Jwt Misconfiguration in Axum

Detecting JWT misconfigurations in Axum requires both static analysis and runtime scanning. middleBrick's black-box scanning approach is particularly effective for Axum applications since it tests the actual API endpoints without requiring source code access.

For Axum applications, middleBrick scans for several JWT-specific vulnerabilities:

Algorithm Confusion Attacks: The scanner tests if your endpoint accepts tokens with weaker algorithms than expected. It submits tokens signed with HS256 when RS256 is expected, attempting to forge valid tokens using the public key as a secret.

Missing Claims Validation: middleBrick verifies whether your Axum endpoints properly validate the aud (audience), iss (issuer), and sub (subject) claims. It crafts tokens with mismatched claims to test if they're accepted.

Weak Key Detection: The scanner analyzes token signatures to detect weak or predictable secrets. For HS256 tokens, it attempts to brute-force short secrets and identifies common weak keys like "secret", "password", or predictable patterns.

Time Validation Bypass: middleBrick tests whether expired tokens are accepted by submitting tokens with exp claims in the past and verifying if they're still processed.

To use middleBrick with your Axum API:

npx middlebrick scan https://yourapi.com/api/auth

The scanner tests all authentication endpoints and protected routes, providing a security score with specific findings. For CI/CD integration:

# GitHub Action example
- name: middleBrick API Security Scan
  uses: middlebrick/middlebrick-action@v1
  with:
    url: https://yourapi.com
    fail-on-score-below: 80

This configuration fails your build if the JWT security score drops below B grade, preventing vulnerable code from reaching production.

Securing JWT in Axum requires proper configuration and validation. Here's how to implement secure JWT handling in Axum applications:

Use Asymmetric Signing: Always prefer RS256 over HS256 in production:

use jsonwebtoken::{decode, DecodingKey, Validation};
use axum::middleware::Next;
use axum::http::{Request, Response};
use std::sync::Arc;
use std::fs;

// Load public key once at startup
let public_key_pem = fs::read_to_string("public_key.pem")?;
let decoding_key = DecodingKey::from_rsa_pem(public_key_pem.as_bytes())?;

async fn auth_middleware(req: Request, next: Next<B>) -> Response {
    let token = req.headers().get("Authorization")
        .and_then(|h| h.to_str().ok())
        .and_then(|h| h.strip_prefix("Bearer "))
        .unwrap_or("");

    let validation = Validation::new(jsonwebtoken::Algorithm::RS256);
    let token_data = decode::<Claims>(&token, &decoding_key, &validation);
    
    match token_data {
        Ok(claims) => {
            // Validate claims
            if claims.claims.aud != "your-api-audience" {
                return Response::builder()
                    .status(401)
                    .body(axum::body::Body::empty())
                    .unwrap();
            }
            let mut req_with_claims = req.clone();
            req_with_claims.extensions_mut().insert(claims);
            next.run(req_with_claims).await
        }
        Err(_) => Response::builder()
            .status(401)
            .body(axum::body::Body::empty())
            .unwrap(),
    }
}

Implement Proper Claims Validation: Always validate all claims:

async fn validate_claims(claims: &Claims) -> Result<(), &'static str> {
    // Validate issuer
    if claims.iss != "expected-issuer" {
        return Err("Invalid issuer");
    }
    
    // Validate audience
    if !claims.aud.contains(&"your-api-audience".to_string()) {
        return Err("Invalid audience");
    }
    
    // Validate subject
    if claims.sub.is_empty() {
        return Err("Missing subject");
    }
    
    Ok(())
}

Secure Token Extraction: Use Axum's typed headers for safer extraction:

use axum::extract::TypedHeader;
use axum::http::headers::authorization::Authorization;
use axum::http::headers::authorization::Bearer;

async fn auth_middleware(req: Request<B>, next: Next<B>) -> Response {
    let TypedHeader(Authorization(Bearer { token })) = 
        TypedHeader::

Add Rate Limiting: Protect against brute-force attacks:

use axum::middleware::Next;
use tower_crate::rate_limit;
use tower_crate::rate_limit::RateLimitLayer;

let rate_limit_layer = RateLimitLayer::new(
    100, // requests
    std::time::Duration::from_secs(60), // per minute
);

let app = axum::Router::new()
    .route(...)
    .layer(rate_limit_layer);

Secure Claims Structure: Define claims with proper validation:

use serde::{Deserialize, Serialize};
use jsonwebtoken::Algorithm;

#[derive(Debug, Serialize, Deserialize)]
struct Claims {
    sub: String,
    aud: Vec<String>, // Validate against multiple audiences
    iss: String,
    exp: usize,
    iat: usize,
    #[serde(default)]
    roles: Vec<String>,
}

impl Claims {
    fn has_role(&self, role: &str) -> bool {
        self.roles.contains(&role.to_string())
    }
}

Testing JWT Security: Use middleBrick in your development workflow to catch misconfigurations before production:

// Test with middleBrick before deployment
npx middlebrick scan https://staging.yourapp.com/api

// Check for specific JWT findings
// Look for: algorithm confusion, weak keys, missing claims validation