mcp-fetch through 1.6.3 Server-Side Request Forgery via Unstripped IPv6 Literal Brackets

{
  "id": "5c3e7759015801056a8f3beb55b2af94d61aaa8b402e847837691f655116eb7f",
  "url": "https://osv.dev/vulnerability/CVE-2026-80347",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "mcp-fetch through 1.6.3 Server-Side Request Forgery via Unstripped IPv6 Literal Brackets",
    "summary": "mcp-fetch checks a fetch target against its SSRF guard without removing the brackets that surround an IPv6 literal. isSafeUrl reads the hostname from the parsed URL, which for a literal such as http://[::1]/ yields the bracketed string, and then tests it with net.isIP. That call returns zero for a bracketed value, so the branch holding the private-address checks is skipped entirely. The guard falls back to resolving the hostname, the bracketed string is not a resolvable name, no addresses are returned, and the target is reported safe. The HTTP client then strips the brackets and connects. Because the address may be given in IPv4-mapped form, the same path reaches any IPv4 target the loopback and private checks were meant to exclude, including link-local metadata endpoints. isPrivateIPv6 also has no case for the ::ffff: prefix, so the mapped form would still pass even if the brackets were removed. The fetch target is supplied as a tool argument, so an attacker who can influence what the model requests can read internal responses back into the model context.",
    "evidence": "osv:CVE-2026-80347",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-80347",
      "aliases": [
        "CVE-2026-80347"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/kazuph/mcp-fetch",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "last_affected": "418e58aad2d0504ebf6ca9ee638feb4b9cbb70c2"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "last_affected": "1.6.3"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StandardRange"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v1.6.3",
            "v1.6.2",
            "v1.6.1",
            "v1.6.0",
            "v1.5.2",
            "v1.5.1",
            "v1.5.0",
            "v1.4.2",
            "v1.4.1",
            "v1.3.3",
            "v1.3.2",
            "v1.3.1",
            "v1.3.0",
            "v1.2.0",
            "v1.1.3",
            "v1.1.2",
            "v1.1.1",
            "v1.1.0",
            "v1.0.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-80347.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:37.939432980Z",
      "provider": "osv",
      "references": [
        "https://www.npmjs.com/package/@kazuph/mcp-fetch",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/80xxx/CVE-2026-80347.json",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-80347",
        "https://www.vulncheck.com/advisories/mcp-fetch-through-1.6.3-server-side-request-forgery-via-unstripped-ipv6-literal-brackets",
        "https://github.com/kazuph/mcp-fetch/issues/16",
        "https://github.com/kazuph/mcp-fetch"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-918"
        ],
        "cna_assigner": "VulnCheck",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/80xxx/CVE-2026-80347.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-80347",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 505273,
  "published_at": "2026-08-26T10:09:47.977+00:00"
}