fix(mcp): parse the Host header before trusting it for the OAuth origin - #305
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Included review availability: Your plan provides up to 10 included reviews per hour; 4 remain after this review. 📝 WalkthroughWalkthroughHost validation now parses host syntax, IPv6 literals, and port ranges before trust checks. Wildcard-port rules use parsed hostnames. OAuth metadata uses the ASGI request scheme and excludes malformed hosts. ChangesHost validation
OAuth metadata
Suggested reviewers: Priority: ⬇️ Low Merge Risk: ⚪ Minimal · up to No merge-blocking risk was identified in the Host-validation and OAuth metadata changes. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
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In `@packages/sie_mcp/src/sie_mcp/auth.py`:
- Around line 47-66: Update _split_host_port to validate the matched ipv6 value
with ipaddress.IPv6Address before returning it; return None when validation
raises ValueError, while preserving valid IPv6 literals and existing port
validation.
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`base_url` advertises the authorization server that MCP clients are told to trust, so it only uses the request's own `Host` when that host is loopback or listed in `SIE_MCP_ALLOWED_HOSTS`. Both trust checks matched on a string prefix instead of a parsed host, so a caller-supplied `Host` could carry extra authority past the part that was checked. A `host:*` allowlist entry was matched with `host.startswith(entry[:-1])`, so with `mcp.example.com:*` configured, any `Host` beginning `mcp.example.com:` was trusted. `mcp.example.com:443@evil.example` then produced the origin `https://mcp.example.com:443@evil.example`, which parses with the configured host as userinfo and `evil.example` as the real host, so the advertised metadata URL pointed at a host the operator does not control. `mcp.example.com:443/evil` injected a path, and `:99999`, `:abc` and a bare trailing colon were accepted as ports. The loopback check had the same shape and needed no configuration at all: `_hostname` truncated a bracketed host at the first `]` and otherwise split on the last `:`, so `[::1]@evil.example` and `localhost:8088@evil.example` both reduced to a loopback name and were trusted. Replace both with a single parse of the `Host` header into hostname and port. A Host header is `uri-host [ ':' port ]` and nothing else (RFC 9110 s7.2), so userinfo, paths, queries and non-numeric or out-of-range ports are now rejected rather than reinterpreted. A wildcard entry matches when the parsed hostname equals the configured host and a valid port is present; exact entries are unchanged.
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@coderabbitai full review The bracketed IPv6 finding is fixed with At |
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The defect
base_url()inpackages/sie_mcp/src/sie_mcp/auth.pyresolves the origin thatthe MCP edge advertises to clients as their authorization server (RFC 9728
resource_metadata, and the OAuth metadata documents). WhenSIE_MCP_PUBLIC_URLis not pinned it falls back to the request's ownHost,but only if that host is loopback or listed in
SIE_MCP_ALLOWED_HOSTS. Itsdocstring states the contract: caller-controlled
HostandX-Forwarded-*values are never advertised.
Both trust checks compared strings by prefix rather than parsing the host, so a
caller could append extra authority after the part that was checked.
1. Wildcard allowlist entries.
host:*was matched withhost.startswith(entry[:-1]), i.e. the literal prefixhost::With
SIE_MCP_ALLOWED_HOSTS=mcp.example.com:*, everyHostbelow was trusted.Measured against the unmodified functions:
Hostheadermcp.example.com:443@evil.examplehttps://mcp.example.com:443@evil.exampleevil.examplemcp.example.com:443/evilhttps://mcp.example.com:443/evilmcp.example.com:99999mcp.example.com:abcmcp.example.com:The first row is the interesting one.
https://mcp.example.com:443@evil.exampleparses with
mcp.example.com:443as userinfo andevil.exampleas the host,so the emitted metadata URL
https://mcp.example.com:443@evil.example/.well-known/oauth-protected-resourcesends the client to an authorization server the operator does not control.
2. The loopback check, which needs no configuration at all.
_hostname()truncated a bracketed host at the first
]and otherwise split on the last:, then compared the remainder against the hardcoded loopback set:So with an empty
SIE_MCP_ALLOWED_HOSTS:Hostheader_hostname()localhost:8088@evil.examplelocalhostevil.example127.0.0.1:8088@evil.example127.0.0.1evil.example[::1]@evil.example[::1][::1].evil.example[::1]Note the asymmetry that makes this easy to miss:
localhost@evil.exampleiscorrectly rejected, because with no colon the whole string survives the split.
Adding a port is what breaks it.
The fix
Parse the
Hostheader once, then make the trust decision on the parsed parts.A Host header is
uri-host [ ":" port ]and nothing else (RFC 9110 s7.2), souserinfo, paths, queries, fragments and non-numeric or out-of-range ports are
rejected rather than reinterpreted.
host:*matches when the parsed hostname equals the configured hostand a syntactically valid port is present (all decimal digits, 1..65535).
