CVE-2026-33252
Cross-Site Tool Execution for HTTP Servers without Authorizatrion in github.com/modelcontextprotocol/go-sdk
Impact
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. #### Impact: A malicious website may have been able to send cross-site POST requests with `Content-Type: text/plain`, which due to CORS-safelisted properties would reach the MCP message handling without any CORS preflight barrier. #### Fix: The SDK was modified to perform `Content-Type` header validation for POST requests and introduced a configurable protection for verifying the origin of the request in commit a433a83. Users are advised to update to v1.4.1 to use this additional protection. Note: v1.4.1 requires Go 1.25 or later. #### Credits: Thank you to Lê Minh Quân for reporting the issue.
Affected packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| go | github.com/modelcontextprotocol/go-sdk | <= 1.4.0 | 1.4.1 |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Lfprojects | Mcp Go Sdk | before 1.4.1 |
Weakness classification
References
- NVD record
- CVE.org record
- GitHub advisory
- https://github.com/modelcontextprotocol/go-sdk/commit/a433a831d6e5d5ac3b9e625a8095aa8eaa040dfc
- https://github.com/modelcontextprotocol/go-sdk/security/advisories/GHSA-89xv-2j6f-qhc8
- https://github.com/advisories/GHSA-89xv-2j6f-qhc8
- https://nvd.nist.gov/vuln/detail/CVE-2026-33252