Add origin/host validation and warning for exposed serves (#1270)

CHANGELOG.md

@@ -44,6 +44,7 @@ Making a new release? Simply add the new header with the version and date undern
 * Fixed the sync fallback scrambling sibling order; replacements are now re-parented ancestors-first and in their original child order. ([#1265])
 * Instances that share a name and class are now robustly matched on resync by comparing their properties, instead of relying on child order alone. ([#1266])
 * Rojo now reports a clear error instead of panicking in several cases, including when the `serve` port is already in use, when a synced file is read-only or locked, when the filesystem watcher can't be created, and when the working directory is inaccessible. ([#1267])
+* `rojo serve` now validates the `Host`/`Origin` headers to protect the local/private server against DNS rebinding, gates `/api/open` to local clients, and warns when bound to a network-reachable address. The accepted hosts can be extended with the `--allowed-hosts` option or a project's `serveAllowedHosts` field, for example to reach a network-exposed server by hostname. ([#1270])
 
 [#1176]: https://github.com/rojo-rbx/rojo/pull/1176
 [#1179]: https://github.com/rojo-rbx/rojo/pull/1179
@@ -58,6 +59,7 @@ Making a new release? Simply add the new header with the version and date undern
 [#1265]: https://github.com/rojo-rbx/rojo/pull/1265
 [#1266]: https://github.com/rojo-rbx/rojo/pull/1266
 [#1267]: https://github.com/rojo-rbx/rojo/pull/1267
+[#1270]: https://github.com/rojo-rbx/rojo/pull/1270
 
 ## [7.7.0-rc.1] (November 27th, 2025)
 

src/cli/serve.rs

@@ -31,6 +31,14 @@ pub struct ServeCommand {
     /// it has none.
     #[clap(long)]
     pub port: Option<u16>,
+
+    /// Extra `Host`/`Origin` values the server will accept, beyond localhost and
+    /// the bind address (for example a hostname like `mypc.lan`). Repeat the
+    /// option or comma-separate to allow several. When given, this overrides the
+    /// project's `serveAllowedHosts`. Listing any host also turns on Host/Origin
+    /// validation for binds where it is otherwise off (such as `0.0.0.0`).
+    #[clap(long, value_delimiter = ',')]
+    pub allowed_hosts: Vec<String>,
 }
 
 impl ServeCommand {
@@ -51,9 +59,17 @@ impl ServeCommand {
             .or_else(|| session.project_port())
             .unwrap_or(DEFAULT_PORT);
 
+        // The CLI flag, when given, replaces the project's list rather than
+        // merging with it, matching how --address and --port override theirs.
+        let allowed_hosts = if self.allowed_hosts.is_empty() {
+            session.serve_allowed_hosts().to_vec()
+        } else {
+            self.allowed_hosts
+        };
+
         let server = LiveServer::new(session);
 
-        server.start((ip, port).into(), || {
+        server.start((ip, port).into(), allowed_hosts, || {
             let _ = show_start_message(ip, port, global.color.into());
         })?;
 
@@ -87,6 +103,25 @@ fn show_start_message(bind_address: IpAddr, port: u16, color: ColorChoice) -> io
 
     writeln!(&mut buffer)?;
 
+    if !bind_address.is_loopback() {
+        let mut warning = ColorSpec::new();
+        warning.set_fg(Some(Color::Yellow)).set_bold(true);
+
+        buffer.set_color(&warning)?;
+        writeln!(
+            &mut buffer,
+            "WARNING: This server is bound to {address_string}, which is reachable from the \
+             network.\n\
+             The serve API is unauthenticated, so anyone who can reach {address_string}:{port} \
+             can read\n\
+             and modify your project's source. Prefer binding to localhost and tunneling (e.g. \
+             SSH,\n\
+             Tailscale, or WireGuard) when you need remote access."
+        )?;
+        buffer.set_color(&ColorSpec::new())?;
+        writeln!(&mut buffer)?;
+    }
+
     buffer.set_color(&ColorSpec::new())?;
     write!(&mut buffer, "Visit ")?;
 

src/project.rs

@@ -106,6 +106,15 @@ pub struct Project {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub serve_address: Option<IpAddr>,
 
+    /// Additional `Host`/`Origin` header values that `rojo serve` will accept
+    /// beyond `localhost` and the bind address, such as a hostname like
+    /// `mypc.lan` used to reach a network-exposed server by name. Listing any
+    /// host also turns on `Host`/`Origin` validation for binds where it is
+    /// otherwise off (such as `0.0.0.0`). The `--allowed-hosts` CLI option
+    /// overrides this field when provided.
+    #[serde(default, skip_serializing_if = "Vec::is_empty")]
+    pub serve_allowed_hosts: Vec<String>,
+
     /// Determines if Rojo should emit scripts with the appropriate `RunContext`
     /// for `*.client.lua` and `*.server.lua` files in the project instead of
     /// using `Script` and `LocalScript` Instances.
@@ -595,6 +604,41 @@ mod test {
         assert!(project.sync_rules[1].include.is_match("init.module.lua"));
     }
 
