Vulnerability · Published Aug 27, 2026 · Assigned by VulnCheck

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

Medium 6.8

In plain English

A medium-severity security flaw in mcp-go from mark3labs. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 13% of all known vulnerabilities).

Severity (CVSS 3.1)6.8Medium

Exploit odds (30 days)0.2%top 87.0% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official 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.

Affected products (as reported)

Vendor	Product	Versions
mark3labs	mcp-go	0 – before 0.56.0

Weakness: CWE-346 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N

References

https://github.com/mark3labs/mcp-go product
https://github.com/mark3labs/mcp-go/pull/921 issue-tracking, patch
mcp-go v0.56.0 Release Notes release-notes
VulnCheck Advisory: mcp-go before 0.56.0 Missing Host Header Validation Enables DNS Rebinding third-party-advisory