CVE-2026-39963
MEDIUMCVE-2026-39963 is a medium-severity (CVSS 6.9) vulnerability in s9y/serendipity. O3 Security confirms whether CVE-2026-39963 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
Serendipity: Host Header Injection enables authentication cookie scoping to an attacker-controlled domain
Blast Radius
s9y/serendipityReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.
Description
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 Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | s9y/serendipity | all versions | 2.6.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for s9y/serendipity. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update s9y/serendipity to 2.6.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-39963 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-39963 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-39963. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-39963 in your dependencies?
O3 detects CVE-2026-39963 across Packagist dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.