CVE-2026-49857

High

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.

SeverityHigh

CVSS7.4

Exploit probability—

ProductAuth-fetch-mcp

PublishedAug 13, 2026

First seenAug 13, 2026

Weakness typeCWE-918Server-side request forgery (SSRF)

How to fix

Update

auth-fetch-mcp3.0.2 or later

Affected products

Product	Affected versions	Fixed in
auth-fetch-mcpnpm	v < 3.0.2	3.0.2

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