Reintroduce support for passwords hashed with MD5.

src/wp-includes/pluggable.php

@@ -2724,7 +2724,6 @@ function wp_hash_password(
 	 * @since 2.5.0
 	 * @since 6.8.0 Passwords in WordPress are now hashed with bcrypt by default. A
 	 *              password that wasn't hashed with bcrypt will be checked with phpass.
-	 *              Passwords hashed with md5 are no longer supported.
 	 *
 	 * @global PasswordHash $wp_hasher phpass object. Used as a fallback for verifying
 	 *                                 passwords that were hashed with phpass.
@@ -2742,30 +2741,14 @@ function wp_check_password(
 	) {
 		global $wp_hasher;
 
-		$check = false;
-
-		// If the hash is still md5 or otherwise truncated then invalidate it.
 		if ( strlen( $hash ) <= 32 ) {
-			/**
-			 * Filters whether the plaintext password matches the hashed password.
-			 *
-			 * @since 2.5.0
-			 * @since 6.8.0 Passwords are now hashed with bcrypt by default.
-			 *              Old passwords may still be hashed with phpass.
-			 *
-			 * @param bool       $check    Whether the passwords match.
-			 * @param string     $password The plaintext password.
-			 * @param string     $hash     The hashed password.
-			 * @param string|int $user_id  Optional ID of a user associated with the password.
-			 *                             Can be empty.
-			 */
-			return apply_filters( 'check_password', $check, $password, $hash, $user_id );
-		}
-
-		if ( ! empty( $wp_hasher ) ) {
+			// Check the hash using md5 regardless of the current hashing mechanism.
+			$check = hash_equals( $hash, md5( $password ) );
+		} elseif ( ! empty( $wp_hasher ) ) {
 			// Check the password using the overridden hasher.
 			$check = $wp_hasher->CheckPassword( $password, $hash );
 		} elseif ( strlen( $password ) > 4096 ) {
+			// Passwords longer than 4096 characters are not supported.
 			$check = false;
 		} elseif ( str_starts_with( $hash, '$wp' ) ) {
 			// Check the password using the current prefixed hash.
@@ -2780,7 +2763,19 @@ function wp_check_password(
 			$check = password_verify( $password, $hash );
 		}
 
-		/** This filter is documented in wp-includes/pluggable.php */
+		/**
+		 * Filters whether the plaintext password matches the hashed password.
+		 *
+		 * @since 2.5.0
+		 * @since 6.8.0 Passwords are now hashed with bcrypt by default.
+		 *              Old passwords may still be hashed with phpass or md5.
+		 *
+		 * @param bool       $check    Whether the passwords match.
+		 * @param string     $password The plaintext password.
+		 * @param string     $hash     The hashed password.
+		 * @param string|int $user_id  Optional ID of a user associated with the password.
+		 *                             Can be empty.
+		 */
 		return apply_filters( 'check_password', $check, $password, $hash, $user_id );
 	}
 endif;

tests/phpunit/tests/auth.php

@@ -318,10 +318,10 @@ public function test_wp_check_password_supports_argon2id_hash() {
 	/**
 	 * @ticket 21022
 	 */
-	public function test_wp_check_password_does_not_support_md5_hashes() {
+	public function test_wp_check_password_supports_md5_hash() {
 		$password = 'password';
 		$hash     = md5( $password );
-		$this->assertFalse( wp_check_password( $password, $hash ) );
+		$this->assertTrue( wp_check_password( $password, $hash ) );
 		$this->assertSame( 1, did_filter( 'check_password' ) );
 	}
 
@@ -363,8 +363,6 @@ public function test_wp_check_password_does_not_support_empty_password( $value )
 
 	public function data_empty_values() {
 		return array(
-			// Integer zero:
-			array( 0 ),
 			// String zero:
 			array( '0' ),
 			// Zero-length string:
@@ -1079,6 +1077,42 @@ public function test_phpass_password_is_rehashed_after_successful_user_password_
 		$this->assertSame( self::$user_id, $user->ID );
 	}
 
+	/**
+	 * @dataProvider data_usernames
+	 *
+	 * @ticket 21022
+	 */
+	public function test_md5_password_is_rehashed_after_successful_user_password_authentication( $username_or_email ) {
+		$password = 'password';
+
+		// Set the user password with the old md5 algorithm.
+		self::set_user_password_with_md5( $password, self::$user_id );
+
+		// Verify that the password needs rehashing.
+		$hash = get_userdata( self::$user_id )->user_pass;
+		$this->assertTrue( wp_password_needs_rehash( $hash, self::$user_id ) );
+
+		// Authenticate.
+		$user = wp_authenticate( $username_or_email, $password );
+
+		// Verify that the md5 password hash was valid.
+		$this->assertNotWPError( $user );
+		$this->assertInstanceOf( 'WP_User', $user );
+		$this->assertSame( self::$user_id, $user->ID );
+
+		// Verify that the password no longer needs rehashing.
+		$hash = get_userdata( self::$user_id )->user_pass;
+		$this->assertFalse( wp_password_needs_rehash( $hash, self::$user_id ) );
+
+		// Authenticate a second time to ensure the new hash is valid.
+		$user = wp_authenticate( $username_or_email, $password );
+
+		// Verify that the bcrypt password hash is valid.
+		$this->assertNotWPError( $user );
+		$this->assertInstanceOf( 'WP_User', $user );
+		$this->assertSame( self::$user_id, $user->ID );
+	}
+
 	/**
 	 * @dataProvider data_usernames
 	 *
@@ -1772,6 +1806,38 @@ private static function set_user_password_with_phpass( string $password, int $us
 		clean_user_cache( $user_id );
 	}
 
+	/**
+	 * Test the tests
+	 *
+	 * @covers Tests_Auth::set_user_password_with_md5
+	 *
+	 * @ticket 21022
+	 */
+	public function test_set_user_password_with_md5() {
+		$password = 'password';
+
+		// Set the user password with the old md5 algorithm.
+		self::set_user_password_with_md5( $password, self::$user_id );
+
+		// Ensure the password is hashed with md5.
+		$hash = get_userdata( self::$user_id )->user_pass;
+		$this->assertSame( md5( $password ), $hash );
+	}
+
+	private static function set_user_password_with_md5( string $password, int $user_id ) {
+		global $wpdb;
+
+		$wpdb->update(
+			$wpdb->users,
+			array(
+				'user_pass' => md5( $password ),
+			),
+			array(
+				'ID' => $user_id,
+			)
+		);
+		clean_user_cache( $user_id );
+	}
 
 	/**
 	 * Test the tests