csrf implemented

src/app.rs

@@ -68,6 +68,12 @@ impl Hooks for App {
             .layer(axum::middleware::from_fn_with_state(
                 ctx.clone(),
                 crate::shared::rbac::inject_subject,
+            ))
+            // CSRF runs outermost so it validates the double-submit token before
+            // any handler sees the request and stamps the cookie on safe ones.
+            .layer(axum::middleware::from_fn_with_state(
+                ctx.clone(),
+                crate::shared::csrf::protect,
             )))
     }
 

src/shared/csrf.rs

@@ -0,0 +1,279 @@
+//! Stateless CSRF protection (signed double-submit cookie).
+//!
+//! Authentication is cookie-based (the `auth_token` JWT cookie, see
+//! [`crate::shared::guard`]), so any state-changing request the browser can be
+//! tricked into making carries the victim's credentials. `SameSite=Lax` on the
+//! auth cookie already blocks the cross-site *form* POST case; this layer is the
+//! defense-in-depth on top of it.
+//!
+//! ## How it works
+//! On every safe request we ensure the browser holds a `csrf_token` cookie whose
+//! value is `<random>.<hmac>` — the HMAC is keyed by the app's JWT secret, so the
+//! server can recognise its own tokens without storing any per-session state
+//! (no session table, survives restarts and multiple instances). On every unsafe
+//! request ([`protect`]) the same token must be echoed back, either as the
+//! `X-CSRF-Token` header (htmx requests) or a `_csrf` form field (native
+//! `<form>` submits, which cannot set a custom header). Because a cross-origin
+//! attacker can neither read the cookie nor forge a valid signature, they cannot
+//! produce a matching echo.
+//!
+//! The browser side lives in the two base templates (`base.html`,
+//! `admin/base.html`): an `htmx:configRequest` hook adds the header and a
+//! `submit` hook injects the hidden `_csrf` field.
+//!
+//! `/api/*` is exempt: that subtree is the token-authenticated JSON API and the
+//! OAuth2 callback, neither of which is driven by the browser session cookie.
+
+use axum::{
+    body::{to_bytes, Body},
+    extract::{Request, State},
+    http::{header, Method, StatusCode},
+    middleware::Next,
+    response::{IntoResponse, Response},
+};
+use axum_extra::extract::cookie::{Cookie, CookieJar, SameSite};
+use bytes::Bytes;
+use hmac::{Hmac, Mac};
+use loco_rs::prelude::*;
+use sha2::Sha256;
+use subtle::ConstantTimeEq;
+use time::Duration as TimeDuration;
+use uuid::Uuid;
+
+/// Cookie that holds the signed token. Deliberately *not* `HttpOnly`: the page
+/// JS has to read it to echo it back in the header / hidden field.
+pub const CSRF_COOKIE: &str = "csrf_token";
+/// Header carrying the echoed token on htmx (and any scripted) requests.
+pub const CSRF_HEADER: &str = "x-csrf-token";
+/// Hidden form field carrying the echoed token on native `<form>` submits.
+pub const CSRF_FIELD: &str = "_csrf";
+
+/// Cookie lifetime. Long enough to outlast a normal browsing session; the token
+/// only needs to be stable, not short-lived (it is not a credential on its own).
+const COOKIE_MAX_AGE_SECS: i64 = 60 * 60 * 24 * 14;
+/// Upper bound on a body we will buffer to find the `_csrf` field. Covers the
+/// largest native multipart submit (a 10 MiB image upload plus the other
+/// fields); anything bigger is rejected rather than buffered.
+const MAX_BODY_BYTES: usize = 16 * 1024 * 1024;
+
+type HmacSha256 = Hmac<Sha256>;
+
+fn to_hex(bytes: &[u8]) -> String {
+    let mut s = String::with_capacity(bytes.len() * 2);
+    for b in bytes {
+        s.push_str(&format!("{b:02x}"));
+    }
+    s
+}
+
+/// HMAC-SHA256 of the random part, keyed by the app secret.
+fn sign(secret: &str, random: &str) -> String {
+    let mut mac =
+        HmacSha256::new_from_slice(secret.as_bytes()).expect("HMAC accepts a key of any length");
