Source

File: wp-includes/pluggable.php

function wp_check_password( $password, $hash, $user_id = '' ) {
		global $wp_hasher;

		// If the hash is still md5...
		if ( strlen( $hash ) <= 32 ) {
			$check = hash_equals( $hash, md5( $password ) );
			if ( $check && $user_id ) {
				// Rehash using new hash.
				wp_set_password( $password, $user_id );
				$hash = wp_hash_password( $password );
			}

			/**
			 * Filters whether the plaintext password matches the encrypted password.
			 *
			 * @since 2.5.0
			 *
			 * @param bool       $check    Whether the passwords match.
			 * @param string     $password The plaintext password.
			 * @param string     $hash     The hashed password.
			 * @param string|int $user_id  User ID. Can be empty.
			 */
			return apply_filters( 'check_password', $check, $password, $hash, $user_id );
		}

		// If the stored hash is longer than an MD5,
		// presume the new style phpass portable hash.
		if ( empty( $wp_hasher ) ) {
			require_once ABSPATH . WPINC . '/class-phpass.php';
			// By default, use the portable hash from phpass.
			$wp_hasher = new PasswordHash( 8, true );
		}

		$check = $wp_hasher->CheckPassword( $password, $hash );

		/** This filter is documented in wp-includes/pluggable.php */
		return apply_filters( 'check_password', $check, $password, $hash, $user_id );
	}