Cross-Site Tool Execution for HTTP Servers without Authorizatrion in github.com/modelcontextprotocol/go-sdk (CVE-2026-33252) | HOL Guard CVE

High · CVSS 7.1CVE-2026-33252GHSA-89XV-2J6F-QHC8

Cross-Site Tool Execution for HTTP Servers without Authorizatrion in github.com/modelcontextprotocol/go-sdk

Answer in brief

CVE-2026-33252 records a High severity (CVSS 7.1) vulnerability in Cross-Site Tool Execution for HTTP Servers without Authorizatrion in github.com/modelcontextprotocol/go-sdk. The current sources do not mark it as known exploited. The current feed maps github.com/modelcontextprotocol/go-sdk (go). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Mar 19, 2026Updated Sep 10, 2026Source checked Sep 10, 2026First seen by HOL Sep 10, 2026Material review Sep 10, 2026

Upstream Advisory

Key facts

Risk: High · CVSS 7.1
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

CVSS is 7.1. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps github.com/modelcontextprotocol/go-sdk (go). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
github.com/modelcontextprotocol/go-sdkgo	>=0 <1.4.1	1.4.1

Recommended response

Check inventory. Check lockfiles and deployed manifests for github.com/modelcontextprotocol/go-sdk.
Review the reported fix. Update github.com/modelcontextprotocol/go-sdk to 1.4.1 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
Mar 19, 2026
Evidence: source:osv:source_dates:source-dates:record
Source modified
Sep 10, 2026
Evidence: source:osv:source_dates:source-dates:record
First seen by HOL
Sep 10, 2026

Sources and claim methodology

Upstream source description

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.

Quoted source text, attributed separately from HOL analysis.