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fix: Disable DNS rebinding protection for containerized deployments

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MCP Python SDK 1.23.0 introduced automatic DNS rebinding protection that
auto-enables when host="127.0.0.1" (the default). This breaks containerized
deployments (Kubernetes, Docker) because the protection rejects requests
with Host headers like "nextcloud-mcp-server.default.svc.cluster.local:8000".

Root cause:
- FastMCP defaults to host="127.0.0.1"
- SDK auto-enables DNS rebinding protection with allowed_hosts=["127.0.0.1:*", "localhost:*", "[::1]:*"]
- K8s/Docker requests use service DNS names or proxied hostnames
- Protection middleware rejects these requests (421 Misdirected Request)

Solution:
- Explicitly pass transport_security=TransportSecuritySettings(enable_dns_rebinding_protection=False)
- Applied to all three FastMCP initializations (OAuth, Smithery, BasicAuth)
- DNS rebinding attacks mitigated by OAuth authentication and network isolation

This fixes issue #373 and enables MCP 1.23.x upgrade in PR #382.

For detailed analysis, see docs/MCP-1.23-DNS-REBINDING-FIX.md
This commit is contained in:
Chris Coutinho
2025-12-12 17:30:22 +01:00
parent bb8a6200aa
commit ec70e70a5d
2 changed files with 127 additions and 2 deletions
Download Patch File Download Diff File Expand all files Collapse all files
docs/MCP-1.23-DNS-REBINDING-FIX.md
+104
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@@ -0,0 +1,104 @@
# MCP 1.23.x DNS Rebinding Protection Fix
## Problem
MCP Python SDK 1.23.0 introduced **automatic DNS rebinding protection** that breaks containerized deployments (Kubernetes, Docker) when the protection is unintentionally auto-enabled.
### Root Cause
From `mcp/server/fastmcp/server.py:177-183` in the Python SDK:
```python
# Auto-enable DNS rebinding protection for localhost (IPv4 and IPv6)
if transport_security is None and host in ("127.0.0.1", "localhost", "::1"):
transport_security = TransportSecuritySettings(
enable_dns_rebinding_protection=True,
allowed_hosts=["127.0.0.1:*", "localhost:*", "[::1]:*"],
allowed_origins=["http://127.0.0.1:*", "http://localhost:*", "http://[::1]:*"],
)
```
### What Was Happening
1. **FastMCP initialization** in `app.py` didn't pass `host` or `transport_security` parameters
2. **Defaults applied**: `host="127.0.0.1"`, `transport_security=None`
3. **Auto-enablement triggered**: Condition `transport_security is None and host == "127.0.0.1"` was TRUE
4. **Protection activated** with `allowed_hosts=["127.0.0.1:*", "localhost:*", "[::1]:*"]`
5. **Kubernetes requests rejected**: `Host: nextcloud-mcp-server.default.svc.cluster.local:8000` didn't match allowed hosts
### Why `--host 0.0.0.0` Didn't Help
The `--host` CLI flag (used in Dockerfile/docker-compose) controls **uvicorn's bind address**, NOT the **FastMCP `host` parameter**. These are separate concerns:
- **Uvicorn bind address** (`--host 0.0.0.0`): Where the HTTP server listens
- **FastMCP host parameter** (defaulted to `"127.0.0.1"`): Used for auto-enablement logic
## Solution
Explicitly disable DNS rebinding protection by passing `transport_security=TransportSecuritySettings(enable_dns_rebinding_protection=False)` to all FastMCP instances.
### Changes Made
Modified `nextcloud_mcp_server/app.py`:
1. **Import** `TransportSecuritySettings` from `mcp.server.transport_security`
2. **Updated all three FastMCP initializations**:
- OAuth mode (line 1015)
- Smithery stateless mode (line 1030)
- BasicAuth mode (line 1040)
Each now includes:
```python
transport_security=TransportSecuritySettings(enable_dns_rebinding_protection=False)
```
## Impact
### ✅ What This Fixes
- **Kubernetes deployments**: Requests with k8s service DNS names now work
