How severe is CVE-2026-39971?

CVE-2026-39971 has a CVSS 3.x base score of 7.2, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is low, integrity low, and availability none.

Is CVE-2026-39971 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (17th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-39971?

CVE-2026-39971 primarily affects S9y Serendipity. In total, 2 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2026-39971?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

Does CVE-2026-39971 have an EU (EUVD) identifier?

Yes. CVE-2026-39971 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-22811.

When was CVE-2026-39971 published?

CVE-2026-39971 was published on 2026-04-15 and last updated on 2026-06-17.

CVE-2026-39971

CVE-2026-39971 is a high-severity vulnerability in S9y Serendipity with a CVSS 3.x base score of 7.2. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-113.

Key facts

Severity: High (CVSS 3.x base score 7.2)
EPSS exploit prediction: 0% (17th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-22811
Weakness: CWE-113
Affected product: S9y Serendipity
Published: 2026-04-15
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2026-39971?: 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.
How severe is CVE-2026-39971?: CVE-2026-39971 has a CVSS 3.x base score of 7.2, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is low, integrity low, and availability none.
Is CVE-2026-39971 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (17th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-39971?: CVE-2026-39971 primarily affects S9y Serendipity. In total, 2 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2026-39971?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
Does CVE-2026-39971 have an EU (EUVD) identifier?: Yes. CVE-2026-39971 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-22811.
When was CVE-2026-39971 published?: CVE-2026-39971 was published on 2026-04-15 and last updated on 2026-06-17.

References

Affected products (2)

cpe:2.3:a:s9y:serendipity:*:*:*:*:*:*:*:*
cpe:2.3:a:s9y:serendipity:2.6.0:beta1:*:*:*:*:*:*

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