CVE-2026-34237Network attack vector

Cross-origin session access via wildcard CORS

Published Mar 31, 2026 · Updated Mar 31, 2026

Permissive CORS in MCP Java SDK mcp-core 1.0.0, 1.1.0, and before 0.18.3 allows remote attackers to access MCP sessions through a browser. The SSE and Streamable HTTP servlet transports hardcode Access-Control-Allow-Origin: *, letting an attacker-controlled page read an internal server's session ID and POST through the victim's browser. Exploitation requires a user to load the page while the browser can reach the MCP server; the advisory reports cross-origin session reads and writes, not service interruption.

CVSS severity6.1
Medium
EPSS probability0.22%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Permissive CORS in MCP Java SDK mcp-core 1.0.0, 1.1.0, and before 0.18.3 allows remote attackers to access MCP sessions through a browser. The SSE and Streamable HTTP servlet transports hardcode Access-Control-Allow-Origin: *, letting an attacker-controlled page read an internal server's session ID and POST through the victim's browser. Exploitation requires a user to load the page while the browser can reach the MCP server; the advisory reports cross-origin session reads and writes, not service interruption.

The record

CVE
CVE-2026-34237
Published
Mar 31, 2026
Updated
Mar 31, 2026
Vendor
Model Context Protocol
Product
MCP Java SDK
Classifications
CWE-942, T1189
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 31, 2026CVE publishedPublication date reported by the CVE source.
  2. Mar 31, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1.0.1
  2. Affected versionversion=< 1.1.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Model Context Protocol · MCP Java SDKversion=< 1.0.1; version=< 1.1.1

Published CVSS scores

6.1GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-942Permissive Cross-domain Security Policy with Untrusted Domains
T1189Drive-by Compromise

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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