What is the CVSS score of CVE-2026-39963?

CVE-2026-39963 has a CVSS 3.1 base score of 6.9 (Medium). CVSS vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:L/A:N. EPSS exploitation probability: 0.0%.

Is there a patch available for CVE-2026-39963?

Yes, a patch is available for CVE-2026-39963. Update the affected software to the latest version immediately.

PHP CVE-2026-39963

MEDIUM

Reliance on Cookies without Validation and Integrity Checking (CWE-565)

2026-04-14 https://github.com/s9y/Serendipity

GHSA-4m6c-649p-f6gf

Privilege Escalation PHP

6.9

CVSS 3.1 · GitHub Advisory

Severity by source

GitHub Advisory PRIMARY

6.9 MEDIUM

AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:L/A:N

Primary rating from GitHub Advisory · only source for this CVE.

CVSS VectorGitHub Advisory

CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:L/A:N

Attack Vector

Network

Attack Complexity

High

Privileges Required

None

User Interaction

Required

Scope

Changed

Confidentiality

High

Integrity

Low

Availability

None

Lifecycle Timeline

Patch released

Apr 15, 2026 - 02:30 nvd

Patch available

Analysis Generated

Apr 15, 2026 - 01:12 vuln.today

Analysis Generated

Apr 14, 2026 - 22:46 vuln.today

CVE Published

Apr 14, 2026 - 22:32 nvd

MEDIUM 6.9

DescriptionGitHub Advisory

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]/serendipity_admin.php 2>&1 | grep -i "set-cookie"

Expected output:

http

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:

php

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,
        ...
    ]);
}

AnalysisAI

Serendipity's serendipity_setCookie() function accepts unsanitized HTTP_HOST header values as the cookie domain parameter, allowing remote attackers to scope authentication cookies (session tokens, auto-login tokens) to attacker-controlled domains and facilitate session hijacking. The vulnerability requires user interaction (victim authentication during poisoned Host header) and man-in-the-middle or reverse proxy misconfiguration to exploit, affecting all versions of Serendipity that use the vulnerable function. A proof-of-concept demonstrating cookie domain poisoning exists; exploitation probability is moderate (EPSS 6.9, CVSS AC:H reflects attack complexity), and no evidence of active exploitation has been identified.

Technical ContextAI

Serendipity is a lightweight PHP blog engine. The vulnerability resides in the serendipity_setCookie() function (include/functions_config.inc.php:726), which directly uses the HTTP_HOST server variable-attacker-controllable in HTTP requests-as the domain parameter to PHP's setcookie() function without validation against the configured server hostname. The root cause (CWE-565: Reliance on Cookies without Validation and Integrity Checking) stems from trusting untrusted input (HTTP_HOST header) for security-critical cookie scope decisions. While the code does strip port numbers when securebyprot=true, this defensive measure is insufficient because the hostname itself remains unvalidated. The function sets cookies with httponly and samesite=Strict flags, which provide some mitigation, but the domain poisoning bypasses the intended audience of the cookie. Affected product: Serendipity (pkg:composer/s9y_serendipity) in versions prior to patching.

RemediationAI

Validate the HTTP_HOST header against the configured serendipity['url'] before using it as a cookie domain. The vendor-suggested fix (referenced in the CVE description) implements hostname parsing and comparison: extract the configured hostname using parse_url($serendipity['url'], PHP_URL_HOST), sanitize the HTTP_HOST header with preg_replace('/:[0-9]+$/', '', ...) to strip port numbers, and use the configured hostname if the request's Host header does not match. Apply the patch provided in the GitHub security advisory (https://github.com/advisories/GHSA-4m6c-649p-f6gf). Alternatively, implement a Content Security Policy and configure reverse proxies to validate and normalize Host headers before forwarding requests to Serendipity. Monitor for signs of session hijacking by reviewing cookie domains in browser developer tools and server logs for mismatched Host headers.