+    #[test]
+    fn project_with_serve_allowed_hosts() {
+        let project_json = r#"{
+            "name": "TestProject",
+            "tree": { "$path": "src" },
+            "serveAllowedHosts": ["mypc.lan", "192.168.1.5"]
+        }"#;
+
+        let project = Project::load_from_slice(
+            project_json.as_bytes(),
+            PathBuf::from("/test/default.project.json"),
+            None,
+        )
+        .expect("Failed to parse project with serveAllowedHosts");
+
+        assert_eq!(project.serve_allowed_hosts, vec!["mypc.lan", "192.168.1.5"]);
+    }
+
+    #[test]
+    fn project_without_serve_allowed_hosts_defaults_to_empty() {
+        let project_json = r#"{
+            "name": "TestProject",
+            "tree": { "$path": "src" }
+        }"#;
+
+        let project = Project::load_from_slice(
+            project_json.as_bytes(),
+            PathBuf::from("/test/default.project.json"),
+            None,
+        )
+        .expect("Failed to parse project");
+
+        assert!(project.serve_allowed_hosts.is_empty());
+    }
+
     #[test]
     fn glob_ignore_paths_negation() {
         let project_json = r#"{

src/serve_session.rs

@@ -207,6 +207,10 @@ impl ServeSession {
         self.root_project.serve_address
     }
 
+    pub fn serve_allowed_hosts(&self) -> &[String] {
+        &self.root_project.serve_allowed_hosts
+    }
+
     pub fn root_dir(&self) -> &Path {
         self.root_project.folder_location()
     }

src/web/api.rs

@@ -1,7 +1,7 @@
 //! Defines Rojo's HTTP API, all under /api. These endpoints generally return
 //! JSON.
 
-use std::{collections::HashMap, fs, path::PathBuf, str::FromStr, sync::Arc};
+use std::{collections::HashMap, fs, net::SocketAddr, path::PathBuf, str::FromStr, sync::Arc};
 
 use futures::{sink::SinkExt, stream::StreamExt};
 use hyper::{body, Body, Method, Request, Response, StatusCode};
@@ -21,6 +21,7 @@ use crate::{
             ServerInfoResponse, SocketPacket, SocketPacketBody, SocketPacketType, SubscribeMessage,
             WriteRequest, WriteResponse, PROTOCOL_VERSION, SERVER_VERSION,
         },
+        origin::canonical,
         util::{deserialize_msgpack, msgpack, msgpack_ok, serialize_msgpack},
     },
     web_api::{
@@ -28,8 +29,12 @@ use crate::{
     },
 };
 
-pub async fn call(serve_session: Arc<ServeSession>, mut request: Request<Body>) -> Response<Body> {
+pub async fn call(
-    let service = ApiService::new(serve_session);
+    serve_session: Arc<ServeSession>,
+    remote_addr: SocketAddr,
+    mut request: Request<Body>,
+) -> Response<Body> {
+    let service = ApiService::new(serve_session, remote_addr);
 
     match (request.method(), request.uri().path()) {
         (&Method::GET, "/api/rojo") => service.handle_api_rojo().await,
@@ -65,11 +70,15 @@ pub async fn call(serve_session: Arc<ServeSession>, mut request: Request<Body>)
 
 pub struct ApiService {
     serve_session: Arc<ServeSession>,
+    remote_addr: SocketAddr,
 }
 
 impl ApiService {
-    pub fn new(serve_session: Arc<ServeSession>) -> Self {
+    pub fn new(serve_session: Arc<ServeSession>, remote_addr: SocketAddr) -> Self {
-        ApiService { serve_session }
+        ApiService {
+            serve_session,
+            remote_addr,
+        }
     }
 
     /// Get a summary of information about the server
@@ -348,6 +357,30 @@ impl ApiService {
 
     /// Open a script with the given ID in the user's default text editor.
     async fn handle_api_open(&self, request: Request<Body>) -> Response<Body> {
+        // Opening a file launches a local program, so it must never be reachable
+        // by a remote client even when the server is bound to an exposed address.
+        //
+        // `remote_addr` is the immediate peer, which is the best locality signal
+        // we have: the legitimate caller is a sandboxed Roblox plugin whose only
+        // credential is being able to reach the port, so there is no secret to
+        // authenticate it with. A connection forwarded over loopback by an
+        // SSH/Tailscale tunnel or a local reverse proxy therefore appears local
+        // and is allowed. That is delegated trust rather than a bypass: by
+        // standing up that tunnel or proxy the user has decided the remote end is
+        // trusted, and reachability is bounded by that hop's own authentication
+        // (e.g. SSH keys or Tailscale ACLs). This gate only stops direct,
+        // unauthenticated peers.
+        //
+        // An IPv4 client reaching a dual-stack (`::`) bind appears as an
+        // IPv4-mapped IPv6 peer (`::ffff:127.0.0.1`), so canonicalize to the bare
+        // IPv4 form before the loopback test, matching `origin`'s handling.
+        if !canonical(self.remote_addr.ip()).is_loopback() {
+            return msgpack(
+                ErrorResponse::forbidden("/api/open is only available to local clients"),
+                StatusCode::FORBIDDEN,
+            );
+        }
+
         let argument = &request.uri().path()["/api/open/".len()..];
         let requested_id = match Ref::from_str(argument) {
             Ok(id) => id,

src/web/interface.rs

@@ -290,6 +290,13 @@ impl ErrorResponse {
         }
     }
 
+    pub fn forbidden<S: Into<String>>(details: S) -> Self {
+        Self {
+            kind: ErrorResponseKind::Forbidden,
+            details: details.into(),
+        }
+    }
+
     pub fn internal_error<S: Into<String>>(details: S) -> Self {
         Self {
             kind: ErrorResponseKind::InternalError,
@@ -302,5 +309,6 @@ impl ErrorResponse {
 pub enum ErrorResponseKind {
     NotFound,
     BadRequest,
+    Forbidden,
     InternalError,
 }

src/web/mod.rs

@@ -5,6 +5,7 @@
 mod api;
 mod assets;
 pub mod interface;
+mod origin;
 mod ui;
 mod util;
 
