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TI-2026-28819 Aliases CVE-2026-39963GHSA-4m6c-649p-f6gf

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

Priority MEDIUM 64CWE-565Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
Required
Attack complexity
High
Impact type
—
Confidentiality · integrity · availability
High · Low · None
Fix status
Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA and GitHub.

Summary

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.

Why it matters now

Exploitation Proof of concept CISA SSVC · not in CISA KEV
EPSS 0.00224 13th percentileScored 2026-11-07
Public exploit Yes 1 exploit referenceTagged by NVD
Fix Fixed Serendipity (s9y): 2.6.0s9y/serendipity (Packagist): 2.6.0

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
Poc
Automatable
No
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
s9ySerendipitybefore 2.6.02.6.0Patch availableCNA
—s9y/serendipity (Packagist)before 2.6.02.6.0Patch availableGHSA-4m6c-649p-f6gf
  • Serendipity Vendors9y Affected before 2.6.0 Fixed in 2.6.0 Patch available CNA
  • s9y/serendipity (Packagist) Vendor— Affected before 2.6.0 Fixed in 2.6.0 Patch available GHSA-4m6c-649p-f6gf

CPE configurations NVD · 2

CPEVersionsRoleVendor
cpe:2.3:a:s9y:serendipity:*:*:*:*:*:*:*:*before 2.6.0Vulnerables9y
cpe:2.3:a:s9y:serendipity:2.6.0:beta1:*:*:*:*:*:*2.6.0 beta1Vulnerables9y
  • cpe:2.3:a:s9y:serendipity:*:*:*:*:*:*:*:* Versions before 2.6.0 Vulnerable ·s9y
  • cpe:2.3:a:s9y:serendipity:2.6.0:beta1:*:*:*:*:*:* Versions 2.6.0 beta1 Vulnerable ·s9y

Remediation

  • Fix · CNA Serendipity (s9y) Fixed in 2.6.0
  • Fix · GHSA-4m6c-649p-f6gf s9y/serendipity (Packagist) Fixed in 2.6.0 Advisory →

Timeline

  1. 2026-10-02 Threatist record created from CVE-2026-39963
  2. 2026-10-03 Alias added: GHSA-4m6c-649p-f6gf

References

Exploit 1

Advisory 1

Other 2

  • TI-2023-30216 Serendipity 2.4.0 Authenticated Remote Code Execution via File Upload CVSS 4.0HIGH 8.7
  • TI-2024-38379 Serendipity 2.5.0 Remote Code Execution via Authenticated Media Upload CVSS 4.0HIGH 8.6
  • TI-2026-28827 Serendipity: Host Header Injection leads to SMTP header injection via unvalidated HTTP_HOST CVSS 3.1HIGH 7.2
  • TI-2023-30215 Serendipity 2.4.0 Stored Cross-Site Scripting via Admin Entry Creation CVSS 4.0MEDIUM 5.1
  • TI-2011-00987 Cross-Site Scripting (XSS) in Xinha, as included in the Serendipity package before 1.5.5… CVSS 3.1CRITICAL 9.8
  • TI-2020-08841 Serendipity before 2.3.4 on Windows allows remote attackers to execute arbitrary code… CVSS 3.1CRITICAL 9.8
See all 57 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences