CVE-2026-34742

Summary

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 Software

Weaknesses

• CWE-1188: CWE-1188: Insecure Default Initialization of Resource

ADP Enrichment

CISA ADP Vulnrichment

• SSVC:
• Exploitation: none
  • Automatable: no
  • Technical Impact: total

github.com/modelcontextprotocol/go-sdk: Model Context Protocol (MCP) Go SDK: DNS rebinding vulnerability allows unauthorized access

Additional References

• https://access.redhat.com/security/cve/CVE-2026-34742
• https://bugzilla.redhat.com/show_bug.cgi?id=2454608
• https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-34742.json
• https://access.redhat.com/errata/RHSA-2026:21772

References

• https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-xw59-hvm2-8pj6
• https://github.com/modelcontextprotocol/go-sdk/pull/760
• https://github.com/modelcontextprotocol/go-sdk/commit/67bd3f2e2b53ce11a16db8d976cdb8ff1e986b6d
• https://github.com/modelcontextprotocol/go-sdk/releases/tag/v1.4.0

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