CVE-2026-34237: Mcp core Security Vulnerability Vulnerability

MODERATE
Advisory Identifier: GHSA-hv2w-8mjj-jw22
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CVSS 6.1
CWE-942
Published
Advisory Summary

CVE-2026-34237 is a MODERATE (CVSS 6.1) security vulnerability vulnerability (CWE-942: Permissive Cross-domain Security Policy with Untrusted Domains) affecting Mcp core libraries in the Maven ecosystem. The issue involves *). Remediation is available by upgrading affected packages to version 1.0.1, 1.1.1, 0.18.3.

CVE-2026-34237 at a Glance

CVE Identifier CVE-2026-34237
GHSA Identifier GHSA-hv2w-8mjj-jw22
Severity MODERATE
CVSS v3.1 Base Score 6.1
CWE Classification CWE-942: Permissive Cross-domain Security Policy with Untrusted Domains
Affected Product Mcp core
Package Ecosystem Maven
Published Date
Last Updated

Security Impact

CVSS: 6.1

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction Required (UI:R)
Scope Changed (S:C)
Confidentiality Low (C:L)
Integrity Low (I:L)
Availability None (A:N)

How to Fix CVE-2026-34237

To remediate this vulnerability, upgrade all affected Mcp core dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

io.modelcontextprotocol.sdk:mcp-core fixed: 1.0.1 
<dependency>
    <groupId>io.modelcontextprotocol.sdk</groupId>
    <artifactId>mcp-core</artifactId>
    <version>1.0.1</version>
</dependency>
io.modelcontextprotocol.sdk:mcp-core fixed: 1.1.1 
<dependency>
    <groupId>io.modelcontextprotocol.sdk</groupId>
    <artifactId>mcp-core</artifactId>
    <version>1.1.1</version>
</dependency>
io.modelcontextprotocol.sdk:mcp-core fixed: 0.18.3 
<dependency>
    <groupId>io.modelcontextprotocol.sdk</groupId>
    <artifactId>mcp-core</artifactId>
    <version>0.18.3</version>
</dependency>
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

mvn dependency:tree -Dincludes=io.modelcontextprotocol.sdk

How to Check if You Are Affected

To determine whether your Java application is affected by CVE-2026-34237, inspect your build dependencies using the Maven Dependency Plugin:

mvn dependency:tree -Dincludes=io.modelcontextprotocol.sdk

Analyze the terminal output to locate any version of Mcp core in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.0.1), your application is affected.

Note: The mvn dependency:tree -Dincludes=io.modelcontextprotocol.sdk command inspects the resolved dependency graph declared in your project's POM files; it does not perform automated security scanning or inspect runtime dynamic classpaths. Always verify runtime container classpaths and shaded JARs.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-942: Permissive Cross-domain Security Policy with Untrusted Domains, where 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. In Mcp core, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

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

CVE-2026-34237 Affected Mcp core Versions

3 component(s)
Fixed Versions:
1.0.11.1.10.18.3
Product / Package Ecosystem Affected Versions Fixed Version
io.modelcontextprotocol.sdk:mcp-core
maven/io.modelcontextprotocol.sdk/mcp-core
Maven
≥ 1.0.0
< 1.0.1
All 1 versions
1.0.0
1.0.1
io.modelcontextprotocol.sdk:mcp-core
maven/io.modelcontextprotocol.sdk/mcp-core
Maven
≥ 1.1.0
< 1.1.1
All 1 versions
1.1.0
1.1.1
io.modelcontextprotocol.sdk:mcp-core
maven/io.modelcontextprotocol.sdk/mcp-core
Maven
< 0.18.3
0.18.3

