CVE-2026-34237 — mcp-core
MEDIUMCVE-2026-34237 is a medium-severity (CVSS 6.1) CWE-942 vulnerability in io.modelcontextprotocol.sdk:mcp-core. A fix is available for io.modelcontextprotocol.sdk:mcp-core — see the affected versions and patch details below.
MCP Java SDK has a Hardcoded Wildcard CORS (Access-Control-Allow-Origin: *)
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-34237.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
io.modelcontextprotocol.sdk:mcp-core☕io.modelcontextprotocol.sdk:mcp-core☕io.modelcontextprotocol.sdk:mcp-coreReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
Description
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
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | io.modelcontextprotocol.sdk:mcp-core | ≥ 1.0.0&&< 1.0.1 | 1.0.1io.modelcontextprotocol.sdk:mcp-core:1.0.1 |
| ☕Maven | io.modelcontextprotocol.sdk:mcp-core | ≥ 1.1.0&&< 1.1.1 | 1.1.1io.modelcontextprotocol.sdk:mcp-core:1.1.1 |
| ☕Maven | io.modelcontextprotocol.sdk:mcp-core | all versions | 0.18.3io.modelcontextprotocol.sdk:mcp-core:0.18.3 |
Affected Products
mcp java sdklfprojectsDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for io.modelcontextprotocol.sdk:mcp-core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update io.modelcontextprotocol.sdk:mcp-core to 1.0.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-34237 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is CVE-2026-34237 in your dependencies?
Find it across Maven, including transitive dependencies.