CVE-2026-33252

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.

Credits:

Thank you to Lê Minh Quân for reporting the issue.

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.

A victim's authenticated browser session is used to submit forged requests to an application that cannot distinguish them from legitimate ones. Typical impact: state-changing actions performed as the victim without their consent.

CVE-2026-33252 has a CVSS score of 7.1 (High). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.4.1); upgrading removes the vulnerable code path.

Kodem intelligence

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.

1. What is CVE-2026-33252? CVE-2026-33252 is a high-severity cross-site request forgery (CSRF) vulnerability in github.com/modelcontextprotocol/go-sdk (go), affecting versions <= 1.4.0. It is fixed in 1.4.1. A victim's authenticated browser session is used to submit forged requests to an application that cannot distinguish them from legitimate ones.
2. How severe is CVE-2026-33252? CVE-2026-33252 has a CVSS score of 7.1 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/modelcontextprotocol/go-sdk are affected by CVE-2026-33252? github.com/modelcontextprotocol/go-sdk (go) versions <= 1.4.0 is affected.
4. Is there a fix for CVE-2026-33252? Yes. CVE-2026-33252 is fixed in 1.4.1. Upgrade to this version or later.
5. Is CVE-2026-33252 exploitable, and should I be worried? Whether CVE-2026-33252 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-33252 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-33252? Upgrade github.com/modelcontextprotocol/go-sdk to 1.4.1 or later.