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TI-2026-28827 Aliases CVE-2026-39971GHSA-458g-q4fh-mj6r

Serendipity: Host Header Injection leads to SMTP header injection via unvalidated HTTP_HOST

Priority HIGH 71CWE-113Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
None
Attack complexity
Low
Impact type
—
Confidentiality · integrity · availability
Low · 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 email sending functionality in include/functions.inc.php inserts $_SERVER['HTTP_HOST'] directly into the Message-ID SMTP header without validation, and the existing sanitization function serendipity_isResponseClean() is not called on HTTP_HOST before embedding it. An attacker who can control the Host header during an email-triggering action such as comment notifications or subscription emails can inject arbitrary SMTP headers into outgoing emails. This enables identity spoofing, reply hijacking via manipulated Message-ID threading, and email reputation abuse through the attacker's domain being embedded in legitimate mail headers. 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 0.00255 17th 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
Yes
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-458g-q4fh-mj6r
  • 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-458g-q4fh-mj6r

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-458g-q4fh-mj6r s9y/serendipity (Packagist) Fixed in 2.6.0 Advisory →

Timeline

  1. 2026-10-02 Threatist record created from CVE-2026-39971
  2. 2026-10-03 Alias added: GHSA-458g-q4fh-mj6r

References

Exploit 1

Patch 1

Advisory 1

Other 1

  • 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-2026-28819 Serendipity: Host Header Injection enables authentication cookie scoping to an attacker-controlled domain CVSS 3.1MEDIUM 6.9
  • 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