@@ -14,7 +15,7 @@ use std::sync::Arc;
 
 use anyhow::Context;
 use hyper::{
-    server::Server,
+    server::{conn::AddrStream, Server},
     service::{make_service_fn, service_fn},
     Body, Request,
 };
@@ -33,22 +34,42 @@ impl LiveServer {
 
     /// Starts the server on the given address, blocking until it stops.
     ///
+    /// `allowed_hosts` are extra `Host`/`Origin` values to accept in addition to
+    /// localhost and the bind address (see [`origin::allowed_hosts`]).
+    ///
     /// `on_listening` is invoked once the server has successfully bound to the
     /// address, so callers can defer printing any "listening" message until
     /// after binding can no longer fail (e.g. due to the port being in use).
-    pub fn start(self, address: SocketAddr, on_listening: impl FnOnce()) -> anyhow::Result<()> {
+    pub fn start(
+        self,
+        address: SocketAddr,
+        allowed_hosts: Vec<String>,
+        on_listening: impl FnOnce(),
+    ) -> anyhow::Result<()> {
         let serve_session = Arc::clone(&self.serve_session);
+        let allowed_hosts = origin::allowed_hosts(address.ip(), address.port(), &allowed_hosts);
 
-        let make_service = make_service_fn(move |_conn| {
+        let make_service = make_service_fn(move |conn: &AddrStream| {
-            let serve_session = Arc::clone(&serve_session);
-
-            async {
-                let service = move |req: Request<Body>| {
             let serve_session = Arc::clone(&serve_session);
+            let allowed_hosts = allowed_hosts.clone();
+            let remote_addr = conn.remote_addr();
 
             async move {
+                let service = move |req: Request<Body>| {
+                    let serve_session = Arc::clone(&serve_session);
+                    let allowed_hosts = allowed_hosts.clone();
+
+                    async move {
+                        // Reject cross-origin requests before doing any work, to
+                        // defend the local server against DNS rebinding.
+                        if let Some(response) =
+                            origin::check_request_origin(&req, allowed_hosts.as_ref())
+                        {
+                            return Ok::<_, Infallible>(response);
+                        }
+
                         if req.uri().path().starts_with("/api") {
-                            Ok::<_, Infallible>(api::call(serve_session, req).await)
+                            Ok::<_, Infallible>(api::call(serve_session, remote_addr, req).await)
                         } else {
                             Ok::<_, Infallible>(ui::call(serve_session, req).await)
                         }

