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MCP Java SDK has a Hardcoded Wildcard CORS (Access-Control-Allow-Origin: *)

Moderate severity GitHub Reviewed Published Mar 30, 2026 in modelcontextprotocol/java-sdk • Updated Mar 30, 2026

Package

maven io.modelcontextprotocol.sdk:mcp-core (Maven)

Affected versions

< 1.0.0
= 1.1.0

Patched versions

1.0.1
1.1.1

Description

Summary

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

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.

Resources

References

@Kehrlann Kehrlann published to modelcontextprotocol/java-sdk Mar 30, 2026
Published to the GitHub Advisory Database Mar 30, 2026
Reviewed Mar 30, 2026
Last updated Mar 30, 2026

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
Required
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

EPSS score

Weaknesses

Permissive Cross-domain Security Policy with Untrusted Domains

The product uses a web-client protection mechanism such as a Content Security Policy (CSP) or cross-domain policy file, but the policy includes untrusted domains with which the web client is allowed to communicate. Learn more on MITRE.

CVE ID

CVE-2026-34237

GHSA ID

GHSA-hv2w-8mjj-jw22

Source code

Credits

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