hyperhive/swarm-nats-auth/src/introspect.rs

//! Validating a presented bearer token against the `IdP`, and learning who
//! presented it.
//!
//! RFC 7662 token introspection: POST the token to authelia, which answers
//! `{"active": true|false, "client_id": "...", ...}`. `active` is the whole
//! **admission** verdict — this responder does not second-guess it, and does
//! not inspect scopes, because a second place that decides who may connect is
//! a second place to get it wrong.
//!
//! # Everything that is not an explicit `active: true` is a denial
//!
//! Network error, timeout, non-2xx, unparseable body, `active: false` — all
//! deny. Not defensive coding but the only safe shape: the failure modes of
//! an HTTP call are exactly when an attacker would like this to fall open.
//!
//! # `client_id` is modelled, and is not a second admission check
//!
//! `sub`/`scope`/`exp` stay unmodelled: modelling a field implies checking
//! it. `client_id` differs in kind — `active` decides **whether** to admit,
//! `client_id` decides **as whom**, which is what lets a grant be scoped to
//! one hive. Admission stays one decision in one place; the identity is
//! returned rather than a bool so "admitted but unscoped" cannot exist.
//!
//! # The timeout is not a tuning knob
//!
//! `swarm-nats.nix` sets `authorization.timeout = "2s"`, so the server stops
//! waiting after two seconds and denies the connection anyway. An
//! introspection call allowed to run longer than that cannot produce a useful
//! answer — it can only hold a task open past the point where the result
//! matters. The budget below is deliberately under the server's.

use std::time::Duration;

use anyhow::{Context, Result};
use serde::Deserialize;
use tokio::time::Instant;

/// Kept under `swarm-nats.nix`'s `authorization.timeout = "2s"` — see the
/// module docs. A slower answer is not a late success, it is a denial that
/// already happened. It bounds a whole request, the re-read of this
/// responder's own secret and the retry after it included, which is why
/// [`identify_caller`] takes a deadline rather than a duration.
pub const INTROSPECTION_TIMEOUT: Duration = Duration::from_millis(1500);

/// What one introspection call said.
#[derive(Debug, PartialEq, Eq)]
pub enum Answer {
    /// A 2xx: the caller, or `None` for every shape that is not an admission.
    Caller(Option<String>),
    /// A non-2xx, with its status: the `IdP` did not answer about the token
    /// at all. A 400 or 401 is how it refuses this responder's own
    /// credential, which `oidc_secret` re-reads and retries once before this
    /// denies.
    Refused(reqwest::StatusCode),
}

/// The two fields this responder acts on — one per decision. Authelia returns
/// more (`sub`, `scope`, `exp`); modelling them would imply we check them.
#[derive(Debug, Deserialize)]
struct IntrospectionResponse {
    /// Whether to admit at all.
    active: bool,
    /// Which client presented the token, i.e. which hive to scope its grant
    /// to. Optional on the wire (RFC 7662 makes every field but `active`
    /// optional), and its absence is a denial rather than a default — see
    /// [`IntrospectionResponse::caller`].
    #[serde(default)]
    client_id: Option<String>,
}

impl IntrospectionResponse {
    /// The identity to admit this token as, or `None` for every shape that is
    /// not an admission.
    ///
    /// An inactive token has no identity here even when the body names one: a
    /// `client_id` on an expired token says who it *was* minted for, which is
    /// not a statement that the bearer may connect.
    ///
    /// An empty `client_id` is treated as absent. It would otherwise become an
    /// empty component in a subject the grant is scoped to, and a scope
    /// assembled from a blank is not a narrower permission, it is a different
    /// one nobody reviewed.
    fn caller(&self) -> Option<&str> {
        if !self.active {
            return None;
        }
        self.client_id.as_deref().filter(|id| !id.is_empty())
    }
}

