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CVE-2026-39971

S9y Serendipity vulnerability

7.2 HIGHPublic exploitEPSS 0.30%
  1. Published15 Apr 2026

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.

CVSS 3.1 breakdown

Attack vectorNetwork
Attack complexityLow
Privileges requiredNone
User interactionNone
ScopeChanged
ConfidentialityLow
IntegrityLow
AvailabilityNone
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

Affected products

S9y Serendipity< 2.6.0 · 2.6.0

Remediation

Fixed in
S9y Serendipity≥ 2.6.0

References

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Frequently asked

Is CVE-2026-39971 being actively exploited?

CVE-2026-39971 is not in the CISA KEV catalog. A public exploit reference exists, so treat it as exploitable. Its EPSS exploit probability is 0.30%.

How severe is CVE-2026-39971?

CVE-2026-39971 has a CVSS 3.1 base score of 7.2, rated high. EPSS estimates a 0.30% probability of exploitation.

Which products are affected by CVE-2026-39971?

Affected products include S9y Serendipity.

How do I fix CVE-2026-39971?

Upgrade affected software to a fixed version, for example S9y Serendipity 2.6.0 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2026-39971 against the real software inventory on every managed device, as each hourly inventory arrives. Rather than a static severity, each match is scored in context: exploit availability, EPSS, the asset's exposure, and business criticality decide where it lands in the queue. If an affected version is present anywhere across your portfolio, it surfaces automatically with a prioritised remediation path.

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.