CVE-2026-49857 auth-fetch-mcp has SSRF Protection Bypass via IPv4-mapped IPv6 Loopback
An SSRF protection bypass in auth-fetch-mcp allows attackers to access loopback services via IPv4-mapped IPv6 addresses, exposing internal data.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-49857?
CVE-2026-49857 is a high-severity server-side request forgery vulnerability in ymw0407 auth-fetch-mcp, published on 13 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official 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.
Weakness: CWE-918
Which versions are affected by CVE-2026-49857?
2 open source packages ship an affected version, including auth-fetch-mcp.
| Package | Affected | Fixed in |
|---|---|---|
auth-fetch-mcp npm | All versions | 3.0.2 |
auth-fetch-mcp npm | All versions | 3.0.2 |
How do I fix CVE-2026-49857?
Upgrade auth-fetch-mcp to 3.0.2 or later.
Upgrade, and send outbound requests through an allow-list that blocks internal addresses. More about Server-side request forgery.
Is CVE-2026-49857 being exploited?
There is no known exploitation of CVE-2026-49857 in the wild and no public exploit code.
Sources
Also known as GHSA-pvrj-8cg3-j5f8.