{
 "public_id": "AIR-2026-0221",
 "incident_id": "2026-09-excel-mcp-server-path-confinement",
 "date_disclosed": "2026-09-04",
 "date_occurred": null,
 "title": "CVE-2026-85661: arbitrary file read/write in excel-mcp-server stdio mode (no path confinement)",
 "class": "responsible_disclosure",
 "attack_surface": "mcp_server",
 "initial_vector": "malicious_tool_arguments",
 "agent_capability_abused": [
  "file_read_tool",
  "file_write_tool"
 ],
 "target_product": "excel-mcp-server",
 "target_vendor": "haris-musa",
 "autonomy_level": "assistant",
 "impact_realized": false,
 "impact_type": [
  "arbitrary_file_read",
  "arbitrary_file_write"
 ],
 "blast_radius": "excel-mcp-server 0.1.8 in stdio mode when EXCEL_FILES_PATH is unset (the default for desktop MCP clients)",
 "cve": [
  "CVE-2026-85661"
 ],
 "taxonomy": {
  "owasp_asi": [
   "ASI04",
   "ASI02"
  ],
  "mitre_atlas": [],
  "labels": [
   "path_traversal",
   "missing_confinement",
   "stdio_transport",
   "mcp_server"
  ]
 },
 "defense_present": "realpath+commonpath confinement (only in SSE/http paths, not stdio)",
 "defense_bypassed": true,
 "fix_shipped": null,
 "primary_sources": [
  "https://cveawg.mitre.org/api/cve/CVE-2026-85661",
  "https://github.com/haris-musa/excel-mcp-server/issues/149",
  "https://cvebrief.com/archive/2026/09/05/"
 ],
 "verified_quote": "excel-mcp-server 0.1.8 fails to enforce path confinement in stdio mode when EXCEL_FILES_PATH is unset, allowing attackers to read and write arbitrary files.",
 "confidence": "high",
 "notes": "CVSS 9.3 (v4.0) / 9.8 (v3.1). Assigned by VulnCheck; reported by George Chen. Also a case-insensitive formula-denylist bypass. Reported to maintainer 2026-06-05 (GHSA-9h7m-8h6c-6jh7) with no response, then filed publicly. PoC/disclosure; no real victim.",
 "risk_category": "unmapped",
 "risk_secondary": [],
 "risk_subcategory": "",
 "risk_sub_secondary": [],
 "ai_entity": "generative",
 "agency": "ai_as_target",
 "agency_debatable": false,
 "agency_reason": "MCP server path flaw; no model decision",
 "vector_family": "malicious_component",
 "vector_detail": "malicious_tool_arguments",
 "guardrail_kind": [
  "unknown"
 ],
 "guardrail_outcome": "silent",
 "xrefs": {
  "aiid": [],
  "atlas": [],
  "cve": [
   "CVE-2026-85661"
  ],
  "ghsa": []
 },
 "primary_asi": "ASI04",
 "agent_framework": "mcp_server",
 "model_provider": "unknown",
 "model_name": "",
 "detection_mechanism": "unknown",
 "tool_access": "unknown",
 "financial_loss_band": "unknown",
 "eai_category": "supply_chain_vulnerabilities",
 "eai_domain": "tool_misuse",
 "eai_secondary": [],
 "eai_reason": "vector default: malicious_component",
 "asi_curated": [],
 "asi_scope": "in_scope",
 "asi_provenance": "rule",
 "asi_reason": "ASI04 (vector_family=malicious_component); ASI02 (impact=tool_misuse)",
 "submitted_by": "EAI Internal"
}