Spring4shell in Axum

Spring4shell is a remote code execution vulnerability that affects Java Spring Framework applications when deserializing untrusted data. While Axum is a Rust web framework and doesn't use Java or Spring, the attack pattern is relevant because many Axum applications integrate with Java services or deserialize data from external sources that might be vulnerable.

The core vulnerability in Spring4shell exploits how Spring's parameter binding can trigger deserialization of malicious objects. In Axum applications, this manifests when:

• Deserializing data from untrusted sources without proper validation
• Using unsafe deserialization libraries that don't validate input
• Accepting serialized objects from external APIs or message queues
• Improperly handling content-type negotiation that could trigger unexpected deserialization
• Using reflection-based object mapping without type safety

Consider an Axum endpoint that accepts JSON data and deserializes it using serde_json:

async fn handle_data(mut payload: Json<serde_json::Value>) -> Result<impl Responder> {
    let data = payload.take();
    // Unsafe: directly using deserialized data without validation
    process_external_data(data)?;
    Ok(StatusCode::OK)
}

The vulnerability pattern emerges when Axum applications deserialize data from external sources and pass it to Java services via gRPC, REST, or message queues. If the Java service is vulnerable to Spring4shell, the attacker can craft payloads that exploit both the Axum application's deserialization and the downstream Java service.

Another manifestation occurs when Axum applications use libraries like bincode or serde_cbor for binary serialization without proper input validation:

async fn deserialize_binary(data: Bytes) -> Result<impl Responder> {
    let deserialized: MyStruct = bincode::deserialize(&data).unwrap();
    // No validation of deserialized content
    process_data(deserialized)?;
    Ok(StatusCode::OK)
}

The attack surface expands when Axum applications act as proxies or API gateways, forwarding requests to vulnerable Java services without proper request sanitization.

Detecting Spring4shell-like vulnerabilities in Axum applications requires examining deserialization patterns and data flow. middleBrick's API security scanner can identify these issues through black-box testing and OpenAPI spec analysis.

middleBrick scans for:

• Endpoints accepting serialized data without validation
• Content-type negotiation that could trigger unsafe deserialization
• Binary data endpoints that might deserialize arbitrary content
• Integration points with external services that could be vulnerable
• Reflection-based object mapping without type constraints

The scanner tests for common deserialization attack patterns by sending crafted payloads that attempt to trigger unsafe deserialization paths. For Axum applications, middleBrick specifically looks for:

#[post("/deserialize")]
async fn deserialize_endpoint(data: Bytes) -> Result<impl Responder> {
    // Potential vulnerability if data is deserialized without validation
    let obj: MyStruct = deserialize_unsafe(&data)?;
}

middleBrick's LLM/AI security module also checks for AI-powered endpoints that might inadvertently process malicious serialized data, though this is more relevant for applications using AI features.

Detection via OpenAPI spec analysis reveals endpoints that accept generic types or use serialization without proper constraints:

paths:
  /api/data:
    post:
      consumes:
        - application/octet-stream
      parameters:
        - in: body
          name: data
          schema:
            type: string
            format: binary
      # Missing validation schema

middleBrick flags these patterns and provides remediation guidance specific to Rust/Axum patterns, such as using serde's type-safe deserialization with validation schemas.

Remediating Spring4shell-like vulnerabilities in Axum applications focuses on safe deserialization practices and input validation. The key is to eliminate unsafe deserialization paths and implement strict type validation.

Safe deserialization pattern using serde with validation:

use serde::Deserialize;
use axum::extract::Json;
use validator::Validate;

#[derive(Deserialize, Validate)]
pub struct UserData {
    #[validate(length(min = 1, max = 255))]
    username: String,
    #[validate(email)]
    email: String,
    #[validate(range(min = 18, max = 120))]
    age: u8,
}

async fn safe_endpoint(Json(user_data): Json<UserData>) -> Result<impl Responder> {
    // Validate before processing
    user_data.validate()?;
    
    // Process validated data
    process_user_data(user_data)?;
    Ok(StatusCode::OK)
}

For binary data endpoints, use type-safe deserialization with schema validation:

use axum::extract::Bytes;
use serde_cbor::from_slice;
use schemars::JsonSchema;

#[derive(Deserialize, JsonSchema)]
pub struct SecureData {
    id: u32,
    data: String,
}

async fn secure_binary_endpoint(data: Bytes) -> Result<impl Responder> {
    // Deserialize with type safety and validate schema
    let secure_data: SecureData = from_slice(&data)
        .map_err(|e| StatusCode::BAD_REQUEST)?;
    
    // Additional validation
    if secure_data.id == 0 {
        return Err(StatusCode::BAD_REQUEST.into());
    }
    
    process_secure_data(secure_data)?;
    Ok(StatusCode::OK)
}

Implementing content-type validation to prevent unexpected deserialization:

use axum::{routing::post, Json, Router};
use axum::http::StatusCode;

async fn validate_content_type(req: &mut axum::http::Request<B>) -> Result<(), StatusCode> {
    let content_type = req.headers().get(axum::http::header::CONTENT_TYPE);
    match content_type {
        Some(t) if t == "application/json" => Ok(()),
        Some(t) if t == "application/x-www-form-urlencoded" => Ok(()),
        _ => Err(StatusCode::UNSUPPORTED_MEDIA_TYPE),
    }
}

let app = Router::new()
    .route("/api/data", post(validate_content_type_then(handle_data)))
    .with_state(MyAppState::new());

middleBrick's continuous monitoring in Pro tier can automatically scan your Axum APIs on a schedule, alerting you if deserialization patterns change or new vulnerabilities are detected in your dependencies.