CVE-2026-34742: CWE-1188: Insecure Default Initialization of Resource in modelcontextprotocol go-sdk
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.
AI Analysis
Technical Summary
The Model Context Protocol (MCP) Go SDK versions before 1.4.0 use Go's standard encoding/json and do not enable DNS rebinding protection by default for HTTP-based servers. When an HTTP-based MCP server runs on localhost without authentication using StreamableHTTPHandler or SSEHandler, a malicious website can exploit DNS rebinding to bypass same-origin policy restrictions. This enables the attacker to send requests to the local MCP server and potentially invoke tools or access exposed resources on the user's behalf. The issue is addressed by enabling DNS rebinding protection by default starting with version 1.4.0.
Potential Impact
An attacker controlling a malicious website can exploit this vulnerability to bypass browser same-origin policy restrictions and send unauthorized requests to a local MCP server running without authentication. This could lead to unauthorized invocation of tools or access to resources exposed by the MCP server, but only under the limited condition that the server is HTTP-based, runs on localhost without authentication, and uses specific handlers (StreamableHTTPHandler or SSEHandler).
Mitigation Recommendations
A fix is available in modelcontextprotocol go-sdk version 1.4.0, which enables DNS rebinding protection by default. Users should upgrade to version 1.4.0 or later to remediate this vulnerability. No other mitigation steps are indicated by the vendor advisory.
CVE-2026-34742: CWE-1188: Insecure Default Initialization of Resource in modelcontextprotocol go-sdk
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.
CVSS v4.0
Score 7.6high
Affected software
pkg:golang/github.com/modelcontextprotocol/go-sdkRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The Model Context Protocol (MCP) Go SDK versions before 1.4.0 use Go's standard encoding/json and do not enable DNS rebinding protection by default for HTTP-based servers. When an HTTP-based MCP server runs on localhost without authentication using StreamableHTTPHandler or SSEHandler, a malicious website can exploit DNS rebinding to bypass same-origin policy restrictions. This enables the attacker to send requests to the local MCP server and potentially invoke tools or access exposed resources on the user's behalf. The issue is addressed by enabling DNS rebinding protection by default starting with version 1.4.0.
Potential Impact
An attacker controlling a malicious website can exploit this vulnerability to bypass browser same-origin policy restrictions and send unauthorized requests to a local MCP server running without authentication. This could lead to unauthorized invocation of tools or access to resources exposed by the MCP server, but only under the limited condition that the server is HTTP-based, runs on localhost without authentication, and uses specific handlers (StreamableHTTPHandler or SSEHandler).
Mitigation Recommendations
A fix is available in modelcontextprotocol go-sdk version 1.4.0, which enables DNS rebinding protection by default. Users should upgrade to version 1.4.0 or later to remediate this vulnerability. No other mitigation steps are indicated by the vendor advisory.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-03-30T19:17:10.224Z
- Cvss Version
- 4.0
- State
- PUBLISHED
- Vendor Advisory Urls
- [{"url":"https://access.redhat.com/security/cve/CVE-2026-34742","vendor":"Red Hat"},{"url":"https://access.redhat.com/errata/RHSA-2026:21772","vendor":"Red Hat"}]
Threat ID: 69cec35ae6bfc5ba1dfb4cd2
Added to database: 04/02/2026, 19:28:26 UTC
Last enriched: 07/15/2026, 09:03:53 UTC
Last updated: 07/31/2026, 19:22:58 UTC
Views: 203
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