Improper Input Validation in ag-ui-protocol ag-ui

Executive Summary

CVE-2026-120236 is an improper input validation vulnerability in the ag-ui client library (versions up to 0.0.58). It stems from a validation asymmetry where protobuf-decoded events bypass schema validation while SSE-decoded events undergo proper validation. This allows malformed or malicious input to corrupt the client state machine, leading to incorrect protocol behavior or unintended data processing.

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Detection Guidance

To detect this vulnerability, check if your system is running ag-ui-protocol ag-ui version 0.0.58 or earlier. Verify if the Transport Decoder Handler component is present and inspect logs for improper input validation errors or malformed events bypassing schema checks.

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Impact Analysis

The vulnerability may allow attackers to inject type-confused or schema-invalid events into the client state, bypassing intended security checks. This could corrupt the client state machine, prevent runs from finishing, or cause downstream logic errors. Since the exploit is public, remote attackers could leverage this to disrupt system operations or manipulate data processing.

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Compliance Impact

The vulnerability allows malformed or malicious input to bypass schema validation, potentially leading to incorrect protocol behavior or unintended data processing. This could result in unauthorized data access, modification, or disclosure, which may violate GDPR's data integrity and confidentiality requirements or HIPAA's safeguards for protected health information.

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Mitigation Strategies

• Upgrade ag-ui to version 2026-10-30 or later to apply the patch 660182b973a44b8eda5484b14b88eb9cc251edc4.
• Verify that the enforcement pipeline in the TypeScript client (@ag-ui/client) properly validates all inputs before processing.
• Ensure protobuf-decoded events undergo the same schema validation as SSE-decoded events to prevent bypassing security checks.

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