medium CVSS 6.9 EPSS <0.1%

CVE-2026-39963·PHP vulnerability

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.

Severity
medium
Software
PHP
Fixed in
2.6.0
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at <0.1% — higher than 17% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Affected versions

Until: 2.6.0

Fixed in: 2.6.0

How to fix this CVE

Upgrade PHP and the Serendipity weblog engine to version 2.6.0 or later to patch the Host header validation flaw in the cookie-setting mechanism. This vulnerability allows attackers to redirect authentication cookies to attacker-controlled domains through HTTP Host header manipulation, potentially compromising admin sessions. Apply the update immediately if you are running Serendipity 2.6-beta2 or earlier, especially on publicly accessible systems.

sudo dnf update php

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST /serendipity/index.php?action=admin (with malicious Host header value not matching server FQDN or expected domain whitelist; followed by Set-Cookie headers with Domain parameter set to attacker-controlled domain)

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement a WAF rule that validates the Host header against a whitelist of known legitimate domains and rejects requests with Host values matching suspicious patterns (e.g., IP addresses of attacker infrastructure, domains known to be used for session theft). Additionally, strip or normalize the Host header before it reaches the application if your infrastructure layer permits.

How to check if you are affected

  1. Check installed PHP version: php -v
  2. Verify Serendipity version in your installation: grep -r "SERENDIPITY_VERSION" /path/to/serendipity/include/ | head -1
  3. Search logs for suspicious Host header values during authentication attempts: grep -i 'login\|auth' /var/log/apache2/access.log | grep -oP 'Host: \K[^ ]+' | sort | uniq -c | sort -rn
  4. Confirm the fix by verifying Serendipity version is 2.6.0 or later and reviewing the serendipity_setCookie() function in include/functions_config.inc.php for proper host validation

FAQ

What is CVE-2026-39963?

A Host header injection vulnerability in Serendipity's cookie-setting function that allows attackers to scope authentication cookies to malicious domains, enabling session hijacking and privilege escalation during admin login.

Is CVE-2026-39963 being actively exploited?

No, this vulnerability is not currently listed on the CISA Known Exploited Vulnerabilities (KEV) catalog, and no public exploits are available. However, the attack vector is practical and should be patched promptly.

What versions of Serendipity are affected by CVE-2026-39963?

Serendipity versions up to and including 2.6-beta2 are vulnerable. Version 2.6.0 and later include the necessary Host header validation.

How do I check if my server is vulnerable to CVE-2026-39963?

Run: grep -A 5 'serendipity_setCookie' /path/to/serendipity/include/functions_config.inc.php | grep -q 'sanitize\|filter\|validate' || echo 'VULNERABLE'; then verify your Serendipity version is below 2.6.0.

Does Defensia detect CVE-2026-39963?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If PHP and Serendipity are installed on a monitored server, CVE-2026-39963 will appear in your dashboard with remediation steps.

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References

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