Verify Cloudflare Access's signed token before trusting the proxy

The proxy sign-in believed Cf-Access-Authenticated-User-Email from any
address in trusted_proxies. On Tower that address is the Docker gateway,
so any container there could name itself anyone (docs/sso.md said as
much, and CLAUDE.md listed it as a known gap).

With [web] access_team and access_aud set, a proxied request must also
carry a Cf-Access-Jwt-Assertion that verifies against Cloudflare's keys
(RS256 only, this application's audience, the team's issuer, not
expired), and the name comes from its email claim. The keys are fetched
at start and again when a token names an unseen key, at most once a
minute, so made-up key ids cannot make every request a request to
Cloudflare. While the keys cannot be had, proxied sign-in is refused;
password and token sign-in are unaffected. Both settings empty, nothing
changes.

jsonwebtoken does the checking, on the aws-lc-rs backend already in the
tree through rustls. Tests sign with throwaway keys in tests/data: a
valid token, another app's audience, expired, a forged signature, HS256,
alg none, the refetch limit, and keys that cannot be fetched. Checked
live on a scratch daemon: the header alone and a forged token got 401,
the admin token still signed in.

vouched_name takes the peer and headers rather than the request: a
&Request held across the new await made the auth middleware's future
unsendable, as a body is not Sync.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/web.rs

@@ -29,6 +29,8 @@ pub struct WebState {
     pub ctx: Arc<Ctx>,
     pub cmds: mpsc::Sender<Command>,
     pub events: broadcast::Sender<Event>,
+    /// Cloudflare Access's signing keys, fetched once and kept.
+    pub access: Arc<crate::access::Keys>,
 }
 
 pub fn router(state: WebState) -> Router {
@@ -99,7 +101,7 @@ async fn auth(State(state): State<WebState>, mut req: Request, next: Next) -> Re
     let token = cfg.web.token.clone();
 
     // 1. A header, but only from a hop we were told to believe.
-    let vouched = vouched_name(&cfg, &req);
+    let vouched = vouched_name(&state, &cfg, peer(&req), req.headers()).await;
 
     let mut set_cookie: Option<String> = None;
     let mut user = None;
@@ -201,20 +203,31 @@ struct Proxied(bool);
 
 /// The name the proxy vouches for, when this request came from one of `trusted_proxies` and
 /// carries `trusted_header`. Anyone able to reach the port could otherwise send the header and
-/// be whoever they liked.
-fn vouched_name(cfg: &crate::config::Config, req: &Request) -> Option<String> {
-    let peer = req
-        .extensions()
-        .get::<axum::extract::ConnectInfo<std::net::SocketAddr>>()
-        .map(|c| c.0.ip().to_string())
-        .unwrap_or_default();
+/// be whoever they liked. With `access_team` and `access_aud` set, it also has to carry a
+/// token Cloudflare Access signed, and the name is the one in the token.
+///
+/// Takes the request's parts rather than the request: a `&Request` held across the await makes
+/// the future unsendable, as a body is not `Sync`.
+async fn vouched_name(state: &WebState, cfg: &crate::config::Config, peer: String, headers: &axum::http::HeaderMap) -> Option<String> {
     if cfg.web.trusted_header.is_empty() || !cfg.web.trusted_proxies.iter().any(|p| p == &peer) {
         return None;
     }
-    req.headers()
-        .get(&cfg.web.trusted_header)
-        .and_then(|v| v.to_str().ok())
-        .and_then(crate::auth::name_from_header)
+    let header = |name: &str| headers.get(name).and_then(|v| v.to_str().ok());
+    let Some((team, aud)) = cfg.web.access() else {
+        return header(&cfg.web.trusted_header).and_then(crate::auth::name_from_header);
+    };
+    // The plain header still has to be there, as it is what switches this path on for a
+    // request; who it names is the token's to say.
+    header(&cfg.web.trusted_header)?;
+    let token = header("Cf-Access-Jwt-Assertion")?;
+    state.access.verify(&state.ctx.client, team, aud, token).await
+}
+
+fn peer(req: &Request) -> String {
+    req.extensions()
+        .get::<axum::extract::ConnectInfo<std::net::SocketAddr>>()
+        .map(|c| c.0.ip().to_string())
+        .unwrap_or_default()
 }
 
 /// Handlers take `User` to say they need one; the auth layer put it there, and nothing
@@ -474,7 +487,7 @@ async fn remove_user(
 /// The password form, except for someone the proxy vouches for: they are signed in already, and
 /// the form only made it look as if they were not.
 async fn login_page(State(state): State<WebState>, req: Request) -> Response {
-    if vouched_name(&state.ctx.cfg(), &req).is_some() {
+    if vouched_name(&state, &state.ctx.cfg(), peer(&req), req.headers()).await.is_some() {
         return Redirect::to("/").into_response();
     }
     ([(header::CACHE_CONTROL, PAGE_CACHE)], Html(include_str!(concat!(env!("OUT_DIR"), "/login.html"))))