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The vulnerability exists within the MCP Apps UI resource component, specifically inside the interactive DataStore table rendered within the host’s webview or iframe.
Communication between this resource and its parent window is established using the standard window.postMessage messaging protocol.
During inbound message processing, the event listener function completely fails to validate the event.origin property of incoming messages.
Because of this missing origin check, any arbitrary window, frame, or malicious domain capable of reaching the document can inject data.
This flaw allows unauthorized entities to inject arbitrary table records and interfere directly with the core MCP UI handshake process.
Additionally, outbound messages transmitted by the UI resource are dispatched using a wildcard target origin value set to ”.
Consequently, the UI’s internal JSON-RPC messages are broadcast openly without any origin restrictions enforced on the receiver end.
If an untrusted context embeds the resource, these sensitive JSON-RPC communication payloads leak directly to that unauthorized parent environment.
Looking at the affected source code in src/resources/datastore-table-ui.ts, the inline HTML listener registers an untrusted message handler.
When messages arrive, the code checks neither origin nor sender authenticity before parsing the event data payload object.
If valid fields or records are identified within the payload, initTable(data) renders the attacker-supplied data directly in the UI.
Because inbound messages matching pending request IDs can resolve or reject promises, attacker frames can spoof initialization responses.
Although actual exploitability depends heavily on the host embedding model, it serves as a critical defense-in-depth hardening issue.
Since a proper MCP Apps host treats the parent window as trusted, remote triggerability is limited, placing the severity at Low.
DailyCVE Form:
Platform: MCP Apps
Version: Unspecified
Vulnerability: Origin flaw
Severity: Low
Date: Today
Prediction: Pending
What Undercode Say
Verify missing origin check via static analysis node poc/verify-no-origin-check.mjs
// PoC code demonstrating cross-origin postMessage injection
window.parent.postMessage({
jsonrpc: '2.0',
id: 'malicious-id',
method: 'ui/initialize',
params: {}
}, '');
Exploit: (Educational Purposes!)
An attacker creates a malicious embedding page (poc/parent-spoof-poc.html) that loads the vulnerable MCP UI resource within an iframe.
Taking advantage of the absent `event.origin` validation in the inbound message listener, the attacker injects arbitrary records and spoofed phishing link URLs.
Simultaneously, since outbound messages are broadcast with a wildcard target origin of ”, the attacker’s wrapper frame intercepts internal JSON-RPC data.
This allows unauthorized manipulation of the handshake lifecycle and visual spoofing of the rendered datastore table.
Protection
Validate `event.origin` against the expected trusted host origin on every inbound message and discard messages from unauthorized sources.
Specify an explicit target origin in all outbound `postMessage` calls instead of using the insecure wildcard “.
Bind communication channels to a secure nonce established during the initial `ui/initialize` handshake phase so only the verified host interacts with the panel.
Impact
Content Spoofing: Attackers can inject arbitrary fields and rows, causing the table to render fabricated data and phishing links.
Handshake Interference: Spoofed responses to pending request IDs can disrupt the initialization handshake and UI stability.
Outbound Data Leakage: Wildcard broadcasting leaks JSON-RPC messages to any embedding parent context.
Risk Level: Classified as a Low severity hardening and defense-in-depth concern dependent on host embedding models.
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Sources:
Reported By: github.com
Extra Source Hub:
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