CVE-2026-39963: Serendipity: Host Header Injection enables authentication cookie scoping to an attacker-controlled domain

Summary The serendipitysetCookie() function uses $SERVER['HTTPHOST'] 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/functionsconfig.inc.php:726: php function serendipitysetCookie($name, $value, $securebyprot = true, ...) { $host = $SERVER['HTTPHOST']; // ← 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 // functionsconfig.inc.php:455-498 serendipitysetCookie('authorautologintoken', $rnd, true, false, true); serendipitysetCookie('authorusername', $user); serendipitysetCookie('authortoken', $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]/serendipityadmin.php 2>&1 | grep -i "set-cookie"

Expected output: http Set-Cookie: serendipity[authortoken]=; 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 — authorautologintoken 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 HTTPHOST against the configured $serendipity['url'] before use: php function serendipitysetCookie($name, $value, ...) { global $serendipity; $configured = parseurl($serendipity['url'], PHPURLHOST); $host = pregreplace('/:[0-9]+$/', '', $SERVER['HTTPHOST']); $host = ($host === $configured) ? $host : $configured;

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