+    mac.update(random.as_bytes());
+    to_hex(&mac.finalize().into_bytes())
+}
+
+/// Mint a fresh `<random>.<hmac>` token.
+pub fn make_token(secret: &str) -> String {
+    let random = Uuid::new_v4().simple().to_string();
+    let sig = sign(secret, &random);
+    format!("{random}.{sig}")
+}
+
+/// True when `token` is well-formed and its signature is the one this server
+/// would produce — i.e. it is one of *our* tokens, not an attacker-injected one.
+fn signature_valid(secret: &str, token: &str) -> bool {
+    let Some((random, sig)) = token.split_once('.') else {
+        return false;
+    };
+    let expected = sign(secret, random);
+    expected.as_bytes().ct_eq(sig.as_bytes()).into()
+}
+
+/// Constant-time equality of two full tokens (the double-submit comparison).
+fn tokens_match(a: &str, b: &str) -> bool {
+    a.as_bytes().ct_eq(b.as_bytes()).into()
+}
+
+fn issue_cookie(token: String) -> Cookie<'static> {
+    Cookie::build((CSRF_COOKIE, token))
+        .path("/")
+        .http_only(false)
+        .same_site(SameSite::Lax)
+        .max_age(TimeDuration::seconds(COOKIE_MAX_AGE_SECS))
+        .build()
+}
+
+fn forbidden(reason: &str) -> Response {
+    tracing::debug!(reason, "CSRF check rejected request");
+    (StatusCode::FORBIDDEN, "CSRF validation failed").into_response()
+}
+
+/// Attach a freshly minted token cookie to an outgoing response.
+fn set_fresh_cookie(res: &mut Response, secret: &str) {
+    let cookie = issue_cookie(make_token(secret));
+    if let Ok(value) = cookie.encoded().to_string().parse() {
+        res.headers_mut().append(header::SET_COOKIE, value);
+    }
+}
+
+/// Pull the `_csrf` value out of an `application/x-www-form-urlencoded` body.
+fn field_from_urlencoded(bytes: &[u8]) -> Option<String> {
+    form_urlencoded::parse(bytes)
+        .find(|(k, _)| k == CSRF_FIELD)
+        .map(|(_, v)| v.into_owned())
+}
+
+/// Pull the `_csrf` value out of a `multipart/form-data` body. Parses a *copy*
+/// of the buffered bytes purely to read the field; the original bytes are still
+/// forwarded to the handler unchanged.
+async fn field_from_multipart(content_type: &str, bytes: Bytes) -> Option<String> {
+    let boundary = multer::parse_boundary(content_type).ok()?;
+    let stream = futures_util::stream::once(async move { Ok::<_, std::io::Error>(bytes) });
+    let mut multipart = multer::Multipart::new(stream, boundary);
+    while let Ok(Some(field)) = multipart.next_field().await {
+        if field.name() == Some(CSRF_FIELD) {
+            return field.text().await.ok();
+        }
+    }
+    None
+}
+
+/// CSRF enforcement middleware. Safe methods get a token cookie (minted if
+/// missing); unsafe methods must echo a valid, matching token. See the module
+/// docs for the full scheme.
+pub async fn protect(
+    State(ctx): State<AppContext>,
+    jar: CookieJar,
+    req: Request,
+    next: Next,
+) -> Response {
+    // The token is keyed by the JWT secret. If no secret is configured we cannot
+    // validate, so fail open rather than 403 the whole site — the same secret is
+    // already required for auth to work at all.
+    let Ok(jwt) = ctx.config.get_jwt_config() else {
+        return next.run(req).await;
+    };
+    let secret = jwt.secret.clone();
+
+    // The token the browser currently holds, accepted only if untampered.
+    let cookie_token = jar
+        .get(CSRF_COOKIE)
+        .map(|c| c.value().to_string())
+        .filter(|t| signature_valid(&secret, t));
+
+    let is_safe = matches!(
+        *req.method(),
+        Method::GET | Method::HEAD | Method::OPTIONS | Method::TRACE
+    );
+    // Token-auth JSON API + OAuth2 callback: not browser-cookie-driven.
+    let exempt = req.uri().path().starts_with("/api/");
+
+    if is_safe || exempt {
+        let mut res = next.run(req).await;
