Spring4shell in Axum with Jwt Tokens

The Spring4shell vulnerability (CVE-2022-22965) exploits a flaw in data binding within Spring MVC and Spring WebFlux when using certain payloads that target specific class hierarchies. In an Axum API secured with JWT-based authentication, the presence of JWT parsing and validation logic does not inherently mitigate server-side request forgery or remote code execution risks if the application exposes endpoints vulnerable to data binding attacks.

When Axum routes are defined to accept JSON payloads that are bound to mutable Java objects (e.g., using Jsonb or Jackson via integrations), an attacker can supply crafted parameters that traverse object graphs unexpectedly. If the application also exposes unauthenticated or weakly authenticated endpoints—such as public token validation or introspection routes—Spring4shell can be chained with JWT handling to bypass intended access controls. For example, a public route that parses a JWT but then binds request data to sensitive beans may inadvertently allow an attacker to escalate privileges or invoke unintended methods through manipulated form fields or JSON properties.

Even when JWT tokens are used to convey roles and permissions, the vulnerability operates at the framework’s data-binding layer, independent of the token’s claims. Therefore, an API that validates JWTs but uses unsafe binding practices remains exposed. Attackers may send maliciously crafted requests that exploit getters/setters or specific constructors, leading to remote code execution or information disclosure despite the presence of JWT-based middleware.

middleBrick detects this class of issue under the BOLA/IDOR and Property Authorization checks, alongside the broader Authentication and Input Validation scans. The scanner does not rely on internal architecture but instead observes runtime behavior, such as whether endpoints reflect unexpected data transformations or expose dangerous constructors when unauthenticated probes are sent.

To reduce risk when using JWT tokens in Axum, apply strict input validation and avoid exposing data-binding endpoints to untrusted input. Ensure that token validation is performed early and that downstream route handlers do not perform unsafe deserialization or reflection-based binding.

Secure JWT validation middleware in Axum

Use a dedicated middleware layer for JWT verification, and ensure that claims are validated before any request data is bound to domain models. Below is a minimal, secure Axum example using the jsonwebtoken crate:

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

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

async fn validate_token(headers: axum::http::HeaderMap) -> Result<Claims, &'static str> {
    let auth_header = headers.get("authorization").ok_or("Missing auth header")?;
    let token = auth_header.to_str().map_err(|_| "Invalid header")?.strip_prefix("Bearer ").ok_or("Bad format")?;
    let decoding_key = DecodingKey::from_secret("secret".as_ref());
    let validation = Validation::new(Algorithm::HS256);
    let token_data: TokenData<Claims> = decode(token, &decoding_key, &validation).map_err(|_| "Invalid token")?;
    Ok(token_data.claims)
}

async fn public_endpoint() -> &'static str {
    "public data"
}

async fn protected_endpoint(claims: Claims) -> String {
    format!("Hello, {}", claims.sub)
}

#[tokio::main]
async fn main() {
    let app = Router::new()
        .route("/public", get(public_endpoint))
        .route("/protected", get(protected_endpoint))
        .layer(axum::middleware::from_fn(|req, next| async move {
            let path = req.uri().path();
            if path == "/protected" {
                validate_token(req.headers().clone()).await.map_err(|_| axum::http::StatusCode::UNAUTHORIZED)?;
            }
            Ok(next.run(req).await)
        }));
    axum::Server::bind(&"0.0.0.0:3000".parse().unwrap()).serve(app.into_make_service()).await.unwrap();
}

Mitigation strategies aligned with OWASP API Top 10

• Apply strict schema validation to all incoming JSON and form data to prevent unexpected property binding.
• Ensure that JWT validation is performed before any business logic and that token payloads are not used to dynamically construct object graphs.
• Leverage Axum’s extractor model to separate authentication from data binding, and avoid using generic mutable structs for public endpoints.
• Use the Pro plan’s continuous monitoring to detect anomalous request patterns that may indicate attempted exploitation of binding or authorization flaws.

middleBrick’s scans can verify whether these controls are effective by checking for missing validation, overly permissive routes, and inconsistencies between declared authentication requirements and actual runtime behavior.