MCP Java SDK has a Hardcoded Wildcard CORS (Access-Control-Allow-Origin: *)
Summary
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
Recommendation
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.
Reference
Software afectado
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| Plataforma | Paquete | Versiones vulnerables | Corregida en |
|---|---|---|---|
| Java / Kotlin (Maven) | io.modelcontextprotocol.sdk:mcp-core | >= 1.0.0, < 1.0.1, >= 1.1.0, < 1.1.1, < 0.18.3 | 1.0.1, 1.1.1, 0.18.3 |
Referencias
- https://github.com/modelcontextprotocol/java-sdk/security/advisories/GHSA-hv2w-8mjj-jw22
- https://nvd.nist.gov/vuln/detail/CVE-2026-34237
- https://cheatsheetseries.owasp.org/cheatsheets/HTTP_Headers_Cheat_Sheet.html#access-control-allow-origin
- https://github.com/modelcontextprotocol/java-sdk
- 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
- Vector CVSS
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N- Debilidad (CWE)
CWE-942- Plataforma
- Java / Kotlin (Maven)
- Paquete
io.modelcontextprotocol.sdk:mcp-core- Publicada
- 30.03.2026
- Actualizado
- 11.10.2026 13:20
Fuentes y licencias
OSV / GitHub Advisory Database - GHSA-hv2w-8mjj-jw22 - CC BY 4.0
Historial de vulnerabilidades: io.modelcontextprotocol.sdk:mcp-core
Todas las vulnerabilidades publicadas de este paquete (Java / Kotlin (Maven)), de la más reciente a la más antigua. Muestra si es un fallo aislado o un paquete que hay que vigilar.
- Entradas en total
- 2
- Primera → última
- 03.2026 04.2026
- Sin corrección
- 0
- Explotada en ataques
- 0
Distribución por gravedad
- Alta 1
- Media 1
Divulgaciones por año
- 2026: 2
-
MCP Java SDK has a Hardcoded Wildcard CORS (Access-Control-Allow-Origin: *) esta entrada
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