Description
The Go MCP SDK used Go's standard encoding/json. Prior to version 1.4.0, the Model Context Protocol (MCP) Go SDK does not enable DNS rebinding protection by default for HTTP-based servers. When an HTTP-based MCP server is run on localhost without authentication with StreamableHTTPHandler or SSEHandler, a malicious website could exploit DNS rebinding to bypass same-origin policy restrictions and send requests to the local MCP server. This could allow an attacker to invoke tools or access resources exposed by the MCP server on behalf of the user in those limited circumstances. This issue has been patched in version 1.4.0.
Affected products
lfprojects: mcp go sdk
References8
- github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA… Vendor Advisory
- github.com/modelcontextprotocol/go-sdk/commit/67bd3f2e2b53ce11a… Patch
- github.com/modelcontextprotocol/go-sdk/pull/760 Issue TrackingPatch
- github.com/modelcontextprotocol/go-sdk/releases/tag/v1.4.0 Release Notes
- access.redhat.com/errata/RHSA-2026:21772
- access.redhat.com/security/cve/CVE-2026-34742
- bugzilla.redhat.com/show_bug.cgi?id=2454608
- security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-34742…
Metrics
- CVSS 3.1
-
8.1
HIGH
Score source: NVD
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N - CVSS 4.0
-
7.6
HIGH
Score source: CNA
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X - EPSS — exploit probability
- 0% percentile: 41 · as of 12 November 2026
- CWE
- CWE-1188
Could this be exploited in your environment?
A patch closes the known hole. A penetration test shows what an attacker can still reach — with proof.