Index: tests/phpunit/tests/rewrite.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- tests/phpunit/tests/rewrite.php	(revision 41026)
+++ tests/phpunit/tests/rewrite.php	(revision )
@@ -389,4 +389,16 @@
 		$this->assertInternalType( 'array', $rewrite_rules );
 		$this->assertNotEmpty( $rewrite_rules );
 	}
+
+	/**
+	 * @ticket 39373
+	 */
+	public function test_url_to_postid_from_external_url(){
+		$this->set_permalink_structure( '/%category%/%postname%/' );
+		$cat1 = wp_create_category( 'something' );
+		self::factory()->post->create( array( 'post_title' => 'External URL', 'post_type' => 'post', 'post_name' => 'external-url', 'post_category' => array( $cat1 ) ) );
+
+		$test_external = 'http://www.external-domain.com/something/external-url/';
+		$this->assertSame( 0, url_to_postid( $test_external ) );
+	}
 }
Index: src/wp-includes/rewrite.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- src/wp-includes/rewrite.php	(revision 41026)
+++ src/wp-includes/rewrite.php	(revision )
@@ -517,6 +517,8 @@
 	if ( !$wp_rewrite->using_index_permalinks() )
 		$url = str_replace( $wp_rewrite->index . '/', '', $url );
 
+	// Before we truncate $url, we temporary store it
+	$temporary_url = $url;
 	if ( false !== strpos( trailingslashit( $url ), home_url( '/' ) ) ) {
 		// Chop off http://domain.com/[path]
 		$url = str_replace(home_url(), '', $url);
@@ -527,6 +529,11 @@
 		$url = preg_replace( sprintf( '#^%s#', preg_quote( $home_path ) ), '', trailingslashit( $url ) );
 	}
 
+	// If the $url is not truncated, that means the $url is not from our current domain. Bail. @ticket 39373
+	if( false !== strpos( $url, $temporary_url ) ){
+		return 0;
+	}
+
 	// Trim leading and lagging slashes
 	$url = trim($url, '/');