more strictly, and requests that are not meant for your site are rejected before the sender's key is fetched.
diff --git a/includes/functions-url.php b/includes/functions-url.php
index 4f926fd4cf..693bb809eb 100644
--- a/includes/functions-url.php
+++ b/includes/functions-url.php
@@ -167,9 +167,14 @@ function fold_authority( $authority ) {
  * @return bool True if the authority names one of this site's hosts, false otherwise.
  */
 function is_signature_host( $authority ) {
+	// The raw value enters the signature base, so it has to be a bare host and port, written exactly so.
+	if ( get_authority( \sprintf( '//%s', $authority ) ) !== (string) $authority ) {
+		return false;
+	}
+
 	$authority = fold_authority( $authority );
 
-	// An unparseable value folds to nothing and must not match an unparseable entry.
+	// A host of only dots or brackets folds to nothing and must not match an unparseable entry.
 	if ( '' === $authority ) {
 		return false;
 	}
diff --git a/includes/signature/class-http-signature-draft.php b/includes/signature/class-http-signature-draft.php
index c69640e27c..a6656b496b 100644
--- a/includes/signature/class-http-signature-draft.php
+++ b/includes/signature/class-http-signature-draft.php
@@ -108,16 +108,17 @@ public function verify( array $headers, $body = null ) {
 			return new \WP_Error( 'activitypub_signature', 'No Key ID present.' );
 		}
 
-		$public_key = Remote_Actors::get_public_key( $parsed['keyId'] );
-		if ( \is_wp_error( $public_key ) ) {
-			return $public_key;
-		}
-
+		// Read before the key is fetched, so a request made out to another host or out of its time window fails without a lookup.
 		$signed_data = $this->get_signed_data( $parsed['headers'], $parsed, $headers );
 		if ( ! $signed_data ) {
 			return new \WP_Error( 'invalid_signed_data', 'Signed data is invalid or expired.' );
 		}
 
+		$public_key = Remote_Actors::get_public_key( $parsed['keyId'] );
+		if ( \is_wp_error( $public_key ) ) {
+			return $public_key;
+		}
+
 		$algorithm = $this->get_signature_algorithm( $parsed, $public_key );
 		if ( \is_wp_error( $algorithm ) ) {
 			return $algorithm;
diff --git a/tests/phpunit/tests/includes/class-test-functions-url.php b/tests/phpunit/tests/includes/class-test-functions-url.php
index 55bad41647..e8c609c968 100644
--- a/tests/phpunit/tests/includes/class-test-functions-url.php
+++ b/tests/phpunit/tests/includes/class-test-functions-url.php
@@ -210,6 +210,14 @@ public function signature_host_provider() {
 			'home host as a subdomain'  => array( 'evil.%s', false ),
 			'empty'                     => array( '', false ),
 			'a port without a host'     => array( ':8080', false ),
+			'home host, trailing dot'   => array( '%s.', true ),
+			'userinfo before home host' => array( 'evil.example@%s', false ),
+			'home host with a path'     => array( '%s/inbox', false ),
+			'home host with a query'    => array( '%s?x=1', false ),
+			'home host with a fragment' => array( '%s#x', false ),
+			'leading whitespace'        => array( ' %s', false ),
+			'empty port'                => array( '%s:', false ),
+			'port with a leading zero'  => array( '%s:08080', false ),
 		);
 	}
 
diff --git a/tests/phpunit/tests/includes/class-test-signature.php b/tests/phpunit/tests/includes/class-test-signature.php
index 7cb5e72db6..f469abf5d2 100644
--- a/tests/phpunit/tests/includes/class-test-signature.php
+++ b/tests/phpunit/tests/includes/class-test-signature.php
@@ -2040,14 +2040,24 @@ public function test_verify_http_signature_rejects_a_foreign_host() {
 		\update_option( 'activitypub_rfc9421_signature', '0' );
 		$mock = $this->mock_remote_key();
 
+		$lookups = 0;
+		$count   = function ( $pre ) use ( &$lookups ) {
+			++$lookups;
+
+			return $pre;
+		};
+		\add_filter( 'activitypub_pre_http_get_remote_object', $count, 5 );
+
 		$request = $this->build_replayed_request( 'evil.example', 'evil.example' );
 		$result  = Signature::verify_http_signature( $request );
 
+		\remove_filter( 'activitypub_pre_http_get_remote_object', $count, 5 );
 		\remove_filter( 'activitypub_pre_http_get_remote_object', $mock );
 		\delete_option( 'activitypub_rfc9421_signature' );
 
 		$this->assertWPError( $result, 'A signature made out to another host must not verify, whatever Host claims.' );
 		$this->assertSame( 'invalid_signed_data', $result->get_error_code() );
+		$this->assertSame( 0, $lookups, 'The key must not be looked up for a request made out to another host.' );
 	}
 
 	/**