Privilege Escalation in Axum with Jwt Tokens

In an Axum application, privilege escalation can occur when JSON Web Tokens (JWT) are used for authorization without properly validating claims, scopes, or roles. JWTs are often trusted solely based on signature validity, but if the token payload is not checked for the intended access level, an attacker can modify claims such as role or scope and gain elevated permissions. For example, a user token with role: "user" can be altered to role: "admin" and replayed to an Axum endpoint that relies on the token for access control without additional server-side authorization checks.

This becomes a critical issue when endpoints in Axum do not enforce authorization based on token claims beyond authentication. An attacker might exploit missing scope validation to perform horizontal or vertical privilege escalation. If the JWT is signed with a weak algorithm such as none or uses a shared secret that is leaked, the token can be forged entirely. Axum middleware that only verifies the signature but does not validate issuer, audience, expiration, or required claims allows an attacker to craft a token that grants administrative access to otherwise restricted routes.

Real-world attack patterns include modifying the sub claim to impersonate another user or changing permissions claims to bypass business logic checks. Without mapping token claims to an authorization model enforced by Axum, the API surface remains vulnerable to token tampering. This risk is compounded when tokens have long lifetimes or are transmitted over unencrypted channels, increasing exposure and replay potential.

Compliance frameworks such as OWASP API Top 10 highlight broken object level authorization and insufficient authorization as common API risks. In the context of JWT usage in Axum, this translates to failing to enforce role-based or scope-based access controls at the handler level. Even if the JWT is valid, missing validation of claims like scope or roles can lead to unauthorized data access or destructive operations. Proper validation and authorization logic must be implemented to ensure that a token is not only authentic but also explicitly permitted to perform the requested action.

Remediation focuses on strict JWT validation and enforcing authorization within Axum handlers. Use a dedicated JWT validation middleware that checks the signature, algorithm, issuer, audience, expiration, and required claims. Enforce scope or role claims at the handler level rather than relying on route-level guards alone.

use axum::{routing::get, Router};
use jsonwebtoken::{decode, Algorithm, DecodingKey, Validation, TokenData, Header};
use serde::{Deserialize, Serialize};
use std::net::SocketAddr;

#[derive(Debug, Serialize, Deserialize)]
struct Claims {
    sub: String,
    role: String,
    scope: String,
    exp: usize,
    iss: String,
    aud: String,
}

async fn validate_jwt(token: &str) -> Result, jsonwebtoken::errors::Error> {
    let validation = Validation {
        algorithms: vec![Algorithm::HS256],
        validate_exp: true,
        validate_nbf: true,
        validate_iat: true,
        validate_iss: true,
        validate_aud: true,
        iss: Some(&"middlebrick-api".to_string()),
        aud: Some(&"api-client".to_string()),
        required_spec_claims: vec!["exp", "iss", "aud", "scope", "role"],
        ..Default::default()
    };
    let key = DecodingKey::from_secret(b"super-secret-key-change-in-production");
    decode::(token, &key, &validation)
}

async fn admin_handler(claims: Claims) -> String {
    if claims.role != "admin" || !claims.scope.contains("admin:read") {
        return String::from("forbidden");
    }
    String::from("admin data")
}

#[tokio::main]
async fn main() {
    let app = Router::new()
        .route("/admin", get(|headers: axum::http::HeaderMap| async move {
            let auth_header = headers.get("authorization")
                .and_then(|v| v.to_str().ok())
                .unwrap_or("");
            let token = auth_header.trim_start_matches("Bearer ");
            match validate_jwt(token).await {
                Ok(token_data) => admin_handler(token_data.claims).await,
                Err(_) => String::from("unauthorized"),
            }
        }));

    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
    axum::Server::bind(&addr)
        .serve(app.into_make_service())
        .await
        .unwrap();
}

This example demonstrates strict validation of issuer, audience, expiration, and required claims including role and scope. The handler explicitly checks the role and scope before allowing access, preventing privilege escalation even if a token is stolen or tampered with. Rotate secrets, use asymmetric keys in production, and ensure tokens are transmitted only over HTTPS to further reduce risk.

Additionally, apply the principle of least privilege by assigning minimal scopes and roles to each token. Avoid using the none algorithm and enforce strong signing keys. Regularly review token validation logic and monitor for unexpected claims or usage patterns. The Pro plan of middleBrick can support continuous monitoring of such configurations, integrating with your CI/CD pipeline via the GitHub Action to fail builds if security thresholds are not met.