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
Chris Coutinho
ec70e70a5d