Heap Overflow in Axum

Heap overflow vulnerabilities in Axum applications typically occur when handling dynamic request data that gets stored in heap-allocated buffers. Unlike stack-based overflows, heap overflows happen when data exceeds the bounds of a dynamically allocated buffer, corrupting adjacent heap metadata or data.

In Axum, heap overflows commonly manifest through:

• Unbounded deserialization of request bodies into Vec or String buffers
• Dynamic memory allocation for JSON payloads without size validation
• Buffer management in custom middleware that processes multipart form data
• Recursive parsing of nested JSON structures without depth limits
• Memory pooling implementations that don't validate allocation sizes

Consider this vulnerable Axum handler:

use axum::extract::Json;
use serde::{Deserialize, Serialize};

#[derive(Deserialize)]
struct Payload {
    data: String,
    nested: Option<Nested>,
}

#[derive(Deserialize)]
struct Nested {
    items: Vec<NestedItem>,
}

#[derive(Deserialize)]
struct NestedItem {
    value: String,
}

async fn handle_request(Json(payload): Json<Payload>) {
    // No validation of payload size or depth
    // Recursive deserialization can cause heap overflow
    process_nested(payload.nested);
}

fn process_nested(nested: Option<Nested>) {
    if let Some(nested) = nested {
        for item in nested.items {
            process_nested(item.nested);
        }
    }
}

This code is vulnerable because serde's deserialization doesn't enforce size limits by default. An attacker can send a JSON payload with millions of nested items, causing excessive heap allocation and potential memory exhaustion or corruption.

Another common pattern involves multipart form data:

use axum::extract::Multipart;

async fn upload_file(mut multipart: Multipart) {
    while let Some(field) = multipart.next_field().await.unwrap() {
        let data = field.bytes().await.unwrap(); // No size validation
        // Process file data without checking size
        process_file_data(data);
    }
}

fn process_file_data(data: Vec<u8>) {
    // Large files can cause heap overflow
    let mut buffer = Vec::with_capacity(data.len());
    buffer.extend_from_slice(&data);
    // No bounds checking on buffer operations
}

The vulnerability here is the lack of size validation before allocating memory for the uploaded file data.

Detecting heap overflow vulnerabilities in Axum requires both static analysis and runtime monitoring. middleBrick's API security scanner can identify these issues through several mechanisms:

Runtime Payload Analysis: middleBrick sends crafted payloads to test for heap overflow vulnerabilities. For JSON endpoints, it sends:

// Test for unbounded deserialization
{
  "data": "A",
  "nested": {
    "items": [
      { "value": "A", "nested": null },
      { "value": "A", "nested": null },
      // ... millions of nested items
    ]
  }
}

Multipart Size Testing: The scanner tests multipart endpoints with oversized files to check for proper size validation:

curl -X POST http://localhost:3000/upload \
  -F "file=@large_file.bin" \
  -F "metadata={"name":"test"}"

middleBrick monitors the server's memory usage and response times. If the server crashes, becomes unresponsive, or shows abnormal memory growth, it flags potential heap overflow vulnerabilities.

Middleware Inspection: middleBrick analyzes your Axum middleware stack to identify custom buffer management code. It looks for patterns like:

// Vulnerable pattern detected
let mut buffer = Vec::new();
buffer.reserve(size); // No validation of 'size'

Configuration Analysis: The scanner checks your Axum configuration for missing size limits:

// Missing limits detected
app.with_state(AppState {})
    .route("/api/*", axum::routing::get(handler))
    // No extractor configuration for size limits

middleBrick provides a detailed report showing:

• Which endpoints are vulnerable to heap overflow
• Specific code patterns that need fixing
• Recommended size limits based on your application's needs
• Compliance mapping to OWASP API Security Top 10 (A1: Broken Object Level Authorization, A4: Lack of Resources & Rate Limiting)

Fixing heap overflow vulnerabilities in Axum requires implementing proper bounds checking and size validation. Here are Axum-specific remediation strategies:

1. Configure Extractor Limits: Use Axum's extractor configuration to set size limits:

use axum::extract::json::JsonConfig;
use axum::Json;

async fn handle_request(
    Json(payload): Json<Payload, JsonConfig>,
) {
    // Payload automatically limited to 1MB
}

// In your main function:
let app = axum::Router::new()
    .route("/api/*", axum::routing::post(handler))
    .with_state(AppState {})
    .layer(
        axum::extract::Extension(JsonConfig::default().limit(1024 * 1024)), // 1MB limit
    );

2. Implement Recursive Depth Limits: Add depth checking to recursive deserialization:

use serde::Deserialize;

#[derive(Deserialize)]
struct Payload {
    data: String,
    #[serde(default)]
    nested: Option<Nested>,
}

#[derive(Deserialize)]
struct Nested {
    items: Vec<NestedItem>,
}

#[derive(Deserialize)]
struct NestedItem {
    value: String,
    #[serde(default)]
    nested: Option<Nested>,
}

async fn handle_request(Json(payload): Json<Payload>) -> Result<impl axum::response::IntoResponse> {
    validate_depth(&payload, 0, 10)?; // Max depth of 10
    Ok(axum::response::Json({"status": "success"}))
}

fn validate_depth(payload: &Payload, current_depth: usize, max_depth: usize) -> Result<()> {
    if current_depth > max_depth {
        return Err(axum::response::Response::from(
            axum::response::Json({"error": "Payload depth exceeded"})
        ));
    }
    
    if let Some(nested) = &payload.nested {
        for item in &nested.items {
            validate_depth(item, current_depth + 1, max_depth)?;
        }
    }
    
    Ok(())
}

3. Validate Multipart Sizes: Use Axum's multipart extractor with size limits:

use axum::extract::multipart::{FormData, MultipartError};
use axum::extract::multipart::Field;

async fn upload_file(mut multipart: FormData) -> Result<impl axum::response::IntoResponse> {
    while let Some(field) = multipart.next_field().await? {
        let field_name = field.name().unwrap();
        let content_type = field.content_type().unwrap_or("application/octet-stream");
        
        // Validate file size before reading
        let size = field.size_hint().unwrap_or(0);
        if size > 10 * 1024 * 1024 { // 10MB limit
            return Err(axum::response::Response::from(
                axum::response::Json({"error": "File too large"})
            ));
        }
        
        let data = field.bytes().await?;
        // Process file data
    }
    
    Ok(axum::response::Json({"status": "success"}))
}

4. Use Safe Buffer Operations: Replace unsafe buffer operations with checked alternatives:

// Vulnerable
let mut buffer = Vec::with_capacity(size);
buffer.extend_from_slice(&data);

// Safe
let mut buffer = Vec::with_capacity(size.min(MAX_SAFE_SIZE));
buffer.extend_from_slice(&data[..data.len().min(MAX_SAFE_SIZE)]);

5. Implement Rate Limiting: Add rate limiting to prevent repeated large payload attacks:

use axum::extract::Extension;
use tower_http::rate_limit::RateLimitLayer;

// In main:
let app = axum::Router::new()
    .route("/api/*", axum::routing::post(handler))
    .layer(RateLimitLayer::new(100, std::time::Duration::from_secs(60)))
    .check_infallible();

These remediation strategies, combined with middleBrick's continuous monitoring, ensure your Axum application is protected against heap overflow attacks while maintaining performance and usability.