Serendipity: Host Header Injection enables authentication cookie scoping to an attacker-controlled domain

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Summary

CVECVE-2026-39963
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-04-15 04:17:39 UTC
Updated2026-04-17 15:38:09 UTC
DescriptionSerendipity 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.

Risk And Classification

Primary CVSS: v3.1 6.9 MEDIUM from [email protected]

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

EPSS: 0.000460000 probability, percentile 0.141430000 (date 2026-04-21)

Problem Types: CWE-565 | CWE-565 CWE-565: Reliance on Cookies without Validation and Integrity Checking

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary6.9MEDIUMCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:L/A:N
3.1CNADECLARED6.9MEDIUMCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:L/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
High
Privileges Required
None
User Interaction
Required
Scope
Changed
Confidentiality
High
Integrity
Low
Availability
None

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

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA S9y Serendipity affected < 2.6.0 Not specified

References

ReferenceSourceLinkTags
github.com/s9y/Serendipity/releases/tag/2.6.0 [email protected] github.com
github.com/s9y/Serendipity/security/advisories/GHSA-4m6c-649p-f6gf 134c704f-9b21-4f2e-91b3-4a467353bcc0 github.com
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