Axum Impl Auth JWT

Axum JWT Authentication

Overview

Stateless JWT authentication in Axum is built from six pieces, mirroring the official tokio-rs/axum examples/jwt crate: a Claims struct, a Keys struct holding an EncodingKey and a DecodingKey, an AuthError enum that implements IntoResponse, a FromRequestParts extractor for Claims, a protected handler, and a login handler that issues a token.

The core mechanism: Claims implements FromRequestParts, so any handler that takes claims: Claims as an argument is automatically protected. A missing, malformed, or expired token makes the extractor return Err(AuthError::InvalidToken), which becomes the HTTP response, and the handler body never runs. Authentication is enforced by the type system, not by a guard call inside the handler.

Two rules dominate correct JWT code:

1. The signing secret MUST be read from an environment variable, NEVER a string literal. A literal bakes the credential into the binary and git history.
2. The Claims struct MUST carry an exp field. Validation::default() checks expiry and requires the exp claim; without it every token fails decode.

Quick Reference

Required crates (verified against examples/jwt/Cargo.toml):

axum = "0.8"
axum-extra = { version = "0.10", features = ["typed-header"] }
jsonwebtoken = { version = "10", features = ["aws_lc_rs"] }
serde = { version = "1.0", features = ["derive"] }
serde_json = "1.0"
tokio = { version = "1.0", features = ["full"] }

Decision Trees

Choosing the signing algorithm

Who signs tokens and who verifies them?
├── ONE service does both        -> HS256 symmetric, EncodingKey/DecodingKey::from_secret
└── one signs, others verify     -> RS256 / ES256 / EdDSA asymmetric,
                                    EncodingKey::from_rsa_pem + DecodingKey::from_rsa_pem

This skill targets the HS256 symmetric default. For the asymmetric case use the PEM/DER constructors and set Validation.algorithms to match.

FromRequest vs FromRequestParts for Claims

Where does the token live?
└── in the Authorization header  -> Claims implements FromRequestParts (ALWAYS)

ALWAYS implement FromRequestParts for Claims. The token is read from a header, never the body, so Claims can sit in any handler argument position. FromRequest would force Claims to be the last argument, consume the body, and is semantically wrong.

Picking the extractor form by Axum version

Which Axum version?
├── 0.8  -> native async fn in trait, NO #[async_trait]
└── 0.7  -> #[async_trait] on the impl block, plus use axum::async_trait

Patterns

Pattern 1: The Claims struct, exp is mandatory

use serde::{Deserialize, Serialize};

#[derive(Debug, Clone, Serialize, Deserialize)]
struct Claims {
    sub: String,
    company: String,
    exp: usize, // Unix timestamp (seconds). MANDATORY.
}

Validation::default() sets validate_exp = true and required_spec_claims = {"exp"}. ALWAYS include exp as a real future Unix timestamp. A Claims struct without exp, or a token whose exp is in the past, is rejected by decode.

Pattern 2: The Keys struct and the env-var secret

use jsonwebtoken::{DecodingKey, EncodingKey};
use std::sync::LazyLock;

struct Keys {
    encoding: EncodingKey,
    decoding: DecodingKey,
}

impl Keys {
    fn new(secret: &[u8]) -> Self {
        Self {
            encoding: EncodingKey::from_secret(secret),
            decoding: DecodingKey::from_secret(secret),
        }
    }
}

// Secret read from the environment at first access. NEVER a string literal.
static KEYS: LazyLock<Keys> = LazyLock::new(|| {
    let secret = std::env::var("JWT_SECRET").expect("JWT_SECRET must be set");
    Keys::new(secret.as_bytes())
});

Both keys derive from the same shared secret because HS256 is a symmetric MAC. ALWAYS read the secret from JWT_SECRET and fail fast with .expect(...) if it is absent. NEVER write Keys::new(b"super-secret"). LazyLock is stable since Rust 1.80; on older toolchains use once_cell::sync::Lazy with the same closure.

Pattern 3: The AuthError enum

use axum::{http::StatusCode, response::{IntoResponse, Response}, Json};
use serde_json::json;

enum AuthError {
    WrongCredentials,
    MissingCredentials,
    TokenCreation,
    InvalidToken,
}

impl IntoResponse for AuthError {
    fn into_response(self) -> Response {
        let (status, msg) = match self {
            AuthError::WrongCredentials => (StatusCode::UNAUTHORIZED, "Wrong credentials"),
            AuthError::MissingCredentials => (StatusCode::BAD_REQUEST, "Missing credentials"),
            AuthError::TokenCreation => (StatusCode::INTERNAL_SERVER_ERROR, "Token creation error"),
            AuthError::InvalidToken => (StatusCode::BAD_REQUEST, "Invalid token"),
        };
        (status, Json(json!({ "error": msg }))).into_response()
    }
}

Because AuthError implements IntoResponse, it is a valid handler return type and the extractor's type Rejection. ALWAYS map jsonwebtoken errors into an AuthError variant; NEVER let a raw crypto error reach the response body.

