CVE-2026-39971

HIGHNVD 7.27.2—
EchelonGraph scoreMEDIUM confidence

This high-severity CVE scores 7.2 under NVD CVSS v3. EPSS exploit probability: 0.3%, top 83% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
Trending — 3 sources updated this weekElevated
7.2
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 0%CVSS: 7.2Exploit: NoneExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

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 v3
7.2
EG Score
7.2(medium)
EG Risk
63(Attend)
EG Risk 63/100SSVC: Attend

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity72% × 45%
Exploitation40% × 40%
Automatability100% × 15%
Action: Remediate soon — notable exploitation risk.
EPSS
17.1%
KEV
Not listed

Published

April 15, 2026

Last Modified

April 23, 2026

Advisory Details (2)

Auto-updated Jun 14, 2026
🔬 Proof of concept available. Patch available. Sources: github_release, github.
github Patch Available🟡 PoC Available

Host Header Injection allows SMTP header injection via unvalidated HTTP_HOST in Message-ID email header · Advisory · s9y/Serendipity · GitHub

https://github.com/s9y/Serendipity/security/advisories/GHSA-458g-q4fh-mj6r
github_release Patch Available

Serendipity 2.6.0

Patch available: s9y/Serendipity 2.6.0

https://github.com/s9y/Serendipity/releases/tag/2.6.0

Affected Packages

(1 across 1 ecosystem)
Packagist(1)
PackageVulnerable rangeFixed inDependents
s9y/serendipity2.0.0 ... 2.6-beta1 (31 versions)2.6.0—

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 8× in last 7d / 35× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

  1. 2026-07-23 03:10 UTCEG score recompute
  2. 2026-07-22 14:08 UTCEPSS rescore
  3. 2026-07-21 15:24 UTCEPSS rescore
  4. 2026-07-20 17:08 UTCEPSS rescore
  5. 2026-07-20 10:04 UTCOSV refresh
  6. 2026-07-19 02:29 UTCEPSS rescore
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  8. 2026-07-16 17:03 UTCEPSS rescore
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  1. 2026-07-01 15:06 UTCEPSS rescore
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  16. 2026-06-19 19:25 UTCEPSS rescore
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  21. 2026-06-15 17:49 UTCEPSS rescore
  22. 2026-06-14 23:18 UTCEPSS rescore
  23. 2026-06-13 23:00 UTCEPSS rescore
  24. 2026-06-12 23:12 UTCEPSS rescore
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  26. 2026-06-11 14:00 UTCEPSS rescore
  27. 2026-06-10 22:18 UTCEPSS rescore
  28. 2026-06-10 13:22 UTCEPSS rescore
  29. 2026-06-08 14:17 UTCEPSS rescore
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  31. 2026-06-07 15:25 UTCEPSS rescore
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  40. 2026-06-04 13:12 UTCEPSS rescore
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  42. 2026-06-02 20:13 UTCEPSS rescore
  43. 2026-06-01 13:52 UTCEPSS rescore
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  46. 2026-05-31 22:30 UTCEPSS rescore
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  51. 2026-05-29 13:44 UTCEPSS rescore
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  53. 2026-05-28 13:44 UTCEPSS rescore
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  56. 2026-05-27 13:40 UTCEPSS rescore
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  62. 2026-05-20 22:38 UTCEPSS rescore
  63. 2026-05-20 22:38 UTCEPSS rescore
  64. 2026-05-20 17:13 UTCEG score recompute
  65. 2026-05-20 11:22 UTCEPSS rescore
  66. 2026-05-20 11:22 UTCEPSS rescore

Frequently asked(5)

What is CVE-2026-39971?
CVE-2026-39971 is a high vulnerability published on April 15, 2026. 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['HTTPHOST'] directly into the Message-ID SMTP header without validation, and the existing sanitization function serendipityisResponseClean() is…
When was CVE-2026-39971 disclosed?
CVE-2026-39971 was first published in the National Vulnerability Database on April 15, 2026, with the most recent update on April 23, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-39971 actively exploited?
CVE-2026-39971 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 17.1% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
What is the CVSS score of CVE-2026-39971?
CVE-2026-39971 has a CVSS v3 base score of 7.2 (NVD).
How do I remediate CVE-2026-39971?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-39971, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

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