+        if cookie_token.is_none() {
+            set_fresh_cookie(&mut res, &secret);
+        }
+        return res;
+    }
+
+    // ---- unsafe, non-exempt: require a valid double-submit ----
+    let Some(expected) = cookie_token else {
+        return forbidden("missing or tampered CSRF cookie");
+    };
+
+    // htmx and other scripted requests send the token as a header; prefer it so
+    // we never have to touch the body.
+    if let Some(header_token) = req.headers().get(CSRF_HEADER).and_then(|v| v.to_str().ok()) {
+        if tokens_match(header_token, &expected) {
+            return next.run(req).await;
+        }
+        return forbidden("CSRF header did not match cookie");
+    }
+
+    // Native <form> submit: the token is a `_csrf` body field. Buffer the body,
+    // read the field from a copy, then forward the original bytes untouched.
+    let content_type = req
+        .headers()
+        .get(header::CONTENT_TYPE)
+        .and_then(|v| v.to_str().ok())
+        .unwrap_or("")
+        .to_string();
+
+    let (parts, body) = req.into_parts();
+    let Ok(bytes) = to_bytes(body, MAX_BODY_BYTES).await else {
+        return forbidden("request body too large to validate CSRF");
+    };
+
+    let submitted = if content_type.starts_with("application/x-www-form-urlencoded") {
+        field_from_urlencoded(&bytes)
+    } else if content_type.starts_with("multipart/form-data") {
+        field_from_multipart(&content_type, bytes.clone()).await
+    } else {
+        None
+    };
+
+    let valid = submitted
+        .as_deref()
+        .is_some_and(|t| tokens_match(t, &expected));
+    if !valid {
+        return forbidden("missing or non-matching CSRF form field");
+    }
+
+    next.run(Request::from_parts(parts, Body::from(bytes))).await
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SECRET: &str = "test-secret";
+
+    #[test]
+    fn fresh_token_validates() {
+        let token = make_token(SECRET);
+        assert!(signature_valid(SECRET, &token));
+    }
+
+    #[test]
+    fn tampered_or_foreign_token_rejected() {
+        let token = make_token(SECRET);
+        // Flip the random part but keep the old signature.
+        let (_, sig) = token.split_once('.').unwrap();
+        let forged = format!("{}.{sig}", Uuid::new_v4().simple());
+        assert!(!signature_valid(SECRET, &forged));
+        // A token minted under a different secret is not ours.
+        assert!(!signature_valid(SECRET, &make_token("other-secret")));
+        // Malformed input never validates.
+        assert!(!signature_valid(SECRET, "no-dot-here"));
+    }
+
+    #[test]
+    fn double_submit_compare() {
+        let token = make_token(SECRET);
+        assert!(tokens_match(&token, &token.clone()));
+        assert!(!tokens_match(&token, &make_token(SECRET)));
+    }
+
+    #[test]
+    fn extract_field_from_urlencoded() {
+        let body = b"email=a%40b.com&_csrf=abc.def&password=x";
+        assert_eq!(field_from_urlencoded(body), Some("abc.def".to_string()));
+        assert_eq!(field_from_urlencoded(b"email=x"), None);
+    }
+
+    #[tokio::test]
+    async fn extract_field_from_multipart() {
+        let boundary = "X-BOUNDARY";
+        let content_type = format!("multipart/form-data; boundary={boundary}");
+        let body = format!(
+            "--{b}\r\nContent-Disposition: form-data; name=\"name\"\r\n\r\nWidget\r\n\
+             --{b}\r\nContent-Disposition: form-data; name=\"_csrf\"\r\n\r\nabc.def\r\n\
+             --{b}--\r\n",
+            b = boundary
+        );
+        let got = field_from_multipart(&content_type, Bytes::from(body)).await;
+        assert_eq!(got, Some("abc.def".to_string()));
+    }
+}

src/shared/mod.rs

@@ -1,5 +1,6 @@
 //! Cross-cutting helpers used across feature slices.
 
+pub mod csrf;
 pub mod guard;
 pub mod money;
 pub mod rbac;