modelcontextprotocol go-sdk up to 1.3.x insecure default initialization of resource

Summaryinfo

A vulnerability was found in modelcontextprotocol go-sdk up to 1.3.x. It has been classified as critical. The affected element is an unknown function. The manipulation leads to insecure default initialization of resource. This vulnerability is documented as CVE-2026-34742. The attack can be initiated remotely. There is not any exploit available. Upgrading the affected component is recommended.

Detailsinfo

A vulnerability was found in modelcontextprotocol go-sdk up to 1.3.x. It has been rated as critical. This issue affects an unknown code block. The manipulation with an unknown input leads to a insecure default initialization of resource vulnerability. Using CWE to declare the problem leads to CWE-1188. The product initializes or sets a resource with a default that is intended to be changed by the administrator, but the default is not secure. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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.

It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-34742 since 03/30/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. It demands that the victim is doing some kind of user interaction. The technical details are unknown and an exploit is not publicly available.

Upgrading to version 1.4.0 eliminates this vulnerability. The upgrade is hosted for download at github.com. Applying the patch 67bd3f2e2b53ce11a16db8d976cdb8ff1e986b6d is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

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Productinfo

Vendor

• modelcontextprotocol

Name

• go-sdk

Version

• 1.0
• 1.1
• 1.2
• 1.3

License

• open-source

Website

• Product: https://github.com/modelcontextprotocol/go-sdk/

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