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Vulnerability record · CVE-2026-49857 · published 13 August 2026

CVE-2026-49857: Server-side request forgery (SSRF) vulnerability

auth-fetch-mcp is an MCP server that lets AI assistants fetch content from authenticated web pages. Version 3.0.1 implements SSRF protection in `assertSafeUrl()` (`src/security.ts`) to block requests to private and loopback addresses. However, the `isPrivateV6()` function fails to detect IPv4-mapped IPv6 loopback addresses in their hex-normalized form. When an attacker supplies a URL such as `http://[::ffff:127.0.0.1]:PORT/`, the Node.js WHATWG URL parser silently normalizes the host to `[::ffff:7f00:1]`. Because `net.isIPv4('7f00:1')` returns `false`, the private-IP check is bypassed and the URL is passed to the browser or HTTP client, allowing the MCP tool to reach loopback services that are supposed to be blocked. The issue is exploitable under default configuration without any special environment variable. Version 3.0.1 patches the issue.

7.4 CVSS 3.1 High EPSS 0.49% · top 60.4% CWE-918 · Server-side request forgery (SSRF) Deferred
7.4CVSS 3.1 base score
0.49%EPSS exploitation probability, 30 days
NoNot in CISA KEV
0Affected product versions listed by NVD
3References
18 Sep 2026Last modified by NVD

Description

auth-fetch-mcp is an MCP server that lets AI assistants fetch content from authenticated web pages. Version 3.0.1 implements SSRF protection in `assertSafeUrl()` (`src/security.ts`) to block requests to private and loopback addresses. However, the `isPrivateV6()` function fails to detect IPv4-mapped IPv6 loopback addresses in their hex-normalized form. When an attacker supplies a URL such as `http://[::ffff:127.0.0.1]:PORT/`, the Node.js WHATWG URL parser silently normalizes the host to `[::ffff:7f00:1]`. Because `net.isIPv4('7f00:1')` returns `false`, the private-IP check is bypassed and the URL is passed to the browser or HTTP client, allowing the MCP tool to reach loopback services that are supposed to be blocked. The issue is exploitable under default configuration without any special environment variable. Version 3.0.1 patches the issue.

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N

References

LinkTags
https://github.com/ymw0407/auth-fetch-mcp/commit/c5c523cd55b24ae1680c5bacef95d59a81ae3450
https://github.com/ymw0407/auth-fetch-mcp/releases/tag/v3.0.1
https://github.com/ymw0407/auth-fetch-mcp/security/advisories/GHSA-pvrj-8cg3-j5f8

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Source: NIST National Vulnerability Database (record CVE-2026-49857), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.