src/web/origin.rs

@@ -0,0 +1,589 @@
+//! Host/Origin validation used to defend the local Rojo server against DNS
+//! rebinding attacks.
+//!
+//! When Rojo is bound to a loopback address (the default), a malicious web page
+//! the developer visits cannot read the API responses directly because of the
+//! browser Same-Origin Policy. However, a DNS rebinding attack can defeat that:
+//! the page points its own hostname at `127.0.0.1` after loading, so the browser
+//! treats requests to the Rojo server as same-origin. Validating that the `Host`
+//! (and, if present, `Origin`) header refers to an address we recognize blocks
+//! this, because the rebound request still carries the attacker's hostname, which
+//! is a domain name rather than one of the IP literals we accept.
+//!
+//! Enforcement covers two kinds of bind:
+//!
+//! * Loopback (the default): only `localhost` and loopback literals are
+//!   accepted.
+//! * A specific private/LAN address: `localhost`, loopback literals, and that exact bind
+//!   IP are accepted. Because the defense works by rejecting any `Host` that
+//!   isn't a recognized IP literal, clients must connect to a private bind by
+//!   IP. A hostname (e.g. `mypc.local`) is indistinguishable from an attacker
+//!   domain and is rejected.
+//!
+//! Enforcement is disabled for unspecified (`0.0.0.0`/`::`) and public binds: the
+//! user has asked for broad, possibly-public exposure where arbitrary hostnames
+//! may legitimately resolve to the server, so we can't build a meaningful
+//! allowlist. Those binds get a startup warning instead. Two consequences worth
+//! being explicit about: such a bind has no rebinding protection even for a
+//! browser on the same machine; and even on a protected private bind this check
+//! does nothing against a hostile peer already on the LAN, who can reach the
+//! unauthenticated API directly. Both are the network-exposure risk the startup
+//! warning addresses, not something the `Host` check is meant to cover.
+//!
+//! The allowlist can be widened with extra hosts (the `--allowed-hosts` CLI
+//! option or a project's `serveAllowedHosts`), for example a hostname like
+//! `mypc.lan` for reaching a network-exposed server by name. Listing any extra
+//! host also turns enforcement back on for an unspecified or public bind that
+//! would otherwise disable it, restricting that bind to localhost, the bind IP,
+//! and the listed hosts.
+
+use std::net::IpAddr;
+
+use hyper::{
+    header::{HOST, ORIGIN},
+    http::uri::{Authority, Uri},
+    Body, Request, Response, StatusCode,
+};
+
+use crate::web::util::response;
+
+/// The set of `Host`/`Origin` values accepted while enforcement is active.
+#[derive(Debug, Clone)]
+pub struct AllowedHosts {
+    port: u16,
+    /// The bind IP accepted in addition to `localhost`/loopback, set only for a
+    /// private (LAN) bind or a specific public bind that has extra hosts. `None`
+    /// for a loopback or unspecified bind, which has no single address to add.
+    bind_ip: Option<IpAddr>,
+    /// Extra `Host`/`Origin` values the user explicitly allowed, via the
+    /// `--allowed-hosts` option or a project's `serveAllowedHosts`. These are
+    /// hostnames such as `mypc.lan`, or IP literals, accepted in addition to
+    /// localhost and the bind IP. Each entry is already passed through
+    /// [`normalize_host`].
+    extra_hosts: Vec<String>,
+}
+
+impl AllowedHosts {
+    /// Returns whether the given host and optional port are allowed. The host is
+    /// accepted if it is `localhost`, a loopback IP literal, (on a private or
+    /// specific public bind) the exact bind IP, or one of the explicitly allowed
+    /// extra hosts. A request with no explicit port is accepted (the host
+    /// already has to be one we recognize).
+    fn allows(&self, host: &str, port: Option<u16>) -> bool {
+        let host = normalize_host(host);
+        let host_ok = host.eq_ignore_ascii_case("localhost")
+            || host
+                .parse::<IpAddr>()
+                .ok()
+                .map(canonical)
+                .is_some_and(|ip| ip.is_loopback() || self.bind_ip == Some(ip))
+            || self.allows_extra(host);
+
+        host_ok && port.is_none_or(|port| port == self.port)
+    }
+
+    /// Returns whether `host` matches one of the explicitly allowed extra hosts.
+    /// Entries that are IP literals are compared as addresses, so equivalent
+    /// forms (such as an IPv4-mapped IPv6 literal) match; everything else is
+    /// compared as a case-insensitive hostname.
+    fn allows_extra(&self, host: &str) -> bool {
+        let host_ip = host.parse::<IpAddr>().ok().map(canonical);
+        self.extra_hosts.iter().any(|allowed| {
+            match (host_ip, allowed.parse::<IpAddr>().map(canonical)) {
+                (Some(host_ip), Ok(allowed_ip)) => host_ip == allowed_ip,
+                _ => host.eq_ignore_ascii_case(allowed),
+            }
+        })
+    }
+}
+
+/// Builds the allowlist for a given bind address and any extra allowed hosts.
+/// Returns `None` (validation disabled) when bound to an unspecified
+/// (`0.0.0.0`/`::`) or public address and no extra hosts were given, where
+/// arbitrary hostnames may legitimately resolve to the server. Listing extra
+/// hosts keeps validation on even for those binds.
+pub fn allowed_hosts(bind: IpAddr, port: u16, extra: &[String]) -> Option<AllowedHosts> {
+    let extra_hosts: Vec<String> = extra
+        .iter()
+        .map(|host| normalize_host(host.trim()).to_owned())
+        .filter(|host| !host.is_empty())
+        .collect();
+
+    if bind.is_loopback() {
+        Some(AllowedHosts {
+            port,
+            bind_ip: None,
+            extra_hosts,
+        })
+    } else if is_private_bind(bind) {
+        Some(AllowedHosts {
+            port,
+            bind_ip: Some(canonical(bind)),
+            extra_hosts,
+        })
+    } else if !extra_hosts.is_empty() {
+        // The bind is unspecified or public, where validation is normally
