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.
References
Configurations
No configuration.
History
No history.
Information
Published : 2026-08-26 10:16
Updated : 2026-08-26 13:19
NVD link : CVE-2026-80347
Mitre link : CVE-2026-80347
CVE.ORG link : CVE-2026-80347
JSON object : View
Products Affected
No product.
CWE
CWE-918
Server-Side Request Forgery (SSRF)