Requiring the port keeps today's semantics exactly: a wildcard entry did not
previously match the bare no-port form, and still does not.
This is the same shape
parse_https_origininpackages/sie_gateway/src/config.rsalready uses: reject userinfo, reject path/query/fragment, validate the port as
ASCII digits, and only then trust the host.
Sibling sweep
The class swept for: an allowlist or trust decision made by prefix/
startswithmatching instead of parsing the value into its components. Host, URL, origin,
redirect-URI and CORS allowlists were all in scope.
On
mainthis reports the two lines fixed here:On this branch those are gone and three hits remain, each checked:
sie_telemetry/src/transport.rs:223— safe, and not an allowlist. The URLhas already been through
reqwest::Url::parse, and the value tested isparsed.host_str(), which has userinfo, path and query stripped by the parser.The
starts_with('[')only decides whether to re-add brackets around an IPv6literal when formatting a diagnostic string. No trust decision is taken.
sie_gateway/src/nats/manager.rs:277andsie_server_sidecar/src/config_subscriber.rs:288— safe by design, anddocumented as such. This is the trusted-producer allowlist matching a
Kubernetes pod name against a configured Deployment name; the prefix branch
requires a literal
-separator, sosie-configmatchessie-config-5f7b6d8c-kxwvrbut notsie-configuration. It compares one wholeidentity against another rather than a structured URL, so there is no userinfo,
port or path component that could be smuggled past the compared prefix. Both
copies are the same intentional rule and are deliberately left unchanged.
Also inspected, no prefix matching present:
_redirect_allowed(packages/sie_mcp/src/sie_mcp/oauth.py:172) testsredirect_uri in config.oauth_redirect_uris— exact membership. Theauthorization-code exchange likewise compares
redirect_urifor equalityagainst the recorded value.
parse_https_origin(packages/sie_gateway/src/config.rs:336) alreadyrejects
@, path, query and fragment and validates the port.CORSMiddleware,allow_origins,allow_origin_regexorAccess-Control-Allow-Origin.The broader sweep for bare
startswith/starts_withreturns 309 hits, all ofwhich are scheme tests, magic-byte sniffing, route namespacing or string
trimming rather than allowlist membership.
Same shape upstream, in the
mcpdependencyThe
host:*syntax originates from the MCP SDK, whoseTransportSecurityMiddlewarethis edge configures from the sameSIE_MCP_ALLOWED_HOSTSlist in_transport_security(
packages/sie_mcp/src/sie_mcp/server.py). That middleware matches the wildcardthe same way, in both
_validate_hostand_validate_origin:Measured against
mcp1.28.1 withallowed_hosts=["mcp.example.com:*"],_validate_hostreturnsTrueformcp.example.com:443@evil.example,mcp.example.com:443/evil,mcp.example.com:abcandmcp.example.com:99999.That is a dependency, so it is not changed here, and the impact is different in
kind: the SDK check is the DNS-rebinding guard deciding whether to serve a
request at all (421 on failure), not a value that gets advertised to clients.
Access control at this edge is the connector secret, and after this change the
advertised origin no longer depends on that check. Worth raising upstream
separately.
One detail from it is load-bearing here:
_validate_hostreturnsFalseformcp.example.comwith no port, so a wildcard entry has never matched the bareno-port form. Requiring a port in the fixed matcher keeps this repo aligned with
that meaning rather than quietly widening it.
Tests
29 parametrized security and compatibility cases cover wildcard ports, loopback
lookalikes, origin authority, malformed bracketed IPv6, and valid IPv6 including
IPv4-mapped literals. Bracketed values are validated with
IPv6Addressbeforean exact or wildcard allowlist entry can establish trust.
The malformed-IPv6 regression produced 8 failures before the address validation
was added. With the complete fix, all 61 auth tests and all 315 MCP tests pass.
The existing authority-injection regression was also mutation-checked: reverting
the original parser fix caused 15 of its 17 added cases to fail.
Six HTTP-level regressions also cover malformed bracketed hosts on all three
OAuth metadata routes, with and without a pinned public URL. Each failed before
reading the scheme from the ASGI scope instead of parsing the untrusted Host
through
request.url. The fixed routes return 503 when unpinned and preservethe exact trusted metadata origin when pinned.
After rebasing onto public
mainat7a68262a, the full MCP suite, scoped Ruffchecks, formatting, and MCP typecheck pass.
Additional sibling sweep for bracketed-host parsing:
The sweep finds this parser and its IPv6 validation, plus the three already
triaged Rust sites listed above. No other matching authority parser remains.
Summary by CodeRabbit