+        // disabled. By listing explicit hosts the user has opted back into it,
+        // so we accept localhost, the bind IP (when it is a specific address),
+        // and those hosts. An unspecified bind (`0.0.0.0`/`::`) has no single
+        // address to add.
+        let bind_ip = (!bind.is_unspecified()).then(|| canonical(bind));
+        Some(AllowedHosts {
+            port,
+            bind_ip,
+            extra_hosts,
+        })
+    } else {
+        None
+    }
+}
+
+/// Collapses an IPv4-mapped IPv6 address (`::ffff:192.168.0.1`) to its IPv4 form
+/// so it classifies and compares consistently with the bare IPv4 address. Shared
+/// with the `/api/open` peer check so it recognizes a mapped loopback peer too.
+pub(crate) fn canonical(ip: IpAddr) -> IpAddr {
+    match ip {
+        IpAddr::V6(v6) => v6.to_ipv4_mapped().map(IpAddr::V4).unwrap_or(ip),
+        v4 => v4,
+    }
+}
+
+/// Returns whether a bind address is a specific private/link-local address, the
+/// case where enforcement stays on with the bind IP added to the allowlist.
+/// Unspecified, loopback, and public addresses are excluded (loopback is handled
+/// separately by [`allowed_hosts`]).
+fn is_private_bind(ip: IpAddr) -> bool {
+    match canonical(ip) {
+        IpAddr::V4(v4) => v4.is_private() || v4.is_link_local(),
+        IpAddr::V6(v6) => {
+            let first = v6.segments()[0];
+            // Unique local (fc00::/7) or link-local (fe80::/10). The std methods
+            // for these are still nightly-only, so test the prefix directly.
+            (first & 0xfe00) == 0xfc00 || (first & 0xffc0) == 0xfe80
+        }
+    }
+}
+
+/// Validates the `Host` and `Origin` headers of an incoming request against the
+/// allowlist. Returns `Some` with a ready-to-send `404` response when the
+/// request should be rejected, or `None` when it is allowed to proceed. When
+/// `allowed` is `None` (unspecified or public bind) every request is accepted.
+pub fn check_request_origin(
+    request: &Request<Body>,
+    allowed: Option<&AllowedHosts>,
+) -> Option<Response<Body>> {
+    let allowed = allowed?;
+
+    // The Host header is mandatory and must refer to a local address.
+    let host_ok = request
+        .headers()
+        .get(HOST)
+        .and_then(|value| value.to_str().ok())
+        .and_then(parse_authority)
+        .is_some_and(|(host, port)| allowed.allows(&host, port));
+
+    if !host_ok {
+        return Some(reject());
+    }
+
+    // The Origin header is optional: non-browser clients such as the Roblox
+    // plugin never send it. When it is present (i.e. a browser made the request)
+    // it must also be local, which rejects a rebound page whose origin is still
+    // its own non-local hostname.
+    if let Some(origin) = request.headers().get(ORIGIN) {
+        let origin_ok = origin
+            .to_str()
+            .ok()
+            .and_then(parse_origin)
+            .is_some_and(|(host, port)| allowed.allows(&host, port));
+
+        if !origin_ok {
+            return Some(reject());
+        }
+    }
+
+    None
+}
+
+/// Normalizes a host literal for parsing/comparison: strips the surrounding
+/// brackets from an IPv6 literal (e.g. `[::1]`) and drops any IPv6 zone id (e.g.
+/// `fe80::1%eth0`), which `Ipv6Addr::from_str` would otherwise reject.
+fn normalize_host(host: &str) -> &str {
+    let host = host
+        .strip_prefix('[')
+        .and_then(|host| host.strip_suffix(']'))
+        .unwrap_or(host);
+
+    host.split('%').next().unwrap_or(host)
+}
+
+/// Parses a `Host` header value (an authority such as `localhost:34872`) into
+/// its host and optional port. Returns `None` if the authority carries userinfo
+/// (e.g. `evil.com@localhost`), so a value whose host looks local only after the
+/// userinfo is stripped can never sneak past the allowlist.
+fn parse_authority(value: &str) -> Option<(String, Option<u16>)> {
+    let authority: Authority = value.parse().ok()?;
+    reject_userinfo(&authority)?;
+    Some((authority.host().to_owned(), authority.port_u16()))
+}
+
+/// Parses an `Origin` header value (an absolute URI such as
+/// `http://localhost:34872`) into its host and optional port. Returns `None` for
+/// origins without a host, such as the opaque `null` origin, or for origins whose
+/// authority carries userinfo (see [`parse_authority`]).
+fn parse_origin(value: &str) -> Option<(String, Option<u16>)> {
+    let uri: Uri = value.parse().ok()?;
+    reject_userinfo(uri.authority()?)?;
+    Some((uri.host()?.to_owned(), uri.port_u16()))
+}
+
+/// Returns `None` (rejecting the value) when an authority contains a userinfo
+/// component, identified by the `@` separator. A bare host or `host:port` never
+/// contains `@`, so this only fires on `userinfo@host` forms.
+fn reject_userinfo(authority: &Authority) -> Option<()> {
+    if authority.as_str().contains('@') {
+        None
+    } else {
+        Some(())
+    }
+}
+
+/// Builds the response sent when a request fails Host/Origin validation. It is a
+/// generic `404` with no Rojo-identifying body: a rejected request may be a
+/// prober (or a DNS-rebound page's same-origin script, which could read the
+/// body), so we reveal nothing rather than confirming a Rojo server is here.
+fn reject() -> Response<Body> {
+    response(StatusCode::NOT_FOUND, "text/plain", "Not Found")
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    use std::net::{Ipv4Addr, Ipv6Addr};
+
+    const PORT: u16 = 34872;
+
+    fn loopback_allowlist() -> Option<AllowedHosts> {
+        allowed_hosts(IpAddr::V4(Ipv4Addr::LOCALHOST), PORT, &[])
+    }
+
+    /// An allowlist for a server bound to a specific private (LAN) address.
+    fn private_allowlist() -> Option<AllowedHosts> {
+        allowed_hosts(IpAddr::V4(Ipv4Addr::new(192, 168, 1, 5)), PORT, &[])
+    }
+
+    /// An allowlist for `bind` with the given extra allowed hosts.
+    fn allowlist_with_hosts(bind: IpAddr, hosts: &[&str]) -> Option<AllowedHosts> {
+        let hosts: Vec<String> = hosts.iter().map(|host| host.to_string()).collect();