- **Docker deployments**: Port-mapped requests (localhost:8000 → container) now work
- **Reverse proxy deployments**: Proxied requests with various Host headers now work
- **Ingress controllers**: Requests via ingress hostnames now work
### 🔒 Security Considerations
DNS rebinding protection defends against attacks where:
1. Attacker controls a DNS domain (e.g., `evil.com`)
2. DNS initially resolves to attacker's IP
3. After victim's browser caches the origin, DNS changes to victim's localhost
4. Attacker's page can now make requests to victim's localhost services
**Why it's safe to disable for this deployment:**
1. **OAuth authentication required** in production deployments (ADR-002, ADR-004)
2. **Network-level isolation** in containerized environments (k8s network policies, Docker networks)
3. **MCP is server-to-server**, not exposed to browsers (no CORS concerns)
4. **Host header validation inappropriate** for multi-tenant k8s environments
If DNS rebinding protection is needed for specific deployments, it can be re-enabled with a custom allowed hosts list:
```python
transport_security=TransportSecuritySettings(
enable_dns_rebinding_protection=True,
allowed_hosts=[
"nextcloud-mcp-server.default.svc.cluster.local:*",
"mcp.example.com:*",
# Add all your expected Host header values
]
)
```
## Testing
- ✅ Ruff linting passes
- ✅ Type checking passes (pre-existing warnings unrelated)
- ✅ Module imports successfully
- ✅ Compatible with MCP 1.23.x
## References
- [MCP Python SDK 1.23.0 Release](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.23.0)
- Commit: `d3a1841` - "Auto-enable DNS rebinding protection for localhost servers"
- Issue #373 (original report of k8s breakage)
- PR #382 (MCP 1.23.x upgrade)
nextcloud_mcp_server/app.py
+23 -2
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@@ -19,6 +19,7 @@ import httpx
from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
from mcp.server.auth.settings import AuthSettings
from mcp.server.fastmcp import Context, FastMCP
from mcp.server.transport_security import TransportSecuritySettings
from pydantic import AnyHttpUrl
from starlette.applications import Starlette
from starlette.middleware.authentication import AuthenticationMiddleware
@@ -1016,6 +1017,11 @@ def get_app(transport: str = "streamable-http", enabled_apps: list[str] | None =
lifespan=oauth_lifespan,
token_verifier=token_verifier,
auth=auth_settings,
# Disable DNS rebinding protection for containerized deployments (k8s, Docker)
# MCP 1.23+ auto-enables this for localhost, breaking k8s service DNS names
transport_security=TransportSecuritySettings(
enable_dns_rebinding_protection=False
),
)
else:
# ADR-016: Use Smithery lifespan for stateless mode, BasicAuth otherwise
@@ -1024,11 +1030,26 @@ def get_app(transport: str = "streamable-http", enabled_apps: list[str] | None =
# json_response=True returns plain JSON-RPC instead of SSE format,
# required for Smithery scanner compatibility
mcp = FastMCP(
"Nextcloud MCP", lifespan=app_lifespan_smithery, json_response=True
"Nextcloud MCP",
lifespan=app_lifespan_smithery,
json_response=True,
# Disable DNS rebinding protection for containerized deployments (k8s, Docker)
# MCP 1.23+ auto-enables this for localhost, breaking k8s service DNS names
transport_security=TransportSecuritySettings(
enable_dns_rebinding_protection=False
),
)
else:
logger.info("Configuring MCP server for BasicAuth mode")
mcp = FastMCP("Nextcloud MCP", lifespan=app_lifespan_basic)
mcp = FastMCP(
"Nextcloud MCP",
lifespan=app_lifespan_basic,
# Disable DNS rebinding protection for containerized deployments (k8s, Docker)
# MCP 1.23+ auto-enables this for localhost, breaking k8s service DNS names
transport_security=TransportSecuritySettings(
enable_dns_rebinding_protection=False
),
)
@mcp.resource("nc://capabilities")
async def nc_get_capabilities():