CVE-2026-39963

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

• 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

CVE-2026-39963 has a CVSS score of 6.9 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.6.0); upgrading removes the vulnerable code path.

Kodem intelligence

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,
        ...
    ]);
}

1. What is CVE-2026-39963? CVE-2026-39963 is a medium-severity security vulnerability in s9y/serendipity (composer), affecting versions < 2.6.0. It is fixed in 2.6.0.
2. How severe is CVE-2026-39963? CVE-2026-39963 has a CVSS score of 6.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of s9y/serendipity are affected by CVE-2026-39963? s9y/serendipity (composer) versions < 2.6.0 is affected.
4. Is there a fix for CVE-2026-39963? Yes. CVE-2026-39963 is fixed in 2.6.0. Upgrade to this version or later.
5. Is CVE-2026-39963 exploitable, and should I be worried? Whether CVE-2026-39963 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-39963 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-39963? Upgrade s9y/serendipity to 2.6.0 or later.