Serendipity is a PHP-powered weblog engine. In versions 2.6-beta2 and below, the serendipity_setCookie() function in include/functions_config.inc.php uses $_SERVER['HTTP_HOST'] without validation as the domain parameter of setcookie(). An attacker who can influence the Host header at login time, such as via MITM, reverse proxy misconfiguration, or load balancer manipulation, can force authentication cookies including session tokens and auto-login tokens to be scoped to an attacker-controlled domain. This enables session fixation, token leakage to attacker-controlled infrastructure, and privilege escalation if an admin logs in under a poisoned Host header. This issue has been fixed in version 2.6.0.

Affected: Packagist: s9y/serendipity

### Summary The `serendipity_setCookie()` function uses `$_SERVER['HTTP_HOST']` without validation as the `domain` parameter of `setcookie()`. An attacker can force authentication cookies — including session tokens and auto-login tokens — to be scoped to an attacker-controlled domain, facilitating session hijacking.

### Details In `include/functions_config.inc.php:726`: ```php function serendipity_setCookie($name, $value, $securebyprot = true, ...) { $host = $_SERVER['HTTP_HOST']; // ← attacker-controlled, no validation

if ($securebyprot) { if ($pos = strpos($host, ":")) { $host = substr($host, 0, $pos); // strips port only } }

setcookie("serendipity[$name]", $value, [ 'domain' => $host, // ← poisoned domain 'httponly' => $httpOnly, 'samesite' => 'Strict' ]); } ```

This function is called during login with sensitive cookies: ```php // functions_config.inc.php:455-498 serendipity_setCookie('author_autologintoken', $rnd, true, false, true); serendipity_setCookie('author_username', $user); serendipity_setCookie('author_token', $hash); ```

If an attacker can influence the `Host` header at login time (e.g. via MITM, reverse proxy misconfiguration, or load balancer), authentication cookies are issued scoped to the attacker's domain instead of the legitimate one.

### PoC ```bash curl -v -X POST \ -H "Host: attacker.com" \ -d "serendipity[user]=admin&serendipity[pass]=admin" \ http://[TARGET]/ser…