+        allowed_hosts(bind, PORT, &hosts)
+    }
+
+    fn request_with(headers: &[(&'static str, &str)]) -> Request<Body> {
+        let mut builder = Request::builder().uri("/api/rojo");
+        for (name, value) in headers {
+            builder = builder.header(*name, *value);
+        }
+        builder.body(Body::empty()).unwrap()
+    }
+
+    #[test]
+    fn loopback_bind_enables_enforcement() {
+        assert!(allowed_hosts(IpAddr::V4(Ipv4Addr::LOCALHOST), PORT, &[]).is_some());
+        assert!(allowed_hosts(IpAddr::V6(Ipv6Addr::LOCALHOST), PORT, &[]).is_some());
+    }
+
+    #[test]
+    fn private_bind_enables_enforcement() {
+        for bind in [
+            IpAddr::V4(Ipv4Addr::new(192, 168, 1, 5)), // 192.168.0.0/16
+            IpAddr::V4(Ipv4Addr::new(10, 0, 0, 1)),    // 10.0.0.0/8
+            IpAddr::V4(Ipv4Addr::new(172, 16, 0, 1)),  // 172.16.0.0/12
+            IpAddr::V4(Ipv4Addr::new(169, 254, 0, 1)), // link-local
+            IpAddr::V6(Ipv6Addr::new(0xfc00, 0, 0, 0, 0, 0, 0, 1)), // unique local
+            IpAddr::V6(Ipv6Addr::new(0xfe80, 0, 0, 0, 0, 0, 0, 1)), // link-local
+        ] {
+            assert!(
+                allowed_hosts(bind, PORT, &[]).is_some(),
+                "private bind {bind} should enable enforcement"
+            );
+        }
+    }
+
+    #[test]
+    fn unspecified_or_public_bind_disables_enforcement() {
+        for bind in [
+            IpAddr::V4(Ipv4Addr::UNSPECIFIED),     // 0.0.0.0
+            IpAddr::V6(Ipv6Addr::UNSPECIFIED),     // ::
+            IpAddr::V4(Ipv4Addr::new(8, 8, 8, 8)), // public
+            IpAddr::V6(Ipv6Addr::new(0x2001, 0x4860, 0, 0, 0, 0, 0, 0x8888)), // public
+        ] {
+            assert!(
+                allowed_hosts(bind, PORT, &[]).is_none(),
+                "bind {bind} should disable enforcement"
+            );
+        }
+    }
+
+    #[test]
+    fn accepts_local_hosts() {
+        let allowed = loopback_allowlist();
+        for host in [
+            format!("localhost:{PORT}"),
+            format!("127.0.0.1:{PORT}"),
+            format!("[::1]:{PORT}"),
+            "localhost".to_owned(),
+        ] {
+            let request = request_with(&[("host", &host)]);
+            assert!(
+                check_request_origin(&request, allowed.as_ref()).is_none(),
+                "host {host} should be allowed"
+            );
+        }
+    }
+
+    #[test]
+    fn rejects_foreign_host() {
+        let allowed = loopback_allowlist();
+        let request = request_with(&[("host", "evil.com")]);
+        assert!(check_request_origin(&request, allowed.as_ref()).is_some());
+    }
+
+    #[test]
+    fn rejects_wrong_port() {
+        let allowed = loopback_allowlist();
+        let request = request_with(&[("host", "localhost:1234")]);
+        assert!(check_request_origin(&request, allowed.as_ref()).is_some());
+    }
+
+    #[test]
+    fn rejects_missing_host() {
+        let allowed = loopback_allowlist();
+        let request = request_with(&[]);
+        assert!(check_request_origin(&request, allowed.as_ref()).is_some());
+    }
+
+    #[test]
+    fn rejects_host_with_userinfo() {
+        let allowed = loopback_allowlist();
+        // The host parses to `localhost`, but the userinfo prefix must keep it
+        // from being treated as a local address.
+        let request = request_with(&[("host", &format!("evil.com@localhost:{PORT}"))]);
+        assert!(check_request_origin(&request, allowed.as_ref()).is_some());
+    }
+
+    #[test]
+    fn rejects_origin_with_userinfo() {
+        let allowed = loopback_allowlist();
+        let request = request_with(&[
+            ("host", &format!("localhost:{PORT}")),
+            ("origin", &format!("http://evil.com@localhost:{PORT}")),
+        ]);
+        assert!(check_request_origin(&request, allowed.as_ref()).is_some());
+    }
+
+    #[test]
+    fn rejects_foreign_origin_even_with_local_host() {
+        let allowed = loopback_allowlist();
+        let request = request_with(&[
+            ("host", &format!("localhost:{PORT}")),
+            ("origin", &format!("http://evil.com:{PORT}")),
+        ]);
+        assert!(check_request_origin(&request, allowed.as_ref()).is_some());
+    }
+
+    #[test]
+    fn rejects_null_origin() {
+        let allowed = loopback_allowlist();
+        let request = request_with(&[("host", &format!("localhost:{PORT}")), ("origin", "null")]);
+        assert!(check_request_origin(&request, allowed.as_ref()).is_some());
+    }
+
+    #[test]
+    fn accepts_local_origin() {
+        let allowed = loopback_allowlist();
+        let request = request_with(&[
+            ("host", &format!("localhost:{PORT}")),
+            ("origin", &format!("http://localhost:{PORT}")),
+        ]);
+        assert!(check_request_origin(&request, allowed.as_ref()).is_none());
+    }
+
+    #[test]
+    fn private_bind_accepts_local_and_bind_ip_hosts() {
+        let allowed = private_allowlist();
+        for host in [
+            format!("192.168.1.5:{PORT}"),
+            format!("localhost:{PORT}"),
+            format!("127.0.0.1:{PORT}"),
+            format!("[::1]:{PORT}"),
+            "192.168.1.5".to_owned(),
+        ] {
+            let request = request_with(&[("host", &host)]);
+            assert!(
+                check_request_origin(&request, allowed.as_ref()).is_none(),
+                "host {host} should be allowed on a private bind"
+            );
+        }
+    }
+
+    #[test]
+    fn private_bind_rejects_other_hosts() {
+        let allowed = private_allowlist();
+        for host in [
+            "evil.com",    // a rebound attacker domain
+            "192.168.1.6", // a different private IP
+            "8.8.8.8",     // a public IP
+        ] {
+            let request = request_with(&[("host", host)]);
+            assert!(
+                check_request_origin(&request, allowed.as_ref()).is_some(),
+                "host {host} should be rejected on a private bind"
+            );
+        }
+    }
+
+    #[test]
