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:

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:

// 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

curl -v -X POST \
  -H "Host: attacker.com" \
  -d "serendipity[user]=admin&serendipity[pass]=admin" \
  http://[TARGET]/serendipity_admin.php 2>&1 | grep -i "set-cookie"

Expected output:

Set-Cookie: serendipity[author_token]=; domain=attacker.com; HttpOnly

Impact

• Session fixation — attacker pre-sets a cookie scoped to their domain,
  then tricks the victim into authenticating, inheriting the poisoned token
• Token leakage — author_autologintoken scoped to wrong domain may be
  sent to attacker-controlled infrastructure
• Privilege escalation — if admin logs in under a poisoned Host header,
  their admin token is compromised

Suggested Fix

Validate HTTP_HOST against the configured $serendipity['url'] before use:

function serendipity_setCookie($name, $value, ...) {
    global $serendipity;
    $configured = parse_url($serendipity['url'], PHP_URL_HOST);
    $host = preg_replace('/:[0-9]+$/', '', $_SERVER['HTTP_HOST']);
    $host = ($host === $configured) ? $host : $configured;

    setcookie("serendipity[$name]", $value, [
        'domain' => $host,
        ...
    ]);
}