Contao: The registration module re-sends activation mails on any unauthenticated POST, with no throttle and no captcha check

{
  "id": "602a2eac3d85443fec74e2f73ea6c1799fb4a4986cf0849ecd366d42573e177d",
  "url": "https://osv.dev/vulnerability/CVE-2026-107843",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Contao: The registration module re-sends activation mails on any unauthenticated POST, with no throttle and no captcha check",
    "summary": "Contao is an Open Source CMS. From version 4.1.0 until 5.3.50 and 5.7.12, ModuleRegistration::compile() enters its follow-up registration branch on any POST to a page containing the registration module without verifying FORM_SUBMIT or the preceding captcha result. resendActivationMail() can then invoke OptInToken::send() without rate limiting, allowing an unauthenticated attacker to cause repeated activation emails to be sent to an address with a pending registration and to determine whether that pending registration exists. The branch is reachable only when reg_activate is enabled and the target has an unconfirmed registration and opt-in token. This issue is fixed in versions 5.3.50 and 5.7.12.",
    "evidence": "osv:CVE-2026-107843",
    "products": [],
    "severity": "medium",
    "structured": {
      "id": "CVE-2026-107843",
      "aliases": [
        "CVE-2026-107843",
        "GHSA-mfxh-vp55-7gc6"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/contao/contao",
              "type": "GIT",
              "events": [
                {
                  "introduced": "a112b68dcb5215b01f2c4c4b8de6bcb3d3b9ae81"
                },
                {
                  "fixed": "a11109b60dedcbd99e32a4e8550afdadb500f2c5"
                },
                {
                  "introduced": "c5e3841e5107ab36ae2bf7795fb8d51d3d30f88b"
                },
                {
                  "fixed": "a6543f49a92187a7368f51b290179c4795386e23"
                },
                {
                  "fixed": "2ea6117f9049db7221679251cfc41e67d941a74b"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "4.1.0"
                      },
                      {
                        "fixed": "5.3.50"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StringRangeExpression"
                  },
                  {
                    "range": [
                      {
                        "introduced": "5.4.0-RC1"
                      },
                      {
                        "fixed": "5.7.12"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StringRangeExpression"
                  },
                  {
                    "range": [
                      {
                        "fixed": "2ea6117f9049db7221679251cfc41e67d941a74b"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  },
                  {
                    "range": [
                      {
                        "fixed": "a11109b60dedcbd99e32a4e8550afdadb500f2c5"
                      }
                    ],
                    "source": "REFERENCES_TAG",
                    "original_tag": "5.3.50"
                  },
                  {
                    "range": [
                      {
                        "fixed": "a6543f49a92187a7368f51b290179c4795386e23"
                      }
                    ],
                    "source": "REFERENCES_TAG",
                    "original_tag": "5.7.12"
                  }
                ]
              }
            }
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-107843.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:37.426454776Z",
      "provider": "osv",
      "references": [
        "https://github.com/contao/contao/releases/tag/5.3.50",
        "https://github.com/contao/contao/releases/tag/5.7.12",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/107xxx/CVE-2026-107843.json",
        "https://github.com/contao/contao/security/advisories/GHSA-mfxh-vp55-7gc6",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-107843",
        "https://github.com/contao/contao/commit/2ea6117f9049db7221679251cfc41e67d941a74b"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-204",
          "CWE-770"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/107xxx/CVE-2026-107843.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-107843",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 504901,
  "published_at": "2026-10-09T19:21:32.011+00:00"
}