CVE-2026-49857: CWE-918: Server-Side Request Forgery (SSRF) in ymw0407 auth-fetch-mcp
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.
AI Analysis
Technical Summary
The auth-fetch-mcp product implements SSRF protection in version 3.0.1 via the assertSafeUrl() function, which blocks requests to private and loopback IP addresses. However, the isPrivateV6() function fails to detect IPv4-mapped IPv6 loopback addresses when they are hex-normalized by the Node.js WHATWG URL parser. This allows an attacker to supply a URL such as http://[::ffff:127.0.0.1]:PORT/, which normalizes to [::ffff:7f00:1]. Since net.isIPv4('7f00:1') returns false, the private IP check is bypassed, enabling SSRF to loopback services that should be blocked. This issue is exploitable under default configuration without special environment variables. The vulnerability is fixed in version 3.0.1.
Potential Impact
An attacker can bypass SSRF protections to make the auth-fetch-mcp server send requests to internal loopback services, potentially exposing sensitive internal resources. The CVSS score of 7.4 indicates high severity with network attack vector, low attack complexity, no privileges required, and user interaction required. The impact is high confidentiality impact with no integrity or availability impact reported.
Mitigation Recommendations
Version 3.0.1 of auth-fetch-mcp patches this vulnerability by correctly detecting IPv4-mapped IPv6 loopback addresses. Users should upgrade to version 3.0.1 or later to remediate this issue. No other mitigations are indicated or required.
CVE-2026-49857: CWE-918: Server-Side Request Forgery (SSRF) in ymw0407 auth-fetch-mcp
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 v3.1
Score 7.4high
Affected software
pkg:github/ymw0407/auth-fetch-mcpRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The auth-fetch-mcp product implements SSRF protection in version 3.0.1 via the assertSafeUrl() function, which blocks requests to private and loopback IP addresses. However, the isPrivateV6() function fails to detect IPv4-mapped IPv6 loopback addresses when they are hex-normalized by the Node.js WHATWG URL parser. This allows an attacker to supply a URL such as http://[::ffff:127.0.0.1]:PORT/, which normalizes to [::ffff:7f00:1]. Since net.isIPv4('7f00:1') returns false, the private IP check is bypassed, enabling SSRF to loopback services that should be blocked. This issue is exploitable under default configuration without special environment variables. The vulnerability is fixed in version 3.0.1.
Potential Impact
An attacker can bypass SSRF protections to make the auth-fetch-mcp server send requests to internal loopback services, potentially exposing sensitive internal resources. The CVSS score of 7.4 indicates high severity with network attack vector, low attack complexity, no privileges required, and user interaction required. The impact is high confidentiality impact with no integrity or availability impact reported.
Mitigation Recommendations
Version 3.0.1 of auth-fetch-mcp patches this vulnerability by correctly detecting IPv4-mapped IPv6 loopback addresses. Users should upgrade to version 3.0.1 or later to remediate this issue. No other mitigations are indicated or required.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-06-01T22:03:19.640Z
- Cvss Version
- 3.1
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 6a7ddec3bf8831d5395cfc29
Added to database: 08/13/2026, 15:12:03 UTC
Last enriched: 08/13/2026, 15:56:57 UTC
Last updated: 08/13/2026, 18:11:59 UTC
Views: 2
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