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

{
  "id": "159ba2d05ef5a7ac2be8ec83cd70ef873da3826003333202f92123072d319354",
  "url": "https://osv.dev/vulnerability/CVE-2026-39971",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Serendipity: Host Header Injection leads to SMTP header injection via unvalidated HTTP_HOST",
    "summary": "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.",
    "evidence": "osv:CVE-2026-39971",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-39971",
      "aliases": [
        "CVE-2026-39971",
        "GHSA-458g-q4fh-mj6r"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/s9y/serendipity",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "87696c92730d75785d17b515125be3862cdfb49e"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "fixed": "2.6.0"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StringRangeExpression"
                  },
                  {
                    "range": [
                      {
                        "fixed": "87696c92730d75785d17b515125be3862cdfb49e"
                      }
                    ],
                    "source": "REFERENCES_TAG",
                    "original_tag": "2.6.0"
                  },
                  {
                    "cpe": "cpe:2.3:a:s9y:serendipity:*:*:*:*:*:*:*:*",
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "fixed": "2.6.0"
                      }
                    ],
                    "source": "CPE_RANGE"
                  }
                ]
              }
            }
          ],
          "versions": [
            "2.6-beta1",
            "2.5.0",
            "2.5-beta1",
            "2.4.0",
            "2.4-beta1",
            "2.3.0",
            "2.3-rc1",
            "2.3-beta1",
            "2.2.1-alpha1",
            "2.1.1",
            "2.1.0",
            "2.1-rc1",
            "2.1-beta3",
            "2.1-beta2",
            "2.1-beta1"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-39971.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:50.735311279Z",
      "provider": "osv",
      "references": [
        "https://github.com/s9y/Serendipity/releases/tag/2.6.0",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/39xxx/CVE-2026-39971.json",
        "https://github.com/s9y/Serendipity/security/advisories/GHSA-458g-q4fh-mj6r",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-39971"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-113"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/39xxx/CVE-2026-39971.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-39971",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 519341,
  "published_at": "2026-04-14T23:35:49.305+00:00"
}