CVE-2026-80347
Received Received - Intake

mcp-fetch SSRF Bypass via IPv6 Brackets

Publication date: 2026-08-26

Last updated on: 2026-08-26

Assigner: VulnCheck

Description
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.

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Meta Information

Published
2026-08-26
Last Modified
2026-08-26
Generated
2026-08-26
AI Q&A
2026-08-26
EPSS Evaluated
N/A
NVD
CVE-2026-80347
EUVD
EUVD-2026-66386

Affected Vendors & Products

Currently, no data is known.

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Exploitability

CWE
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KEV
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CWE ID Description
CWE-918 The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.

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Executive Summary

This vulnerability is a Server-Side Request Forgery (SSRF) guard bypass in the mcp-fetch library. It occurs because the SSRF protection fails to handle IPv6 literal addresses enclosed in brackets (e.g., [::1]). The isSafeUrl function checks the hostname without removing these brackets, causing net.isIP() to incorrectly identify the address as non-IP. This skips IP-based safety checks, allowing the request to proceed. Attackers can exploit this to bypass protections and access internal services or metadata endpoints.

Detection Guidance

To detect this vulnerability, check if your mcp-fetch server is running version 1.6.3 or earlier. Run: curl -s https://raw.githubusercontent.com/kazuph/mcp-fetch/main/package.json | grep version. If the version is <=1.6.3, the system is vulnerable. Additionally, monitor network logs for unusual requests to internal or private IP addresses, especially IPv6 literals like [::1] or IPv4-mapped IPv6 addresses such as [::ffff:127.0.0.1].

Impact Analysis

An attacker who can influence the model's requests could exploit this to read internal responses into the model context. This includes accessing restricted resources like loopback addresses, private networks, or cloud metadata endpoints (e.g., http://[::ffff:169.254.169.254]/). The vulnerability allows bypassing SSRF protections, potentially exposing sensitive internal data.

Compliance Impact

This vulnerability could lead to unauthorized access to internal systems, potentially violating data protection requirements under GDPR (e.g., unauthorized data access) and HIPAA (e.g., exposure of protected health information). Organizations using mcp-fetch may fail to meet compliance standards due to insufficient SSRF protections.

Mitigation Strategies

Immediately upgrade mcp-fetch to version 1.6.4 or later, which includes fixes for IPv6 bracket handling and IPv4-mapped address validation. If upgrading is not possible, implement network-level restrictions to block outbound requests to private or loopback addresses. Review and restrict tool arguments to prevent attackers from supplying malicious URLs.

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