CVE-2026-34237: Model Context Protocol Java SDK hardcoded wildcard CORS policy

CVE-2026-34237 · Severity: medium · CVSS 6.1 · Published 2026-03-31

The official Java SDK for the Model Context Protocol (MCP) contains a security flaw that allows unauthorized websites to interact with MCP servers. By default, the software permits any website to read sensitive session information if a user visits a malicious page while connected to an internal MCP server. This could allow an attacker to hijack sessions and perform actions on the server using the victim's browser as a relay.

Vendors: Maven, Model Context Protocol.

Quick answers

Is CVE-2026-34237 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-34237 being exploited in the wild.
How severe is CVE-2026-34237?
Medium severity, with a CVSS score of 6.1, published 31 March 2026.
Which products and versions are affected?
io.modelcontextprotocol.sdk:mcp-core (Maven) ECOSYSTEM:introduced=1.0.0,fixed=1.0.1; Model Context Protocol Core < 0.18.3, 1.0.0, 1.1.0; Model Context Protocol Java SDK < 0.83.0, < 1.0.1, < 1.1.1.
Is there a fix for CVE-2026-34237?
Yes. io.modelcontextprotocol.sdk:mcp-core (Maven): upgrade to 1.0.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-34237 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-34237?
Check whether you run io.modelcontextprotocol.sdk:mcp-core (Maven), Model Context Protocol Core or Model Context Protocol Java SDK at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • io.modelcontextprotocol.sdk:mcp-core (Maven): upgrade to 1.0.1 or later
    # set io.modelcontextprotocol.sdk:mcp-core to 1.0.1 in your pom.xml or build.gradle

Technical details

A hardcoded wildcard CORS policy (Access-Control-Allow-Origin: *) exists in the HttpServletSseServerTransportProvider and HttpServletStreamableServerTransportProvider components of the MCP Java SDK. This vulnerability (CWE-942) allows an attacker-controlled web page to bypass the Same-Origin Policy (SOP) and read Server-Sent Events (SSE) streams, which contain session identifiers. With these identifiers, an attacker can perform cross-origin POST requests to the server's endpoints using the victim's browser. The issue is resolved in versions 0.83.0, 1.0.1, and 1.1.1 by removing the hardcoded header and deferring CORS policy management to the server implementer.

Affected products

  • Model Context Protocol MCP Java SDK (mcp-core) < 0.83.0, < 1.0.1, < 1.1.1

Identifiers

  • CVE-2026-34237
  • GHSA-hv2w-8mjj-jw22

Timeline

  • 2026-03-30: advisory: GitHub Security Advisory GHSA-hv2w-8mjj-jw22 published
  • 2026-03-31: disclosed: CVE-2026-34237 published to NVD

References

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