{"id":"CVE-2026-80347","summary":"mcp-fetch through 1.6.3 Server-Side Request Forgery via Unstripped IPv6 Literal Brackets","details":"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.","modified":"2026-08-28T11:47:33.730784240Z","published":"2026-08-26T10:09:47.977Z","database_specific":{"cwe_ids":["CWE-918"],"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/80xxx/CVE-2026-80347.json","cna_assigner":"VulnCheck"},"references":[{"type":"WEB","url":"https://www.npmjs.com/package/@kazuph/mcp-fetch"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/80xxx/CVE-2026-80347.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-80347"},{"type":"ADVISORY","url":"https://www.vulncheck.com/advisories/mcp-fetch-through-1.6.3-server-side-request-forgery-via-unstripped-ipv6-literal-brackets"},{"type":"REPORT","url":"https://github.com/kazuph/mcp-fetch/issues/16"},{"type":"PACKAGE","url":"https://github.com/kazuph/mcp-fetch"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/kazuph/mcp-fetch","events":[{"introduced":"0"},{"last_affected":"418e58aad2d0504ebf6ca9ee638feb4b9cbb70c2"}],"database_specific":{"extracted_events":[{"introduced":"0"},{"last_affected":"1.6.3"}],"source":"AFFECTED_FIELD"}}],"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"}}],"schema_version":"1.9.0","severity":[{"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"}]}