Pattern 4: The FromRequestParts extractor

This is what makes any handler taking Claims protected.

// axum 0.8 : native async fn in trait, NO #[async_trait]
use axum::{extract::FromRequestParts, http::request::Parts, RequestPartsExt};
use axum_extra::{
    headers::{authorization::Bearer, Authorization},
    TypedHeader,
};
use jsonwebtoken::{decode, Validation};

impl<S> FromRequestParts<S> for Claims
where
    S: Send + Sync,
{
    type Rejection = AuthError;

    async fn from_request_parts(parts: &mut Parts, _state: &S) -> Result<Self, Self::Rejection> {
        let TypedHeader(Authorization(bearer)) = parts
            .extract::<TypedHeader<Authorization<Bearer>>>()
            .await
            .map_err(|_| AuthError::InvalidToken)?;

        let token_data = decode::<Claims>(bearer.token(), &KEYS.decoding, &Validation::default())
            .map_err(|_| AuthError::InvalidToken)?;

        Ok(token_data.claims)
    }
}

// axum 0.7 : #[async_trait] is REQUIRED
use axum::async_trait;

#[async_trait]
impl<S> FromRequestParts<S> for Claims
where
    S: Send + Sync,
{
    type Rejection = AuthError;

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

The only difference between versions is the #[async_trait] attribute (required on 0.7, rejected on 0.8) and the TypedHeader import path (axum::TypedHeader behind the headers feature on 0.7, axum_extra::TypedHeader with the typed-header feature on 0.8). The extractor body, trait bounds, and method signature are otherwise identical.

Pattern 5: The protected handler

async fn protected(claims: Claims) -> Result<String, AuthError> {
    Ok(format!("Welcome. Your data: {claims:?}"))
}

The claims: Claims argument alone makes this route require a valid token. No guard call inside the body is needed or correct.

Pattern 6: The login handler that issues a token

use jsonwebtoken::{encode, Header};

async fn authorize(Json(payload): Json<AuthPayload>) -> Result<Json<AuthBody>, AuthError> {
    if payload.client_id.is_empty() || payload.client_secret.is_empty() {
        return Err(AuthError::MissingCredentials);
    }
    if payload.client_id != "foo" || payload.client_secret != "bar" {
        return Err(AuthError::WrongCredentials); // real code queries a database here
    }

    let claims = Claims {
        sub: "b@b.com".to_owned(),
        company: "ACME".to_owned(),
        exp: now_plus_seconds(3600), // a real future timestamp, not a literal
    };

    let token = encode(&Header::default(), &claims, &KEYS.encoding)
        .map_err(|_| AuthError::TokenCreation)?;

    Ok(Json(AuthBody::new(token)))
}

Header::default() selects HS256. Compute exp as "now plus token lifetime" with the time or chrono crate. An encode failure maps to AuthError::TokenCreation.

Router wiring:

use axum::{routing::{get, post}, Router};

let app = Router::new()
    .route("/protected", get(protected))
    .route("/authorize", post(authorize));

Pattern 7: Validating audience or issuer

Validation::default() checks expiry only. To constrain audience or issuer, build a non-default Validation:

let mut validation = Validation::default();
validation.set_audience(&["my-api"]);
validation.set_issuer(&["my-auth-server"]);
let token_data = decode::<Claims>(token, &KEYS.decoding, &validation)?;

Reference Links

• references/methods.md : complete signatures for encode, decode, Header, EncodingKey, DecodingKey, Validation, TokenData, and the FromRequestParts trait, with 0.7 / 0.8 differences.
• references/examples.md : the full end-to-end JWT example (Cargo.toml, all six pieces, both extractor forms), version-annotated.
• references/anti-patterns.md : real mistakes with the exact symptom and WHY each fails, including the hardcoded-secret security hole.

Related Skills

• axum-syntax-custom-extractors : the FromRequestParts trait in depth.
• axum-impl-authorization : role and permission checks on top of authentication.
• axum-errors-handling : the IntoResponse error-enum pattern in general.