CVE-2026-39963
S9Y Serendipity ≤ 2.6.0
Raw vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:L/A:NSummary
CVE-2026-39963 is a medium-severity Reliance on Cookies without Validation and Integrity Checking (CWE-565) vulnerability in S9Y Serendipity. Its CVSS base score is 6.9 (Medium).
Operationally, exploitation aligns with the MITRE ATT&CK technique Web Session Cookie (T1550.004); ranked at the 13th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.
The strongest mitigations our analysis identified map to AC-3 (Access Enforcement) and SC-23 (Session Authenticity) — see the control section below for these in your framework.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-22809
Vulnerability Data
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…
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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(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Mitigating Controls (NIST 800-53 r5) AI
Access Enforcement requires authorizations to be enforced by the system rather than by trusting client-supplied cookie values.
Session Authenticity directly requires protecting the integrity and authenticity of session tokens such as cookies, eliminating blind reliance on them.
Transmission Confidentiality and Integrity mandates cryptographic or equivalent protection for data in transit, which covers cookie values exchanged over HTTP.
Software, Firmware, and Information Integrity can detect unauthorized modification of cookie-based data after the fact via integrity verification.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Cookies commonly carry identity assertions; requiring their protection, conveyance, and verification directly eliminates the weakness.
Strong authentication mechanisms reduce reliance on unvalidated cookies for identity and access decisions.
Cryptographic integrity for data-in-transit directly mitigates tampering of cookies sent over the network.
Enforcing access policy and least privilege limits damage from cookie misuse but does not address cookie validation itself.
Integrity protections for data-at-rest can apply to cookie stores but do not cover validation during use.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Security testing can detect cookie-validation flaws but does not itself implement the required controls.
Application security requirements can mandate cookie validation, integrity protection, and server-side session handling.
Secure coding practices directly eliminate reliance on unvalidated cookies by requiring proper integrity checks and server-side verification.
Secure authentication mechanisms can enforce server-side validation and integrity checks that prevent reliance on untrusted cookies.