References

GitHub Advisory / Wiki ADVISORY
https://github.com/modelcontextprotocol/java-sdk/security/advisories/GHSA-hv2w-8mjj-jw22
NVD — CVE-2026-34237 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-34237
cheatsheetseries.owasp.org/cheatsheets/http_he... WEB
https://cheatsheetseries.owasp.org/cheatsheets/HTTP_Headers_Cheat_Sheet.html#access-control-allow-origin
Mcp core Source Repository REPOSITORY
https://github.com/modelcontextprotocol/java-sdk
Mcp core Source Repository REPOSITORY
https://github.com/modelcontextprotocol/java-sdk/blob/main/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletSseServerTransportProvider.java#L289
Mcp core Source Repository REPOSITORY
https://github.com/modelcontextprotocol/java-sdk/blob/main/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletStreamableServerTransportProvider.java#L525

Frequently Asked Questions

What is CVE-2026-34237?
CVE-2026-34237 is a security vulnerability in Mcp core involving *) under CWE-942: Permissive Cross-domain Security Policy with Untrusted Domains.
Which Mcp core versions are affected?
Affected versions include releases prior to 1.0.1, 1.1.1, 0.18.3 across affected packages: io.modelcontextprotocol.sdk:mcp-core.
What version fixes CVE-2026-34237?
Upgrading to 1.0.1, 1.1.1, 0.18.3 (or the latest patched release for your respective branch) resolves the vulnerability.
Is mcp-core affected?
Yes, io.modelcontextprotocol.sdk:mcp-core is affected in releases prior to 1.0.1, 1.1.1, 0.18.3. Upgrade your dependency to 1.0.1, 1.1.1, 0.18.3 or later.
What is the CVSS score of CVE-2026-34237?
CVE-2026-34237 has a CVSS v3.1 base score of 6.1 (MODERATE) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.
What CWE is associated with CVE-2026-34237?
CVE-2026-34237 is classified under CWE-942: 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.).
How can I check Maven dependencies for CVE-2026-34237?
Run the dependency tree command: 'mvn dependency:tree -Dincludes=io.modelcontextprotocol.sdk'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-hv2w-8mjj-jw22",
  "modified" : "2026-06-09T18:39:02Z",
  "published" : "2026-03-30T17:26:44Z",
  "aliases" : [ "CVE-2026-34237" ],
  "summary" : "MCP Java SDK has a Hardcoded Wildcard CORS (Access-Control-Allow-Origin: *)",
  "details" : "### Summary\n\n**Hardcoded Wildcard CORS (Access-Control-Allow-Origin: * )**\n\n- https://github.com/modelcontextprotocol/java-sdk/blob/main/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletSseServerTransportProvider.java#L289\n- https://github.com/modelcontextprotocol/java-sdk/blob/main/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletStreamableServerTransportProvider.java#L525\n\n### Attack Scenario\nAn 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.\n\n### Comparison with python-sdk\nNo Access-Control-Allow-Origin header is emitted by either Python transport. The browser's default same-origin policy remains in full effect.\nhttps://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/sse.py\nhttps://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http.py\n\n### Recommendation\nIn 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.\n\n### Reference\n\n- https://cheatsheetseries.owasp.org/cheatsheets/HTTP_Headers_Cheat_Sheet.html#access-control-allow-origin",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "io.modelcontextprotocol.sdk:mcp-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.0.0"
      }, {
        "fixed" : "1.0.1"
      } ]
    } ],
    "versions" : [ "1.0.0" ]
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "io.modelcontextprotocol.sdk:mcp-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.1.0"
      }, {
        "fixed" : "1.1.1"
      } ]
    } ],
    "versions" : [ "1.1.0" ]
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "io.modelcontextprotocol.sdk:mcp-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.18.3"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/java-sdk/security/advisories/GHSA-hv2w-8mjj-jw22"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-34237"
  }, {
    "type" : "WEB",
    "url" : "https://cheatsheetseries.owasp.org/cheatsheets/HTTP_Headers_Cheat_Sheet.html#access-control-allow-origin"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/modelcontextprotocol/java-sdk"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/java-sdk/blob/main/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletSseServerTransportProvider.java#L289"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/modelcontextprotocol/java-sdk/blob/main/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/HttpServletStreamableServerTransportProvider.java#L525"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-942" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-30T17:26:44Z",
    "nvd_published_at" : "2026-03-31T16:16:32Z"
  }
}