CVE-2026-49857 in auth-fetch-mcp
Summary
by MITRE • 08/13/2026
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.
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Analysis
by VulDB Data Team • 08/13/2026
The auth-fetch-mcp server represents a critical security vulnerability in its version 3.0.1 implementation where the SSRF protection mechanism contains a fundamental flaw in IPv6 address handling. This vulnerability stems from the inadequate detection of IPv4-mapped IPv6 loopback addresses within the `isPrivateV6()` function, which is designed to block access to private and loopback network addresses. The flaw specifically manifests when attackers provide URLs using the IPv4-mapped IPv6 format such as `http://[::ffff:127.0.0.1]:PORT/`, exploiting a gap in the security validation logic.
The technical execution of this vulnerability relies on Node.js WHATWG URL parser behavior where it silently normalizes IPv4-mapped IPv6 addresses from their dotted-decimal notation to hexadecimal format. In the case of `::ffff:127.0.0.1`, the parser converts it to `[::ffff:7f00:1]` in its normalized form. The security check function `isPrivateV6()` fails to properly identify this normalized representation because `net.isIPv4('7f00:1')` returns false, causing the validation to incorrectly pass the address as non-private. This normalization bypass allows malicious requests to traverse the SSRF protection mechanism and access loopback services that should be blocked.
The operational impact of this vulnerability is significant as it enables attackers to perform server-side request forgery attacks against internal services that are typically protected by network-level restrictions. The vulnerability operates under default configuration without requiring any special environment variables or conditions, making it particularly dangerous in production environments where the MCP tool might have access to sensitive internal resources. Attackers can potentially access internal APIs, databases, or other loopback services that should remain isolated from external access.
This security flaw aligns with CWE-918, Server-Side Request Forgery, and maps to ATT&CK technique T1190 - Exploit Public-Facing Application, as it allows attackers to leverage the MCP server's legitimate functionality to access internal resources. The vulnerability demonstrates a classic case of improper input validation where address normalization is not properly accounted for in security checks. The patch implemented in version 3.0.1 addresses this by enhancing the `isPrivateV6()` function to properly detect and block IPv4-mapped IPv6 loopback addresses in their normalized hexadecimal form, ensuring comprehensive protection against similar SSRF attack vectors.