CVE-2026-33252 MCP Go SDK Allows Cross-Site Tool Execution for HTTP Servers without Authorizatrion
An attacker can force a victim's browser to execute tools on a local MCP server by bypassing origin checks in the Go SDK's HTTP transport.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-33252?
CVE-2026-33252 is a high-severity vulnerability in modelcontextprotocol go-sdk, published on 23 Mar 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
The Go MCP SDK used Go's standard encoding/json. Prior to version 1.4.1, the Go SDK's Streamable HTTP transport accepted browser-generated cross-site `POST` requests without validating the `Origin` header and without requiring `Content-Type: application/json`. In deployments without Authorization, especially stateless or sessionless configurations, this allows an arbitrary website to send MCP requests to a local server and potentially trigger tool execution. Version 1.4.1 contains a patch for the issue.
Weakness: CWE-352
Which versions are affected by CVE-2026-33252?
3 open source packages ship an affected version, including github.com/modelcontextprotocol/go-sdk. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
github.com/modelcontextprotocol/go-sdk Go | All versions | 1.4.1 |
github.com/modelcontextprotocol/go-sdk Go | All versions | 1.4.1 |
github.com/modelcontextprotocol/go-sdk Go | All versions | 1.4.1 |
Products include Lfprojects Mcp Go Sdk.
How do I fix CVE-2026-33252?
Upgrade github.com/modelcontextprotocol/go-sdk to 1.4.1 or later.
Is CVE-2026-33252 being exploited?
There is no known exploitation of CVE-2026-33252 in the wild and no public exploit code.
Other vulnerabilities in modelcontextprotocol go-sdk
Sources
Also known as CLEANSTART-2026-RU81320, CLEANSTART-2026-YF18656, GHSA-89xv-2j6f-qhc8, GO-2026-4773, MINI-3xjf-pmwg-9r77, MINI-95p6-8pr9-2cc7 and 2 more.