fix(mcp): validate Host and Origin headers on the HTTP transport - #1120
fix(mcp): validate Host and Origin headers on the HTTP transport#1120Pablo (pablo-airbyte) wants to merge 6 commits into
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📝 WalkthroughWalkthroughThe MCP HTTP server now supports configurable bind hosts and allowed hosts. New ASGI middleware validates Host and Origin headers, rejects invalid requests, and preserves non-HTTP scopes. Unit tests cover resolution, validation, duplicate headers, and server wiring. ChangesMCP HTTP transport security
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🟠 High · up to The new host validation can accept a request when its first Host header is allowed but a later Host header is not, allowing crafted requests to bypass the intended protection and potentially re-enable unauthorized browser access. This security issue should be fixed before merging. Sequence Diagram(s)sequenceDiagram
participant MCPClient
participant HostOriginGuardMiddleware
participant MCPHTTPApp
MCPClient->>HostOriginGuardMiddleware: HTTP request with Host and Origin headers
HostOriginGuardMiddleware->>HostOriginGuardMiddleware: Match normalized hosts against the allowlist
HostOriginGuardMiddleware->>MCPHTTPApp: Forward valid request
MCPHTTPApp-->>MCPClient: HTTP response
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Inline comments:
In `@airbyte/mcp/_transport_security.py`:
- Around line 73-77: Update _request_host to collect Host headers and return a
host only when exactly one allowed Host header is present; reject duplicate Host
headers so the request produces 421, while preserving existing validation for
missing or invalid hosts. Add a regression test covering two Host headers and
asserting a 421 response.
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airbyte/mcp/__init__.pyairbyte/mcp/_transport_security.pyairbyte/mcp/http_main.pytests/unit_tests/test_mcp_auth.pytests/unit_tests/test_mcp_transport_security.py
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Approving to merge when ready, pending other bot feedback.
Also: I suggest after this does merge, we trigger a follow-up to move the wiring upstream into fastmcp-extensions, and then another PR to refactor to use the upstream. Devin, you can tag me with the HITL tools afterwards when that is ready and I'm happy to review. (Not a blocker though for this phase.)
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👍 On it (follow-ups noted, nothing to change here). Thanks. Bot feedback is settled: CodeRabbit's one actionable finding (duplicate Agreed on the sequencing for the follow-ups: land this as the local guard, then move the |
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Pull request overview
This PR adds an ASGI middleware guard in PyAirbyte’s MCP HTTP entrypoint to validate Host and (when present) Origin headers against an allowlist, addressing a DNS-rebinding/browser-drive risk for locally or privately run airbyte-mcp-http servers. It also introduces configuration knobs for the HTTP bind host and allowed hosts, plus unit tests covering the new behavior.
Changes:
- Added
HostOriginGuardMiddlewareandresolve_allowed_hosts()to enforceHost/Originallowlisting for HTTP requests. - Wrapped the HTTP transport app in
http_mainwith the new guard and added env var support for bind host and allowed host patterns. - Added/updated unit tests to cover allowlist behavior, missing/duplicate headers, wildcards, and server URL hostname inclusion.
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.
Show a summary per file
| File | Description |
|---|---|
airbyte/mcp/_transport_security.py |
Introduces host/origin guard middleware and allowlist resolution logic. |
airbyte/mcp/http_main.py |
Wires the middleware into the HTTP MCP startup path; adds env var docs and bind-host override. |
airbyte/mcp/__init__.py |
Documents the new env vars for HTTP mode. |
tests/unit_tests/test_mcp_transport_security.py |
Adds unit tests for host/origin validation and allowlist resolution. |
tests/unit_tests/test_mcp_auth.py |
Adjusts delegation test to assert the new wrapper produces the guard middleware. |
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| for host in _DEFAULT_ALLOWED_HOSTS: | ||
| add(host) | ||
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| hostname = urlparse(server_url).hostname | ||
| if hostname and not _is_unspecified_address(hostname): | ||
| add(hostname) | ||
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| for host in os.getenv(ALLOWED_HOSTS_ENV, "").split(","): | ||
| add(host) |
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🙋 Human Input Needed: the behavior is real, but I don't think it's a bypass of what this guard is for, and narrowing it would break hosted deployments. Asking before changing it.
