# 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.

- **Severity:** High 7.4 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N`)
- **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:** Yes
- **Published:** 13 Aug 2026
- **Updated:** 13 Aug 2026
- **Also known as:** GHSA-pvrj-8cg3-j5f8

## 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 (Server-Side Request Forgery)

## Which versions are affected by CVE-2026-49857?

2 open source packages ship an affected version, including auth-fetch-mcp.


| Package | Ecosystem | 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.

## Is CVE-2026-49857 being exploited?

There is no known exploitation of CVE-2026-49857 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-49857 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-49857 in the wild and no public exploit code.

Affected packages and fixed versions:
- auth-fetch-mcp (npm): affected all versions; fixed in 3.0.2
- auth-fetch-mcp (npm): affected all versions; fixed in 3.0.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-49857/ (Markdown: https://insights.sbomify.com/CVE-2026-49857/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-49857
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-49857
- OSV: https://osv.dev/vulnerability/CVE-2026-49857

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-49857/
Summaries are generated and may contain mistakes; the official description above is the record.