Index: src/wp-includes/pluggable.php
===================================================================
--- src/wp-includes/pluggable.php	(revision 29710)
+++ src/wp-includes/pluggable.php	(working copy)
@@ -669,7 +669,9 @@
 	$pass_frag = substr($user->user_pass, 8, 4);
 
 	$key = wp_hash( $username . '|' . $pass_frag . '|' . $expiration . '|' . $token, $scheme );
-	$hash = hash_hmac( 'sha256', $username . '|' . $expiration . '|' . $token, $key );
+	// If ext/hash is not present, compat.php's hash_hmac() does not support sha256.
+	$algo = function_exists( 'hash' ) ? 'sha256' : 'sha1';
+	$hash = hash_hmac( $algo, $username . '|' . $expiration . '|' . $token, $key );
 
 	if ( ! hash_equals( $hash, $hmac ) ) {
 		/**
@@ -734,7 +736,9 @@
 	$pass_frag = substr($user->user_pass, 8, 4);
 
 	$key = wp_hash( $user->user_login . '|' . $pass_frag . '|' . $expiration . '|' . $token, $scheme );
-	$hash = hash_hmac( 'sha256', $user->user_login . '|' . $expiration . '|' . $token, $key );
+	// If ext/hash is not present, compat.php's hash_hmac() does not support sha256.
+	$algo = function_exists( 'hash' ) ? 'sha256' : 'sha1';
+	$hash = hash_hmac( $algo, $user->user_login . '|' . $expiration . '|' . $token, $key );
 
 	$cookie = $user->user_login . '|' . $expiration . '|' . $token . '|' . $hash;
 
Index: src/wp-includes/session.php
===================================================================
--- src/wp-includes/session.php	(revision 29710)
+++ src/wp-includes/session.php	(working copy)
@@ -61,7 +61,12 @@
 	 * @return string A hash of the session token (a verifier).
 	 */
 	final private function hash_token( $token ) {
-		return hash( 'sha256', $token );
+		// If ext/hash is not present, use sha1() instead.
+		if ( function_exists( 'hash' ) ) {
+			return hash( 'sha256', $token );
+		} else {
+			return sha1( $token );
+		}
 	}
 
 	/**