How severe is CVE-2026-39963?

CVE-2026-39963 has a CVSS 3.x base score of 6.9, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and user interaction. Impact on confidentiality is high, integrity low, and availability none.

Is CVE-2026-39963 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (13th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-39963?

CVE-2026-39963 primarily affects S9y Serendipity. In total, 2 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2026-39963?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

Does CVE-2026-39963 have an EU (EUVD) identifier?

Yes. CVE-2026-39963 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-22809.

When was CVE-2026-39963 published?

CVE-2026-39963 was published on 2026-04-15 and last updated on 2026-06-17.

CVE-2026-39963

CVE-2026-39963 is a medium-severity vulnerability in S9y Serendipity with a CVSS 3.x base score of 6.9. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-565.

Key facts

Severity: Medium (CVSS 3.x base score 6.9)
EPSS exploit prediction: 0% (13th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-22809
Weakness: CWE-565
Affected product: S9y Serendipity
Published: 2026-04-15
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2026-39963?: 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.
How severe is CVE-2026-39963?: CVE-2026-39963 has a CVSS 3.x base score of 6.9, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and user interaction. Impact on confidentiality is high, integrity low, and availability none.
Is CVE-2026-39963 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (13th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-39963?: CVE-2026-39963 primarily affects S9y Serendipity. In total, 2 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2026-39963?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-39963 have an EU (EUVD) identifier?: Yes. CVE-2026-39963 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-22809.
When was CVE-2026-39963 published?: CVE-2026-39963 was published on 2026-04-15 and last updated on 2026-06-17.

References

Affected products (2)

cpe:2.3:a:s9y:serendipity:*:*:*:*:*:*:*:*
cpe:2.3:a:s9y:serendipity:2.6.0:beta1:*:*:*:*:*:*

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