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
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-34237 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,333 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
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 |
Detection & 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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-34237 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-34237. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-34237 in your dependencies?
O3 Security finds CVE-2026-34237 across Maven dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.