Bug 2454608 (CVE-2026-34742)

Summary: CVE-2026-34742 github.com/modelcontextprotocol/go-sdk: Model Context Protocol (MCP) Go SDK: DNS rebinding vulnerability allows unauthorized access
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: high Docs Contact:
Priority: high    
Version: unspecifiedCC: anpicker, bparees, dfreiber, dhanak, drosa, drow, dsimansk, hasun, jburrell, jfula, jkoehler, jowilson, kbempah, kingland, kverlaen, lphiri, mnovotny, nyancey, ometelka, ptisnovs, sausingh, sdawley, solenoci, syedriko, vkumar, xdharmai
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Model Context Protocol (MCP) Go SDK. When an HTTP-based MCP server is run on localhost without authentication, a malicious website can exploit a DNS rebinding vulnerability. This allows the attacker to bypass same-origin policy restrictions and send requests to the local MCP server. Consequently, an attacker could invoke tools or access resources exposed by the MCP server on behalf of the user.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
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Bug Depends On: 2454683, 2454684    
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Description OSIDB Bzimport 2026-04-02 19:07:03 UTC
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.