CVE-2026-39971
Serendipity: Host Header Injection leads to SMTP header injection via unvalidated HTTP_HOST
Description
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.
INFO
Published Date :
April 15, 2026, 4:17 a.m.
Last Modified :
April 23, 2026, 1:59 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | HIGH | MITRE-CVE | ||||
| CVSS 3.1 | HIGH | [email protected] |
Solution
- Update Serendipity to version 2.6.0 or later.
- Apply vendor patches if available.
- Validate all user inputs carefully.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-39971.
| URL | Resource |
|---|---|
| https://github.com/s9y/Serendipity/releases/tag/2.6.0 | Patch |
| https://github.com/s9y/Serendipity/security/advisories/GHSA-458g-q4fh-mj6r | Exploit Vendor Advisory Mitigation |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-39971 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-39971
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-39971 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-39971 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
Initial Analysis by [email protected]
Apr. 23, 2026
Action Type Old Value New Value Added CPE Configuration OR *cpe:2.3:a:s9y:serendipity:*:*:*:*:*:*:*:* versions up to (excluding) 2.6.0 *cpe:2.3:a:s9y:serendipity:2.6.0:beta1:*:*:*:*:*:* Added Reference Type GitHub, Inc.: https://github.com/s9y/Serendipity/releases/tag/2.6.0 Types: Patch Added Reference Type GitHub, Inc.: https://github.com/s9y/Serendipity/security/advisories/GHSA-458g-q4fh-mj6r Types: Exploit, Mitigation, Vendor Advisory -
New CVE Received by [email protected]
Apr. 15, 2026
Action Type Old Value New Value Added Description 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. Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N Added CWE CWE-113 Added Reference https://github.com/s9y/Serendipity/releases/tag/2.6.0 Added Reference https://github.com/s9y/Serendipity/security/advisories/GHSA-458g-q4fh-mj6r