auth-fetch-mcp has SSRF Protection Bypass via IPv4-mapped IPv6 Loopback

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-49857
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-08-13 15:19:41 UTC
Updated2026-08-13 18:17:30 UTC
Descriptionauth-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.

Risk And Classification

Primary CVSS: v3.1 7.4 HIGH from [email protected]

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

Problem Types: CWE-918 | CWE-918 CWE-918: Server-Side Request Forgery (SSRF)

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary7.4HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N
3.1CNADECLARED7.4HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
Required
Scope
Changed
Confidentiality
High
Integrity
None
Availability
None

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

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Ymw0407 Auth-fetch-mcp affected < 3.0.2 Not specified

References

ReferenceSourceLinkTags
github.com/ymw0407/auth-fetch-mcp/security/advisories/GHSA-pvrj-8cg3-j5f8 [email protected] github.com
github.com/ymw0407/auth-fetch-mcp/releases/tag/v3.0.1 [email protected] github.com
github.com/ymw0407/auth-fetch-mcp/commit/c5c523cd55b24ae1680c5bacef95d59... [email protected] github.com
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