What happened
NVD published this CVSS 6.3 Medium vulnerability on Sept 16, 2026 in the ag-ui agent event protocol's origin-validation logic.
Why it matters
ag-ui is a protocol for agent-to-UI event streaming; an origin-validation bypass could allow cross-origin event injection into agent conversation state, a narrow but relevant agentic-protocol integrity issue.
Attack vector
Manipulation of the TEXT_MESSAGE_START argument in prepareRunAgentInput (agent/agent.ts, Event Application Layer) causes an origin validation error, enabling remote exploitation though attack complexity is reported as high.
Affected systems
ag-ui-protocol/ag-ui 1.0
Mitigation
Track the ag-ui-protocol project for a fix; validate event origin independently of client-supplied event fields.