+    fn private_bind_keeps_origin_strict() {
+        // A different private IP as Origin must be rejected even though the Host
+        // is the valid bind IP: the Origin check is not widened to arbitrary
+        // private addresses.
+        let allowed = private_allowlist();
+        let request = request_with(&[
+            ("host", &format!("192.168.1.5:{PORT}")),
+            ("origin", &format!("http://192.168.1.6:{PORT}")),
+        ]);
+        assert!(check_request_origin(&request, allowed.as_ref()).is_some());
+    }
+
+    #[test]
+    fn private_bind_accepts_bind_ip_origin() {
+        let allowed = private_allowlist();
+        let request = request_with(&[
+            ("host", &format!("192.168.1.5:{PORT}")),
+            ("origin", &format!("http://192.168.1.5:{PORT}")),
+        ]);
+        assert!(check_request_origin(&request, allowed.as_ref()).is_none());
+    }
+
+    #[test]
+    fn accepts_ipv4_mapped_bind_ip() {
+        // `::ffff:192.168.1.5` is the IPv4-mapped form of the bind IP and must
+        // be treated as equal to it.
+        let allowed = private_allowlist();
+        let request = request_with(&[("host", &format!("[::ffff:192.168.1.5]:{PORT}"))]);
+        assert!(check_request_origin(&request, allowed.as_ref()).is_none());
+    }
+
+    #[test]
+    fn canonical_collapses_ipv4_mapped_loopback() {
+        // The `/api/open` peer gate relies on this: an IPv4 loopback client
+        // reaching a dual-stack (`::`) bind arrives as `::ffff:127.0.0.1`, which
+        // is only recognized as loopback after canonicalization.
+        let mapped: IpAddr = "::ffff:127.0.0.1".parse().unwrap();
+        assert!(!mapped.is_loopback());
+        assert!(canonical(mapped).is_loopback());
+    }
+
+    #[test]
+    fn normalize_host_strips_brackets_and_zone_id() {
+        // Brackets and an IPv6 zone id must be removed so the result parses as an
+        // `IpAddr` (`Ipv6Addr::from_str` rejects zone ids).
+        assert_eq!(normalize_host("[::1]"), "::1");
+        assert_eq!(normalize_host("[fe80::1%eth0]"), "fe80::1");
+        assert_eq!(normalize_host("localhost"), "localhost");
+        assert!(normalize_host("[fe80::1%eth0]").parse::<IpAddr>().is_ok());
+    }
+
+    #[test]
+    fn allowed_hosts_extend_the_allowlist() {
+        // A hostname listed as an allowed host is accepted on a loopback bind,
+        // with the usual port check still applied; an unlisted host is not.
+        let allowed = allowlist_with_hosts(IpAddr::V4(Ipv4Addr::LOCALHOST), &["mypc.lan"]);
+
+        let ok = request_with(&[("host", &format!("mypc.lan:{PORT}"))]);
+        assert!(check_request_origin(&ok, allowed.as_ref()).is_none());
+
+        let wrong_port = request_with(&[("host", "mypc.lan:1234")]);
+        assert!(check_request_origin(&wrong_port, allowed.as_ref()).is_some());
+
+        let other = request_with(&[("host", &format!("other.lan:{PORT}"))]);
+        assert!(check_request_origin(&other, allowed.as_ref()).is_some());
+    }
+
+    #[test]
+    fn allowed_hosts_apply_to_origin() {
+        let allowed = allowlist_with_hosts(IpAddr::V4(Ipv4Addr::LOCALHOST), &["mypc.lan"]);
+
+        let ok = request_with(&[
+            ("host", &format!("mypc.lan:{PORT}")),
+            ("origin", &format!("http://mypc.lan:{PORT}")),
+        ]);
+        assert!(check_request_origin(&ok, allowed.as_ref()).is_none());
+
+        let foreign_origin = request_with(&[
+            ("host", &format!("mypc.lan:{PORT}")),
+            ("origin", &format!("http://evil.com:{PORT}")),
+        ]);
+        assert!(check_request_origin(&foreign_origin, allowed.as_ref()).is_some());
+    }
+
+    #[test]
+    fn allowed_hosts_enable_enforcement_on_exposed_bind() {
+        // Binding to 0.0.0.0 normally disables validation, but listing a host
+        // turns it back on: localhost and the listed host are accepted while
+        // everything else is rejected.
+        let allowed = allowlist_with_hosts(IpAddr::V4(Ipv4Addr::UNSPECIFIED), &["mypc.lan"]);
+        assert!(allowed.is_some());
+
+        for host in [format!("mypc.lan:{PORT}"), format!("localhost:{PORT}")] {
+            let request = request_with(&[("host", &host)]);
+            assert!(
+                check_request_origin(&request, allowed.as_ref()).is_none(),
+                "host {host} should be allowed"
+            );
+        }
+
+        let evil = request_with(&[("host", "evil.com")]);
+        assert!(check_request_origin(&evil, allowed.as_ref()).is_some());
+    }
+
+    #[test]
+    fn allowed_hosts_on_public_bind_accept_the_bind_ip() {
+        // A specific public bind with an allowed host accepts localhost, the
+        // listed host, and the bind IP itself, but nothing else. (203.0.113.0/24
+        // is the TEST-NET-3 documentation range, treated here as a public IP.)
+        let bind = IpAddr::V4(Ipv4Addr::new(203, 0, 113, 5));
+        let allowed = allowlist_with_hosts(bind, &["mypc.lan"]);
+
+        for host in [
+            format!("203.0.113.5:{PORT}"),
+            format!("mypc.lan:{PORT}"),
+            format!("localhost:{PORT}"),
+        ] {
+            let request = request_with(&[("host", &host)]);
+            assert!(
+                check_request_origin(&request, allowed.as_ref()).is_none(),
+                "host {host} should be allowed"
+            );
+        }
+
+        let evil = request_with(&[("host", "evil.com")]);
+        assert!(check_request_origin(&evil, allowed.as_ref()).is_some());
+    }
+
+    #[test]
+    fn disabled_allowlist_accepts_foreign_host() {
+        for bind in [
+            IpAddr::V4(Ipv4Addr::UNSPECIFIED),
+            IpAddr::V4(Ipv4Addr::new(8, 8, 8, 8)),
+        ] {
+            let allowed = allowed_hosts(bind, PORT, &[]);
+            let request = request_with(&[("host", "evil.com")]);
+            assert!(
+                check_request_origin(&request, allowed.as_ref()).is_none(),
+                "disabled allowlist for bind {bind} should accept any host"
+            );
+        }
+    }
+}

