CVE-2026-39971Public exploitNetwork attack vector

Email spoofing via unvalidated Host header

Published Apr 14, 2026 · Updated Apr 14, 2026

SMTP header injection in Serendipity before 2.6.0 allows remote attackers to spoof outgoing email identities through a crafted Host header. The mail builder in include/functions.inc.php appends HTTP_HOST to Message-ID without calling serendipity_isResponseClean(), so SMTP delimiters become new headers. Exploitation requires an installation that accepts arbitrary Host values and an email-triggering request; injected headers enable BCC insertion, spoofing, and thread hijacking.

CVSS severity7.2
High
EPSS probability0.26%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

SMTP header injection in Serendipity before 2.6.0 allows remote attackers to spoof outgoing email identities through a crafted Host header. The mail builder in include/functions.inc.php appends HTTP_HOST to Message-ID without calling serendipity_isResponseClean(), so SMTP delimiters become new headers. Exploitation requires an installation that accepts arbitrary Host values and an email-triggering request; injected headers enable BCC insertion, spoofing, and thread hijacking.

The record

CVE
CVE-2026-39971
Published
Apr 14, 2026
Updated
Apr 14, 2026
Vendor
Serendipity Team
Product
Serendipity
Classifications
CWE-113, T1684.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 14, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 14, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 2.6.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Serendipity Team · Serendipityversion=< 2.6.0

Published CVSS scores

7.2GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-113Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')
T1684.002Social Engineering: Email Spoofing

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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