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

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Summary

CVECVE-2026-39971
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-04-15 04:17:39 UTC
Updated2026-04-17 15:38:09 UTC
DescriptionSerendipity 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.

Risk And Classification

Primary CVSS: v3.1 7.2 HIGH from [email protected]

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

EPSS: 0.000380000 probability, percentile 0.114500000 (date 2026-04-21)

Problem Types: CWE-113 | CWE-113 CWE-113: Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary7.2HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N
3.1CNADECLARED7.2HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA S9y Serendipity affected < 2.6.0 Not specified

References

ReferenceSourceLinkTags
github.com/s9y/Serendipity/security/advisories/GHSA-458g-q4fh-mj6r [email protected] github.com
github.com/s9y/Serendipity/releases/tag/2.6.0 [email protected] github.com
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