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VulnerabilityPublished

CVE-2026-39971: s9y Serendipity · HTTP Request/Response Splitting

Affects s9y SerendipitySource CVE Database V5

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Not known to be exploited · CVSS 7.2 High : upgrade to 2.6.0 soon.

Data as of 30 Oct 2026, 06:59 UTC

CVSS 3.1 base score
7.2High
How severe the flaw is on the 0–10 CVSS scale.
EPSS
0.2% chance of exploitation in 30 days – higher than 17% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch
Available – fixed in 2.6.0
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 14 Apr 2026

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponized: no evidence on record.

    No public exploit code indexed in Metasploit or Exploit-DB

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq Mirage: no evidence on record.

    No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

What to do

Upgrade Serendipity and s9y/Serendipity to 2.6.0.

Affected software

ProductAffectedFixed in
s9ySerendipityAffected
  • <2.6.0
Fixed in
  • 2.6.0
GitHubs9y/Serendipitypkg:github/s9y/SerendipityOther threatsAffected
  • <2.6.0
Fixed in
  • 2.6.0

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Upgrade Serendipity to version 2.6.0 or later, where this vulnerability is fixed. There is no indication of an official patch or temporary workaround for earlier versions. Users running affected versions should prioritize updating to the fixed release to prevent exploitation.

Description

Serendipity versions prior to 2.6.0 contain a vulnerability where the HTTP Host header is improperly sanitized before being inserted into the Message-ID SMTP header during email sending. This allows an attacker controlling the Host header to inject arbitrary SMTP headers into outgoing emails, potentially enabling identity spoofing, reply hijacking, and email reputation abuse. The issue is fixed in version 2.6.0.

References

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