DeepChat Mermaid XSS to RCE via Electron IPC MCP Server Registration (CVE-2025-66481 / GHSA-h9f5-7hhf-fqm4) - ATR-2026-01967 (c2930205-65ee-53ff-8b1e-d85f33fc245e)
Detects CVE-2025-66481 (CVSS 9.6 CRITICAL): DeepChat <= 0.5.1 incompletely sanitizes Mermaid diagram content in MermaidArtifact.vue. The sanitizer regex /on\w+\s=\s["'][^"']*["']/ only strips QUOTED event-handler attributes, so an unquoted handler (e.g. <audio src=x onerror=...>) survives and executes in the Electron renderer. The PoC handler invokes window.electron.ipcRenderer.invoke ('presenter:call','mcpPresenter','addMcpServer',...) then 'startServer' to register and launch a malicious stdio MCP server (command:'calc.exe'), escalating stored XSS to remote code execution. This rule keys on the unquoted-onerror + IPC presenter:call mcpPresenter addMcpServer/startServer tokens, not on Mermaid alone.