Compliance Impact

The vulnerability involves a hardcoded wildcard CORS policy allowing any origin to access Server-Sent Events (SSE) endpoints, which can lead to unauthorized access to sensitive data such as session IDs. This exposure of sensitive information could potentially violate data protection requirements under regulations like GDPR and HIPAA, which mandate strict controls over personal and sensitive data to prevent unauthorized access and data breaches.

Because the vulnerability permits cross-origin requests from untrusted domains, it increases the risk of unauthorized data disclosure and session hijacking, undermining confidentiality and integrity protections required by these standards.

The recommended mitigation is to remove CORS enforcement from the SDK transport layer and instead apply CORS policies at the server implementation level, aligning with best practices to maintain compliance with security standards.

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Executive Summary

CVE-2026-34237 is a vulnerability in the MCP Java SDK versions prior to 1.0.1 and 1.1.1 caused by a hardcoded wildcard Cross-Origin Resource Sharing (CORS) policy. Specifically, the SDK transport layer sets the HTTP header Access-Control-Allow-Origin to *, allowing any web origin to access the MCP server endpoints.

This means that an attacker-controlled web page can perform cross-origin Server-Sent Events (SSE) GET requests to internal MCP server endpoints and receive sensitive data such as session IDs. The attacker can then use the victim's browser to send POST requests to the MCP server, potentially exploiting the victim's session.

The vulnerability arises from permissive cross-domain security policies that allow untrusted domains to access sensitive MCP server resources, classified as CWE-942.

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Impact Analysis

This vulnerability can lead to unauthorized access and manipulation of MCP server sessions by attackers. Because the wildcard CORS policy allows any origin to access the server, an attacker can craft a malicious web page that interacts with the MCP server through the victim's browser.

The attacker can receive sensitive information such as session IDs via Server-Sent Events and use the victim's browser to send POST requests to the MCP server, potentially hijacking or manipulating the victim's session.

The impact includes limited confidentiality and integrity loss, as indicated by the CVSS score, but no availability impact.

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Detection Guidance

This vulnerability can be detected by checking for the presence of the hardcoded wildcard CORS header `Access-Control-Allow-Origin: *` in the HTTP responses from the MCP Java SDK server endpoints, especially the SSE endpoint (commonly `/sse`).

You can use network inspection tools or command-line utilities to observe HTTP headers returned by the server. For example, using curl to inspect the headers of the SSE endpoint:

• curl -i -N https://your-mcp-server/sse

Look for the `Access-Control-Allow-Origin: *` header in the response. Its presence indicates the vulnerable configuration.

Alternatively, you can use browser developer tools to inspect the network requests to the MCP server's SSE endpoint and check the CORS headers.

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Mitigation Strategies

The immediate mitigation is to upgrade the MCP Java SDK to versions 1.0.1 or 1.1.1 or later, where the hardcoded wildcard CORS vulnerability has been patched.

If upgrading is not immediately possible, remove or disable the CORS policy enforcement in the SDK transport layer to avoid sending the wildcard `Access-Control-Allow-Origin: *` header.

Instead, implement CORS filtering at the servlet filter or Spring Security layers on the server side, applying strict and appropriate CORS policies according to your security requirements.

This approach aligns with best practices recommended by OWASP and prevents unauthorized cross-origin access.

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