Null Pointer Dereference in Axum with Jwt Tokens

In Axum, a null pointer dereference can occur when a JWT token validation step returns an empty or None value and the handler proceeds to access fields or methods on that value. This typically happens when the token is malformed, expired, or missing required claims, and the application does not explicitly guard against a None result before unwrapping or chaining method calls.

During request processing, Axum extracts the JWT from headers, verifies its signature, and maps claims into a struct. If the verification fails or the claim extraction returns an Option that is not handled, a subsequent call such as claims.user_id on a None value can trigger a null pointer dereference at runtime. This is especially risky when using libraries that return Option or Result types without enforcing non-null expectations downstream.

The attack surface is exposed when endpoints accept unauthenticated requests and perform unchecked unwrapping of JWT-derived data. For example, using .expect() or the ? operator without proper error mapping can propagate a None or Err into handler code that assumes a concrete object. This can lead to crashes or information leakage depending on how the framework logs or surfaces the error.

In the context of middleBrick’s 12 security checks, this scenario may surface under Property Authorization and Input Validation, where missing or malformed JWT tokens result in unexpected behavior rather than graceful rejection. The scanner tests whether the API responds safely to missing or invalid authentication tokens without exposing internal state or causing service disruption.

Real-world patterns include using unwrap() on a decoded JWT payload or chaining methods like claims.get_claim("sub").unwrap() without ensuring the Option is Some. These patterns are vulnerable when the token payload does not conform to expectations. OWASP API Top 10 A01:2023 (Broken Object Level Authorization) and A02:2023 (Cryptographic Failures) are relevant when authentication data is not consistently validated.

To prevent null pointer dereference with JWT tokens in Axum, always handle Option and Result types explicitly and avoid unwrapping without validation. Use pattern matching or combinators such as map, and_then, and ok_or to convert Option into Result with meaningful error types before accessing fields.

Below is a concrete Axum handler example that safely extracts and validates a JWT token using the jsonwebtoken crate and ensures no None or Err values are dereferenced unsafely.

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

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

async fn handler(headers: axum::http::HeaderMap) -> Result {
    let token = headers.get("authorization")
        .and_then(|v| v.to_str().ok())
        .and_then(|s| s.strip_prefix("Bearer "))
        .ok_or((axum::http::StatusCode::UNAUTHORIZED, "Missing or malformed token".to_string()))?;

    let decoded = decode::(
        token,
        &DecodingKey::from_secret("secret".as_ref()),
        &Validation::new(Algorithm::HS256),
    ).map_err(|e| (axum::http::StatusCode::UNAUTHORIZED, format!("Invalid token: {}", e)))?;

    let user_id = decoded.claims.sub;
    Ok(format!(\"User ID: {}\", user_id))
}

#[tokio::main]
async fn main() {
    let app = Router::new().route("/profile", get(handler));
    let addr = SocketAddr::from(([127, 0, 0, 1], 3000));
    axum::Server::bind(&addr).serve(app.into_make_service()).await.unwrap();
}

This approach ensures that missing headers, invalid formats, and decoding failures are converted into proper HTTP responses rather than causing a null pointer dereference. The use of ok_or and map_err transforms Option and Result into a consistent error type that Axum can handle without panicking.

In a production setup, you can integrate middleBrick’s CLI tool using middlebrick scan <url> to verify that your endpoints handle invalid or missing JWT tokens safely. For teams requiring continuous oversight, the Pro plan enables continuous monitoring and can integrate checks into CI/CD pipelines via the GitHub Action to fail builds if risky patterns are detected.