Why it isn't a bypass of the intended protection: this guard defends the browser-driven rebinding path, where the malicious page cannot choose the Host header (the browser sets it to the rebound attacker hostname), so the allowlist stops it. A non-browser attacker who can already open a TCP connection to the port can of course send Host: localhost, but that attacker never needed a Host trick in the first place; the control for network reachability is the bind address, which this PR makes configurable via AIRBYTE_MCP_HTTP_HOST, plus the auth layer for hosted deployments.
Why the narrowing has a cost: dropping loopback when MCP_SERVER_URL is a public URL would 421 anything that legitimately dials the container over loopback or an IP literal, which in practice is health probes and sidecars. That's the same class of breakage already called out for probes sending an IP-literal Host.
Aaron ("AJ") Steers (@aaronsteers), if you'd rather have the stricter behavior, the shape I'd pick is an opt-out (AIRBYTE_MCP_ALLOWED_HOSTS= explicitly empty means "no loopback defaults") rather than making it conditional on MCP_SERVER_URL, so the hosted rollout can turn it on once probes are confirmed. Happy to add that here or as a follow-up. Leaving as-is unless you want it.
Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
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☑️ Correction to my design note above, after reading fastmcp 3.4.5's guard more closely ( I said upstream's same-origin fallback makes the Also: |
Summary
The HTTP MCP entrypoint accepts any
HostandOriginheader, so a browser page can drive a locally or privately runairbyte-mcp-httpserver (CWE-346, reported externally by a third party researcher; requested by Pablo Vega). The pinnedfastmcp3.2.0 constructsStreamableHTTPSessionManagerwithoutTransportSecuritySettingsand ships no host/origin guard, so there is no flag to flip; this adds the check as an ASGI middleware in PyAirbyte instead of bumping a public dependency.HostOriginGuardMiddlewareinairbyte/mcp/_transport_security.pywraps the whole app, outside the existing client-credentials wrapper:Host->421Origin->403; absentOriginis allowed (non-browser clients)fnmatchon the hostname only, port-stripped, bracket-stripped, lowercasedThere is deliberately no same-origin fallback (accepting
Origin == scheme://<request Host>, as upstream fastmcp's newer guard does). Both headers are attacker-controlled in the rebinding scenario, so that fallback makes the Origin check bypassable whenever the Host allowlist is permissive.The allowlist resolves from three sources, none of them request-derived: loopback defaults (
127.0.0.1,localhost,::1), the hostname parsed fromMCP_SERVER_URL(already injected with the public URL by the hosted deployment, so hosted traffic keeps working with no new config), and the newAIRBYTE_MCP_ALLOWED_HOSTS(comma-separated, patterns like*.run.appallowed) for ingress hostnames, in-cluster Service DNS names, or anything else the process cannot infer about itself.AIRBYTE_MCP_HTTP_HOSTnow overrides the previously hard-coded bind address. The default stays0.0.0.0so hosted deployments are unaffected; the guard, not the bind address, is what closes LAN and browser access here.Before rolling this out to a deployment, confirm every legitimate
Hostvalue is covered. Load-balancer or kubelet health probes that send an IP-literalHost, and direct*.run.apphits where ingress still permits them, will get a421unless they are added toAIRBYTE_MCP_ALLOWED_HOSTSor pointed at an unguarded path with an explicitHost.Test plan
tests/unit_tests/test_mcp_transport_security.pydrives the middleware throughstarlette.testclient.TestClient: allowed loopback hosts pass;Host: attacker.example:8080gets421;Origin: http://attacker.example:8080with an allowedHostgets403(the regression test for the reported vector); missingOriginpasses; missingHostgets421;AIRBYTE_MCP_ALLOWED_HOSTSentries and pattern entries pass;resolve_allowed_hostsincludes theMCP_SERVER_URLhostname. The existinghttp_maindelegation test now asserts the wrapper produces the guard.Run:
pytest tests/unit_tests/test_mcp_transport_security.py tests/unit_tests/test_mcp_auth.py(38 passed locally), plusruff format --check .,ruff check ., andmypyon the changed modules. Full-repomypyis blocked by a preexisting duplicatesetupmodule error in the test fixture sources, unrelated to this branch.Link to Devin session: https://app.devin.ai/sessions/d15a969d92424c7990d399577f916d89
Requested by: Pablo (@pablo-airbyte)
Summary by CodeRabbit
New Features
AIRBYTE_MCP_ALLOWED_HOSTS.Documentation
Tests