diff --git a/src/wp-includes/rewrite.php b/src/wp-includes/rewrite.php
index 0a6b84b75e534..976d2a014b9db 100644
--- a/src/wp-includes/rewrite.php
+++ b/src/wp-includes/rewrite.php
@@ -502,7 +502,8 @@ function url_to_postid( $url ) {
 	$url_host = parse_url( $url, PHP_URL_HOST );
 
 	if ( is_string( $url_host ) ) {
-		$url_host = str_replace( 'www.', '', $url_host );
+		// Only a leading 'www.' is optional. Removing it anywhere else would match a different host.
+		$url_host = preg_replace( '|^www\.|', '', $url_host );
 	} else {
 		$url_host = '';
 	}
@@ -510,7 +511,7 @@ function url_to_postid( $url ) {
 	$home_url_host = parse_url( home_url(), PHP_URL_HOST );
 
 	if ( is_string( $home_url_host ) ) {
-		$home_url_host = str_replace( 'www.', '', $home_url_host );
+		$home_url_host = preg_replace( '|^www\.|', '', $home_url_host );
 	} else {
 		$home_url_host = '';
 	}
diff --git a/tests/phpunit/tests/rewrite.php b/tests/phpunit/tests/rewrite.php
index 24c7e4e1459fc..a45bb8e2ec913 100644
--- a/tests/phpunit/tests/rewrite.php
+++ b/tests/phpunit/tests/rewrite.php
@@ -167,7 +167,7 @@ public function test_url_to_postid_of_http_site_when_current_site_uses_https() {
 	 * @param string      $url         The complete home URL including scheme and path.
 	 * @param string      $path        Path relative to the home URL. Blank string if no path is specified.
 	 * @param string|null $orig_scheme Scheme to give the home URL context.
-	 * @param int|null    $_blog_id    Site ID, or null for the current site.
+	 * @param int|null    $blog_id     Site ID, or null for the current site.
 	 * @return string                  The complete home URL including scheme and path.
 	 */
 	public function filter_http_home_url( $url, $path, $orig_scheme, $_blog_id ) {
@@ -262,6 +262,48 @@ public function test_url_to_postid_url_has_only_path() {
 		$this->assertSame( 0, url_to_postid( '/example/' ) );
 	}
 
+	/**
+	 * Only a leading 'www.' is optional when comparing the URL's host to the site's.
+	 *
+	 * A 'www.' elsewhere in the host belongs to a different domain, which an attacker
+	 * can register: stripping it everywhere makes 'exwww.ample.com' match 'example.com'.
+	 *
+	 * @ticket 65016
+	 *
+	 * @covers ::url_to_postid
+	 *
+	 * @dataProvider data_url_to_postid_host_matching
+	 *
+	 * @param string $host     Host of the URL to resolve.
+	 * @param bool   $is_local Whether the host should be treated as this site.
+	 */
+	public function test_url_to_postid_matches_www_prefix_only( $host, $is_local ) {
+		update_option( 'home', 'https://example.com' );
+		update_option( 'siteurl', 'https://example.com' );
+
+		$post_id = self::factory()->post->create();
+
+		$expected = $is_local ? $post_id : 0;
+
+		$this->assertSame( $expected, url_to_postid( "https://$host/?p=$post_id" ) );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_url_to_postid_host_matching() {
+		return array(
+			'the site host'              => array( 'example.com', true ),
+			'the site host with www'     => array( 'www.example.com', true ),
+			'www inside the domain'      => array( 'exwww.ample.com', false ),
+			'www inside the TLD'         => array( 'example.cwww.om', false ),
+			'an unrelated host'          => array( 'evil.com', false ),
+			'the site host as subdomain' => array( 'example.com.evil.com', false ),
+		);
+	}
+
 	/**
 	 * @covers ::url_to_postid
 	 */