/// Ask the `IdP` whether `token` is currently valid, and as whom.
///
/// Returns `Ok(Answer::Caller(Some(client_id)))` **only** on a 2xx whose
/// body says `active: true` and names the client. Every other 2xx is
/// `Ok(Answer::Caller(None))` with the reason logged, a non-2xx is
/// `Ok(Answer::Refused(_))`, and `Err` is a call that could not be made at all
/// or did not finish by `deadline` — callers must treat all but the first as
/// a denial.
///
/// `own_client_id` / `own_client_secret` are *this responder's* credential for
/// the introspection endpoint. They are not the caller's identity and must
/// never be used as one; the caller's is the return value.
///
/// The token is never logged, not even truncated: a prefix is enough to
/// correlate across logs, and a credential that is 80% redacted is still a
/// credential in a journal.
pub async fn identify_caller(
    http: &reqwest::Client,
    url: &str,
    own_client_id: &str,
    own_client_secret: &str,
    token: &str,
    deadline: Instant,
) -> Result<Answer> {
    let resp = http
        .post(url)
        .basic_auth(own_client_id, Some(own_client_secret))
        .form(&[("token", token)])
        .timeout(deadline.saturating_duration_since(Instant::now()))
        .send()
        .await
        .context("introspection request")?;

    let status = resp.status();
    if !status.is_success() {
        tracing::warn!(%status, "introspection returned non-2xx");
        return Ok(Answer::Refused(status));
    }
    let body: IntrospectionResponse = match resp.json().await {
        Ok(b) => b,
        Err(e) => {
            tracing::warn!(error = ?e, "introspection body did not parse; denying");
            return Ok(Answer::Caller(None));
        }
    };
    if body.active && body.caller().is_none() {
        // Loud on purpose: this is a token the `IdP` says is good, refused by
        // us. Without a line here the operator sees a working credential
        // rejected for no visible reason.
        tracing::warn!("introspection said active but named no client; denying");
    }
    Ok(Answer::Caller(body.caller().map(str::to_owned)))
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn the_timeout_stays_under_the_servers() {
        // `swarm-nats.nix` sets `authorization.timeout = "2s"`. If someone
        // raises this constant past that, the extra time buys nothing: the
        // server has already denied the connection. This test is the only
        // thing tying the two numbers together, since they live in different
        // languages in different files.
        assert!(
            INTROSPECTION_TIMEOUT < Duration::from_secs(2),
            "introspection budget must stay under the server's auth_callout timeout"
        );
    }

    fn parse(body: &str) -> IntrospectionResponse {
        serde_json::from_str(body).expect("introspection body")
    }

    #[test]
    fn only_active_true_deserializes_to_a_grant() {
        assert!(parse(r#"{"active":true,"client_id":"hive-alpha"}"#).active);
        assert!(!parse(r#"{"active":false}"#).active);
        // A body that omits `active` is not a grant with a default - it is a
        // response we do not understand, and it must fail to parse rather
        // than deserialize to `false` quietly under some future `#[serde(default)]`.
        assert!(serde_json::from_str::<IntrospectionResponse>(r#"{"sub":"someone"}"#).is_err());
    }

    #[test]
    fn an_active_token_is_admitted_as_the_client_that_holds_it() {
        assert_eq!(
            parse(r#"{"active":true,"client_id":"hive-alpha","scope":"x"}"#).caller(),
            Some("hive-alpha")
        );
    }

    #[test]
    fn active_without_a_client_id_is_a_denial_not_an_unscoped_grant() {
        // The invariant the whole scoping design rests on. If this ever
        // returns `Some`, a token the `IdP` could not attribute would be
        // admitted with whatever scope the caller assembles from nothing -
        // which is how a per-hive permission becomes a swarm-wide one.
        assert_eq!(parse(r#"{"active":true}"#).caller(), None);
        assert_eq!(parse(r#"{"active":true,"client_id":""}"#).caller(), None);
    }

    #[test]
    fn an_inactive_token_has_no_identity_even_when_the_body_names_one() {
        // Admission is decided by `active` alone. A `client_id` alongside
        // `active: false` says who the token *was* for, and reading it as an
        // identity would turn an expired credential into a working one.
        assert_eq!(
            parse(r#"{"active":false,"client_id":"hive-alpha"}"#).caller(),
            None
        );
    }
}