Axum Impl Authorization

axum-impl-authorization

Overview

Authorization answers "is this known user allowed to do this". It is a separate concern from authentication, which answers "who are you". Authentication is handled by axum-impl-auth-jwt and produces a verified Claims value. Authorization always runs AFTER authentication and reads that Claims value.

Axum has no built-in authorization. It provides the building blocks: FromRequestParts extractors, from_fn and from_fn_with_state middleware, and the route_layer application method. There are two mechanisms, and they are NOT interchangeable:

1. Middleware guard: from_fn or from_fn_with_state applied with route_layer. Protects a GROUP of routes. The requirement lives on the router.
2. Newtype guard extractor: a custom FromRequestParts type such as AdminClaims. Protects a SINGLE handler. The requirement lives in the handler signature.

Two rules dominate this skill:

• A failed authorization check ALWAYS returns 403 Forbidden, NEVER 401.
• A guard is ALWAYS applied with .route_layer(), NEVER .layer().

The authorization mechanisms are version-stable: from_fn, from_fn_with_state, Next, route_layer, and layer have the same shape in Axum 0.7 and 0.8. Only the newtype guard's FromRequestParts impl differs between versions (see the version note in the patterns below).

Quick Reference

Status code rule:

.layer() versus .route_layer():

Middleware guard argument order (from_fn / from_fn_with_state):

[zero or more FromRequestParts extractors], [exactly one FromRequest extractor], Next

State and Claims are FromRequestParts and come first. Request is the single FromRequest extractor and is second to last. Next is always last.

Core rules:

• ALWAYS apply an authorization guard with .route_layer(). NEVER .layer().
• ALWAYS return 403 Forbidden on a failed role or permission check.
• ALWAYS register routes BEFORE attaching .route_layer(); it only wraps routes that already exist.
• ALWAYS reuse the Claims extractor for identity. NEVER re-verify the token inside a guard.
• ALWAYS check a permission (articles:write) rather than a coarse role when the role set may grow.
• NEVER implement both FromRequest and FromRequestParts for a guard newtype; the official docs warn this makes the extractor unusable.

Decision Trees

Middleware guard or newtype extractor

How many routes does this requirement protect?
  a whole group (every route under /admin)
    -> middleware guard: from_fn(_) or from_fn_with_state(_) + .route_layer()
  exactly one handler
    -> newtype guard extractor: a FromRequestParts type in the handler signature
  one handler, and the requirement must be visible in the signature
    -> newtype guard extractor (self-documenting, impossible to forget)

from_fn or from_fn_with_state

Does the authorization decision need application state?
  no, it only reads fields already inside Claims (claims.role, claims.permissions)
    -> from_fn(guard)
  yes, it queries a permissions database, a policy cache, or a role-to-permission table
    -> from_fn_with_state(state.clone(), guard), and call .with_state(state) on the router

Which status code

Why is the request being rejected?
  no credentials, or the token is invalid or expired
    -> 401 Unauthorized (this is the Claims extractor's job, not the guard's)
  the user is known but the role or permission does not match
    -> 403 Forbidden (the guard returns this)
NEVER return 401 from an authorization guard.

Patterns

Pattern: role-based middleware guard with route_layer

from_fn turns an async fn into a Layer. The guard reads the verified Claims, checks the role, and either returns 403 or calls next.run.

use axum::{
    extract::Request,
    http::StatusCode,
    middleware::{self, Next},
    response::Response,
    routing::get,
    Router,
};

// `Claims` is the verified JWT extractor from axum-impl-auth-jwt.
// It is FromRequestParts, so it runs first and a missing or invalid
// token already rejects with 401 before this guard body runs.
async fn require_admin(
    claims: Claims,    // FromRequestParts: runs first
    request: Request,  // FromRequest: the single body extractor
    next: Next,        // Next: always last
) -> Result<Response, StatusCode> {
    if claims.role != "admin" {
        // authenticated but not authorized: 403, NEVER 401
        return Err(StatusCode::FORBIDDEN);
    }
    Ok(next.run(request).await)
}

let admin_routes = Router::new()
    .route("/admin/users", get(list_users))
    .route("/admin/settings", get(settings))
    // route_layer, NOT layer: a non-existent /admin/* path stays 404
    .route_layer(middleware::from_fn(require_admin));

Pattern: from_fn_with_state for a state-dependent permission check

When the decision needs application state (a permission store, a policy cache), use from_fn_with_state. Verified signature:

pub fn from_fn_with_state<F, S, T>(state: S, f: F) -> FromFnLayer<F, S, T>

State surfaces inside the guard as a State<S> extractor.

use axum::{
    extract::{Request, State},
    http::StatusCode,
    middleware::{self, Next},
    response::Response,
    routing::get,
    Router,
};

