high CVSS 7.2

CVE-2026-39971·PHP vulnerability

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.

Severity
high
Software
PHP
Fixed in
2.6.0
Published
2026-04-15

Affected versions

Until: 2.6.0

Fixed in: 2.6.0

How to fix this CVE

Update PHP and Serendipity to version 2.6.0 or later to patch the Host header injection vulnerability in email header processing. This vulnerability allows attackers to manipulate SMTP headers through the HTTP Host field, potentially spoofing sender identity and hijacking email threads. Apply the update immediately if you run Serendipity-based applications or have PHP components that process email headers.

sudo dnf update php

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST|GET requests with manipulated Host headers followed by email-triggering actions (comment submission, subscription confirmation); look for entries in mail logs containing unusual domain names or CRLF sequences in the Message-ID header: (Message-ID:.*\r\n|Host:.*[^a-zA-Z0-9.-])

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement Host header validation rules: whitelist legitimate Host values and block requests with Host headers containing CRLF characters (\r\n, \x0d\x0a) or unusual domain suffixes. Additionally, restrict email-triggering endpoints to authenticated users only and enforce strict input validation on comment/subscription forms.

How to check if you are affected

  1. Check installed PHP version: php -v and verify it corresponds to Serendipity 2.6.0 or later
  2. Inspect include/functions.inc.php in your Serendipity installation for direct $_SERVER['HTTP_HOST'] usage in email headers without validation
  3. Search application logs for suspicious Host header values during email-sending operations: grep -r 'HTTP_HOST' /path/to/serendipity/logs/ | grep -i 'message-id'
  4. Verify the fix by checking the source code: grep -n 'serendipity_isResponseClean' include/functions.inc.php | grep -i host to confirm sanitization is now applied

FAQ

What is CVE-2026-39971?

CVE-2026-39971 is a Host header injection vulnerability in Serendipity PHP blogging engine that allows attackers to inject malicious SMTP headers into outgoing emails by controlling the HTTP Host field, enabling email spoofing and thread hijacking attacks.

Is CVE-2026-39971 being actively exploited?

No, this vulnerability is not currently listed on the CISA Known Exploited Vulnerabilities catalog, and no public exploits are available, though the attack vector is network-based and requires minimal privileges.

What versions of PHP are affected by CVE-2026-39971?

Serendipity versions 2.6-beta2 and all earlier releases are affected; the vulnerability was patched in version 2.6.0. The underlying PHP component (functions.inc.php) is the vulnerable code, so any Serendipity deployment below 2.6.0 running on PHP requires remediation.

How do I check if my server is vulnerable to CVE-2026-39971?

Run serendipity_get_config_var('version') or check your Serendipity admin panel for the installed version; if it is below 2.6.0, your system is vulnerable. Additionally, inspect include/functions.inc.php for unvalidated $_SERVER['HTTP_HOST'] in email header construction.

Does Defensia detect CVE-2026-39971?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If PHP and Serendipity are installed on a monitored server, CVE-2026-39971 will appear in your dashboard with remediation steps.

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References

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