Stack Overflow in Axum

Stack Overflow vulnerabilities in Axum applications typically occur when user-controlled input is processed without proper bounds checking, allowing attackers to overwrite the call stack. In Axum, this often manifests through unsafe deserialization, buffer operations on user input, or recursive request processing.

A common Axum-specific scenario involves parsing JSON payloads in extractors. Consider this vulnerable endpoint:

use axum::extract::Json;
use axum::routing::post;
use axum::Router;

#[derive(serde::Deserialize)]
struct UserInput {
    data: String,
}

async fn vulnerable_endpoint(Json(input): Json<UserInput>) -> String {
    // No bounds checking on input.data length
    let mut buffer = [0u8; 512];
    let bytes = input.data.as_bytes();
    
    // This can overflow if bytes.len() > 512
    buffer[..bytes.len()].copy_from_slice(bytes);
    
    String::from_utf8_lossy(&buffer).to_string()
}

let app = Router::new().route("/vulnerable", post(vulnerable_endpoint));

The issue here is that copy_from_slice doesn't validate that bytes.len() fits within the 512-byte buffer. An attacker can send a payload exceeding this size, causing a stack overflow that crashes the service or potentially allows code execution.

Another Axum-specific vector is through recursive middleware chains. If middleware processes requests without depth limits, deeply nested requests can exhaust the stack:

use axum::middleware::Next;
use axum::response::IntoResponse;

async fn recursive_middleware(
    mut req: axum::http::Request,
    next: Next,
) -> impl IntoResponse {
    // No recursion depth limit
    if req.extensions().get::().is_none() {
        req.extensions_mut().insert(true);
        return next.run(req).await;
    }
    
    // This can recurse indefinitely
    recursive_middleware(req, next).await
}

Stack overflows can also occur in Axum when processing deeply nested JSON structures through extractors, especially when combined with recursive data structures in Rust code.

Detecting stack overflow vulnerabilities in Axum requires both static analysis and runtime testing. middleBrick's black-box scanner can identify several Axum-specific patterns that indicate stack overflow risks.

middleBrick scans for stack overflow by testing endpoint boundaries and analyzing response patterns. For Axum applications, it specifically looks for:

• Endpoints that accept unbounded JSON payloads without size limits
• Extractors that deserialize into recursive data structures
• Middleware chains that could recurse without limits
• Buffer operations on user input without bounds checking
• Recursive request processing patterns

The scanner tests these by sending progressively larger payloads and monitoring for crashes, timeouts, or abnormal responses. For Axum applications, middleBrick's LLM/AI security module also checks for any AI-powered components that might have additional stack overflow vectors through prompt processing.

To manually test for stack overflows in your Axum application, you can use tools like curl or hey to send oversized payloads:

# Test with oversized JSON payload
curl -X POST http://localhost:3000/vulnerable \
  -H "Content-Type: application/json" \
  -d "{\"data\": \"$(python -c 'print(\"A\" * 10000)')\"}"

If your service crashes or behaves unexpectedly with large inputs, you likely have stack overflow vulnerabilities. middleBrick's continuous monitoring (Pro plan) can automatically detect these issues as they emerge in production.

Remediating stack overflow vulnerabilities in Axum requires a combination of input validation, safe coding practices, and Axum's built-in safety features. Here are Axum-specific fixes for the patterns discussed:

For the buffer overflow example, use safe operations with explicit bounds checking:

use axum::extract::Json;
use axum::routing::post;
use axum::Router;
use axum::http::StatusCode;

#[derive(serde::Deserialize)]
struct UserInput {
    data: String,
}

async fn safe_endpoint(Json(input): Json<UserInput>) -> Result<String, StatusCode> {
    // Validate input length first
    if input.data.len() > 512 {
        return Err(StatusCode::BAD_REQUEST);
    }
    
    // Use safe operations
    let mut buffer = [0u8; 512];
    let bytes = input.data.as_bytes();
    
    // This is safe because we validated length
    buffer[..bytes.len()].copy_from_slice(bytes);
    
    Ok(String::from_utf8_lossy(&buffer).to_string())
}

let app = Router::new().route("/safe", post(safe_endpoint));

For recursive middleware, implement depth limits using request extensions:

use axum::middleware::Next;
use axum::response::IntoResponse;
use axum::http::StatusCode;

const MAX_RECURSION_DEPTH: usize = 10;

async fn safe_recursive_middleware(
    mut req: axum::http::Request,
    next: Next,
) -> impl IntoResponse {
    let depth = *req.extensions().get::().unwrap_or(&0);
    
    if depth >= MAX_RECURSION_DEPTH {
        return (StatusCode::TOO_MANY_REQUESTS, "Recursion depth exceeded").into_response();
    }
    
    req.extensions_mut().insert(depth + 1);
    next.run(req).await
}

For JSON deserialization, use #[serde(deny_unknown_fields)] and validate nested structures:

#[derive(serde::Deserialize)]
#[serde(deny_unknown_fields)]
struct SafeInput {
    data: String,
    #[serde(default)]
    nested: NestedData,
}

#[derive(serde::Deserialize)]
struct NestedData {
    value: String,
    #[serde(default)]
    children: Vec<NestedData>, // Limited by Vec capacity, not recursion
}

middleBrick's Pro plan includes continuous monitoring that can detect when new stack overflow vulnerabilities are introduced through code changes, providing alerts before they reach production.