#[derive(Clone)]
struct AppState {
    perms: PermissionStore,
}

async fn require_users_write(
    State(state): State<AppState>, // FromRequestParts: first
    claims: Claims,                // FromRequestParts: still before Request
    request: Request,              // FromRequest: second to last
    next: Next,                    // Next: last
) -> Result<Response, StatusCode> {
    let allowed = state
        .perms
        .has(&claims.sub, "users:write")
        .await
        .map_err(|_| StatusCode::INTERNAL_SERVER_ERROR)?;
    if !allowed {
        return Err(StatusCode::FORBIDDEN);
    }
    Ok(next.run(request).await)
}

let state = AppState { perms: PermissionStore::new() };

let app = Router::new()
    .route("/admin/users", get(list_users))
    // the same state value is passed twice: clone for the layer
    .route_layer(middleware::from_fn_with_state(state.clone(), require_users_write))
    .with_state(state);

ALWAYS pass the state twice: state.clone() into from_fn_with_state and the original state into .with_state(). The official docs example uses exactly this route_layer plus double-pass shape.

Pattern: permission-based check is the preferred default

A role check hardcodes a fixed hierarchy into every endpoint. A permission check names the CAPABILITY the endpoint needs, independent of which roles hold it. The mechanism is identical; only the predicate changes.

// BRITTLE: every editor-or-above guard must repeat the role list.
// Adding a "moderator" role means editing every one of these.
async fn require_editor(claims: Claims, request: Request, next: Next)
    -> Result<Response, StatusCode>
{
    if claims.role != "editor" && claims.role != "admin" {
        return Err(StatusCode::FORBIDDEN);
    }
    Ok(next.run(request).await)
}

// STABLE: the endpoint names the capability, not the roles that have it.
// Adding a "moderator" role only changes the role-to-permission mapping.
async fn require_articles_write(claims: Claims, request: Request, next: Next)
    -> Result<Response, StatusCode>
{
    if !claims.permissions.contains("articles:write") {
        return Err(StatusCode::FORBIDDEN);
    }
    Ok(next.run(request).await)
}

The Claims struct SHOULD carry a permission set (HashSet<String> or Vec<String>) using a resource:action convention (users:read, users:write, billing:refund). The authentication step expands a role into its permission set when minting the token, so guards only ever check permissions.

Pattern: newtype guard extractor (AdminClaims)

For a SINGLE protected handler, a newtype FromRequestParts extractor makes the requirement visible in the signature. It composes WITH the Claims extractor by calling Claims::from_request_parts, never re-verifying the token.

use axum::{
    extract::FromRequestParts,
    http::{request::Parts, StatusCode},
};

// A handler that takes `AdminClaims` is provably admin-only.
pub struct AdminClaims(pub Claims);

// axum 0.8: native async fn in trait, no attribute needed.
impl<S> FromRequestParts<S> for AdminClaims
where
    S: Send + Sync,
{
    type Rejection = StatusCode;

    async fn from_request_parts(parts: &mut Parts, state: &S)
        -> Result<Self, Self::Rejection>
    {
        // Reuse the existing Claims extractor for identity.
        let claims = Claims::from_request_parts(parts, state)
            .await
            .map_err(|_| StatusCode::UNAUTHORIZED)?; // bad/no token: 401
        if claims.role != "admin" {
            return Err(StatusCode::FORBIDDEN);        // known, not admin: 403
        }
        Ok(AdminClaims(claims))
    }
}

// axum 0.7: the SAME impl block must be annotated with #[async_trait].
#[async_trait::async_trait]
impl<S> FromRequestParts<S> for AdminClaims
where
    S: Send + Sync,
{
    type Rejection = StatusCode;

    async fn from_request_parts(parts: &mut Parts, state: &S)
        -> Result<Self, Self::Rejection>
    { /* identical body as the axum 0.8 version above */ }
}

Used in a handler, the requirement is self-documenting:

use axum::Json;

// The argument type PROVES this handler is admin-only.
async fn delete_user(AdminClaims(claims): AdminClaims)
    -> Result<Json<()>, StatusCode>
{
    tracing::info!(admin = %claims.sub, "admin deleting user");
    Ok(Json(()))
}

NEVER implement both FromRequest and FromRequestParts for the newtype; the official docs warn this makes the extractor unusable. A guard inspects request parts (headers, claims) only, so FromRequestParts alone is correct.

Reference Links

• references/methods.md: verified signatures for from_fn, from_fn_with_state, Next, route_layer, layer, with_state, and FromRequestParts, plus the middleware argument-order rule.
• references/examples.md: full working programs, a permission store wired through from_fn_with_state, a generic RequirePermission extractor, and the 0.7 versus 0.8 newtype-guard forms.
• references/anti-patterns.md: the seven recurring authorization mistakes with the root cause of each and the fix.

Related skills:

• axum-impl-auth-jwt: defines the Claims extractor that every guard here reuses for identity.
• axum-impl-middleware: from_fn argument-order rules and layer ordering.
• axum-syntax-custom-extractors: the FromRequestParts trait the newtype guard implements.