tests/rojo_test/serve_util.rs

@@ -126,6 +126,10 @@ impl TestServeSession {
         &self.project_path
     }
 
+    pub fn port(&self) -> usize {
+        self.port
+    }
+
     /// Waits for the `rojo serve` server to come online with expontential
     /// backoff.
     pub fn wait_to_come_online(&mut self) -> ServerInfoResponse {
@@ -241,6 +245,39 @@ impl TestServeSession {
 
         Ok(deserialize_msgpack(&body).expect("Server returned malformed response"))
     }
+
+    /// Sends a GET to `/api/rojo` with the given extra request headers and
+    /// returns the full response. Used to exercise the Host/Origin allowlist that
+    /// guards against DNS rebinding, including asserting that a rejection reveals
+    /// nothing about the server.
+    pub fn api_rojo_response_with_headers(
+        &self,
+        headers: &[(&str, &str)],
+    ) -> reqwest::blocking::Response {
+        let client = reqwest::blocking::Client::new();
+        let url = format!("http://localhost:{}/api/rojo", self.port);
+
+        let mut request = client.get(url);
+        for (name, value) in headers {
+            request = request.header(*name, *value);
+        }
+
+        request.send().expect("Failed to send request")
+    }
+
+    /// Sends a POST to `/api/open/<id>` and returns the response status code.
+    /// Used to verify that the local-only gate on `/api/open` admits loopback
+    /// peers (the test harness always connects over loopback).
+    pub fn api_open_status(&self, id: &str) -> reqwest::StatusCode {
+        let client = reqwest::blocking::Client::new();
+        let url = format!("http://localhost:{}/api/open/{}", self.port, id);
+
+        client
+            .post(url)
+            .send()
+            .expect("Failed to send request")
+            .status()
+    }
 }
 
 fn serialize_msgpack<T: Serialize>(value: T) -> Result<Vec<u8>, rmp_serde::encode::Error> {

tests/tests/serve.rs

@@ -10,6 +10,74 @@ use crate::rojo_test::{
 
 use librojo::web_api::SocketPacketType;
 
+#[test]
+fn rejects_dns_rebinding_requests() {
+    run_serve_test("empty", |session, _redactions| {
+        let port = session.port();
+        let local_host = format!("localhost:{port}");
+
+        // A request carrying a local Host header is served normally.
+        assert_eq!(
+            session
+                .api_rojo_response_with_headers(&[("host", &local_host)])
+                .status(),
+            reqwest::StatusCode::OK,
+        );
+
+        // A request whose Host is a foreign hostname, as a DNS-rebound page
+        // would send, is rejected with a generic 404 that reveals nothing about
+        // the server.
+        assert_rejected(session.api_rojo_response_with_headers(&[("host", "evil.com")]));
+
+        // Even with a local Host, a present-but-foreign Origin is rejected.
+        let foreign_origin = format!("http://evil.com:{port}");
+        assert_rejected(
+            session.api_rojo_response_with_headers(&[
+                ("host", &local_host),
+                ("origin", &foreign_origin),
+            ]),
+        );
+    });
+}
+
+/// Asserts that a Host/Origin rejection is a generic 404 whose body and
+/// content-type do not identify the server as Rojo.
+fn assert_rejected(response: reqwest::blocking::Response) {
+    assert_eq!(response.status(), reqwest::StatusCode::NOT_FOUND);
+
+    let content_type = response
+        .headers()
+        .get(reqwest::header::CONTENT_TYPE)
+        .and_then(|value| value.to_str().ok())
+        .unwrap_or_default()
+        .to_owned();
+    assert!(
+        !content_type.contains("msgpack"),
+        "rejection should not use the msgpack API content-type, got {content_type:?}",
+    );
+
+    let body = response.text().expect("Failed to read response body");
+    let body_lower = body.to_lowercase();
+    assert!(
+        !body_lower.contains("rojo") && !body_lower.contains("rebinding"),
+        "rejection body should not identify the server, got {body:?}",
+    );
+}
+
+#[test]
+fn allows_api_open_from_loopback_peer() {
+    run_serve_test("empty", |session, _redactions| {
+        // The harness always connects over loopback, so the local-only gate on
+        // /api/open must let the request through. A bogus instance id then fails
+        // id parsing with 400, which confirms we got past the gate rather than
+        // being rejected with 403.
+        assert_eq!(
+            session.api_open_status("not-a-real-ref"),
+            reqwest::StatusCode::BAD_REQUEST,
+        );
+    });
+}
+
 #[test]
 fn empty() {
     run_serve_test("empty", |session, mut redactions| {