CVE-2026-34742High· 8.1▾ TwilightDNS Rebinding Protection Disabled by Default in Model Context Protocol Go SDK for Servers Running on Localhost
▾ Twilight zone — High severity, or a signal on a lesser flaw
impact 44.6 · likelihood 0.1 · exploitation 0
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Exploit-prediction probability, daily snapshots since Jul 21.
Disclosure to exploitation, from the record and what we observed since indexing it.
Disclosed via OSV
Last analysed / modified upstream
0.5%
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.
Note that running HTTP-based MCP servers locally without authentication is not recommended per MCP security best practices. This issue does not affect servers using stdio transport.
Servers created via StreamableHTTPHandler or SSEHandler now have this protection enabled by default when binding to localhost. Users are advised to update to version 1.4.0 to receive this automatic protection.
github.com/modelcontextprotocol/go-sdk < 1.4.0Upgrade to a patched release:
github.com/modelcontextprotocol/go-sdk 1.4.0Connected by shared product, vendor, weakness, or advisory.
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