---
id: CVE-2026-80347
title: >-
  mcp-fetch checks a fetch target against its SSRF guard without removing the
  brackets that surround an IPv6 literal
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, …
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N'
cwe:
  - CWE-918
published: '2026-08-26'
updated: '2026-09-24'
sourceUpdated: '2026-09-24T20:43:32.537'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-80347'
references:
  - url: 'https://github.com/kazuph/mcp-fetch'
    label: disclosure@vulncheck.com
  - url: 'https://github.com/kazuph/mcp-fetch/issues/16'
    label: disclosure@vulncheck.com
  - url: 'https://www.npmjs.com/package/@kazuph/mcp-fetch'
    label: disclosure@vulncheck.com
  - url: >-
      https://www.vulncheck.com/advisories/mcp-fetch-through-1.6.3-server-side-request-forgery-via-unstripped-ipv6-literal-brackets
    label: disclosure@vulncheck.com
  - url: 'https://github.com/kazuph/mcp-fetch/issues/16'
    label: 134c704f-9b21-4f2e-91b3-4a467353bcc0
tags:
  - nvd
epss: 0.00482
epssPercentile: 0.3887
ingestedAt: '2026-09-24T20:51:40.230Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.