Is CVE-2026-39963 in the CISA KEV catalog?

No. CVE-2026-39963 is not currently on the CISA Known Exploited Vulnerabilities list.

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

CVE-2026-39963 has a CVSS v3 base score of 6.9 (MEDIUM).

What is the EPSS score for CVE-2026-39963?

CVE-2026-39963 has an EPSS score of 0.22% (13th percentile), estimating the probability of exploitation in the next 30 days.

How do I fix CVE-2026-39963?

Upgrade serendipity to 2.6.0 or later.

CVE & CISA-KEV Catalog

CVE-2026-39963

MEDIUM

6.9

CVSS v3

NVD

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.

CWE-565

How to fix

Remediation Available

serendipityNVD

Affected:< 2.6.0Fixed in:2.6.0CVE-2026-39963derived from NVD

Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.

CVSS v3 Vector

Exploitability

Attack VectorNetwork

Attack ComplexityHigh

Privileges RequiredNone

User InteractionRequired

ScopeChanged

Impact

ConfidentialityHigh

IntegrityLow

AvailabilityNone

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

Exploit Intelligence

0.22%probability of exploitation in 30 days

13thpercentile

Low risk: more likely to be exploited than 13% of all known CVEs.

References

Exploit1

github.com

Product1

github.com

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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-04-23.