CVE-2026-39963 | Serendipity: Host Header Injection enables authentication cookie scoping to an attacker-controlled domain

Exp

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.

Published: 2026-04-15 Last update: 2026-04-23 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2026-39963 is rated Exploit Available (50/100): CVSS Medium severity, with low exploitation likelihood (EPSS 0.05%). 1 public exploit reference(s) are indexed (Exploit-DB). Public exploits are available—assess exposure, apply mitigations, and prioritize patching.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Public exploit references (Exploit-DB) for CVE-2026-39963

EDB-ID Source Kind Published Link
— nvd_ref exploit_tag Exploit-DB ↗

Exploit prediction scoring system (EPSS) score for CVE-2026-39963

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-04-24 0.04% 0.05% +0.01%
2 2026-04-15 — 0.04% —

Full EPSS history (2 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2026-39963

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
6.9 3.1 MEDIUM
CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:L/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:H)
Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:R)
A real person has to do something—click, install, enable—otherwise it doesn’t land.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:L)
Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N)
Service keeps running; no real outage angle.
 1.6 4.7 [email protected]

Weakness enumeration for CVE-2026-39963

GitHub Security Advisory for CVE-2026-39963

GHSA-4m6c-649p-f6gf · Severity: medium · Ecosystem: composer — Serendipity has a Host Header Injection allows authentication cookie scoping to attacker-controlled domain in functions_config.inc.php

Affected software / configurations for CVE-2026-39963

Vendor Product Version Raw CPE
s9y serendipity < 2.6.0 cpe:2.3:a:s9y:serendipity:*:*:*:*:*:*:*:*
s9y serendipity 2.6.0 cpe:2.3:a:s9y:serendipity:2.6.0:beta1:*:*:*:*:*:*

References for CVE-2026-39963

cvelogic Threat Intelligence