CVE-2026-34237

Hardcoded Wildcard CORS (Access-Control-Allow-Origin: * )

• https://github.com/modelcontextprotocol/java-sdk/blob/main/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletSseServerTransportProvider.java#L289
• https://github.com/modelcontextprotocol/java-sdk/blob/main/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletStreamableServerTransportProvider.java#L525

Attack Scenario

An attacker-controlled web page instructs the victim's browser to open GET https://internal-mcp-server/sse. Because Access-Control-Allow-Origin: * allows cross-origin SSE reads, the attacker's page receives the endpoint event, which contains the session ID. The attacker can then POST to that endpoint from their page using the victim's browser as a relay.

Comparison with python-sdk

No Access-Control-Allow-Origin header is emitted by either Python transport. The browser's default same-origin policy remains in full effect.
https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/sse.py
https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http.py

Reference

• https://cheatsheetseries.owasp.org/cheatsheets/HTTP_Headers_Cheat_Sheet.html#access-control-allow-origin

CVE-2026-34237 has a CVSS score of 6.1 (Medium). 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 (1.1.1, 1.0.1, 0.18.3); upgrading removes the vulnerable code path.

Kodem intelligence

In the SDK, the transport layer should not own CORS policy. Server implementors who need cross-origin access can add a CORS filter at the servlet filter or Spring Security layer.

1. What is CVE-2026-34237? CVE-2026-34237 is a medium-severity security vulnerability in io.modelcontextprotocol.sdk:mcp-core (maven), affecting versions = 1.1.0. It is fixed in 1.1.1, 1.0.1, 0.18.3.
2. How severe is CVE-2026-34237? CVE-2026-34237 has a CVSS score of 6.1 (Medium). 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 io.modelcontextprotocol.sdk:mcp-core are affected by CVE-2026-34237? io.modelcontextprotocol.sdk:mcp-core (maven) versions = 1.1.0 is affected.
4. Is there a fix for CVE-2026-34237? Yes. CVE-2026-34237 is fixed in 1.1.1, 1.0.1, 0.18.3. Upgrade to this version or later.
5. Is CVE-2026-34237 exploitable, and should I be worried? Whether CVE-2026-34237 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-2026-34237 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-2026-34237?
   • Upgrade io.modelcontextprotocol.sdk:mcp-core to 1.1.1 or later
   • Upgrade io.modelcontextprotocol.sdk:mcp-core to 1.0.1 or later
   • Upgrade io.modelcontextprotocol.sdk:mcp-core to 0.18.3 or later