Injection Flaws in Axum with Mutual Tls

Injection flaws in Axum applications using mutual Transport Layer Security (mTLS) often arise when server-side request handling does not sufficiently validate or sanitize inputs, even though the transport itself is authenticated and encrypted. mTLS ensures that both client and server present valid certificates, which strongly authenticates the peer. However, authentication at the transport layer does not automatically protect against injection attacks at the application layer, such as SQL injection, command injection, or template injection, especially when the server processes untrusted data from authenticated clients.

In Axum, a common pattern is to extract typed request parts (JSON body, headers, path parameters) and pass them to business logic or database layers. If these inputs are concatenated into queries or system commands without proper parameterization or escaping, an authenticated client with a valid mTLS certificate can still trigger injection. For example, an API endpoint that builds a SQL query by interpolating a user_id from a JSON payload, even when the request uses mTLS, remains vulnerable if the input is not validated or parameterized. Attackers who possess valid certificates (possibly obtained via compromised internal systems or stolen client credentials) can craft payloads designed to manipulate query structure.

Additionally, mTLS setups in Axum may inadvertently encourage trust in request metadata. Headers such as X-Forwarded-For or custom authorization tokens might be assumed safe because the connection is mTLS-secured. If Axum routes or logs these values and they later appear in error messages or system commands, injection via these channels becomes feasible. The risk is compounded when Axum applications integrate with external services or generate dynamic queries using data that originated from authenticated but potentially malicious clients.

Real-world injection patterns observed in Axum services include unsanitized string formatting for SQL, unsafe deserialization, and unchecked template variables. These map to OWASP API Top 10 categories such as Broken Object Level Authorization (BOLA) when injection enables privilege escalation across mTLS-authenticated sessions. Notably, mTLS does not protect against injection within the application logic; it only secures the channel. Therefore, robust input validation, strict type constraints, and parameterized interactions remain essential regardless of transport security.

middleBrick scans Axum endpoints to detect injection risks even when mTLS is in place, checking for unsafe data handling in authenticated contexts. The tool evaluates how Axum routes and extractors handle potentially malicious inputs from certificate-authenticated requests, highlighting areas where validation or parameterization is insufficient. By correlating runtime behavior with OpenAPI specifications, it can identify mismatches between declared security expectations and actual implementation, including scenarios where mTLS presence might obscure injection surfaces.

To remediate injection risks in Axum with mTLS, focus on strict input validation, parameterized queries, and safe handling of request data irrespective of transport security. Below are concrete Axum code examples demonstrating secure practices alongside insecure patterns to avoid.

Secure Axum endpoint with proper input validation and SQL parameterization

use axum::{
    routing::get,
    Router,
    extract::Query,
    response::IntoResponse,
};
use serde::Deserialize;
use sqlx::PgPool;

#[derive(Deserialize)]
struct UserParams {
    user_id: i32,
}

async fn get_user_handler(
    Query(params): Query,
    pool: &axum::extract::State,
) -> impl IntoResponse {
    // Use parameterized query to prevent SQL injection
    let user = sqlx::query_as!(User, "SELECT id, name FROM users WHERE id = $1", params.user_id)
        .fetch_optional(pool.inner())
        .await
        .unwrap_or_default();
    // Safe response handling
}

fn main() {
    let pool = PgPool::connect("postgres://user:pass@localhost/db").await.unwrap();
    let app = Router::new()
        .route("/users", get(get_user_handler))
        .with_state(pool);
}

Insecure Axum endpoint vulnerable to injection despite mTLS

// Avoid: string interpolation in SQL
async fn unsafe_user_handler(
    Query(params): Query>,
) -> String {
    let user_id = params.get("user_id").unwrap_or(&"1".to_string());
    format!("SELECT * FROM users WHERE id = {}", user_id) // Injection risk
}

Handling headers and external data safely

Even with mTLS, sanitize and validate any data extracted from headers or forwarded values before using them in commands or logs. For example, avoid directly interpolating header values into shell commands:

use std::process::Command;

// Risky: unsanitized header used in command
let raw_header = req.headers().get("X-Custom-Input").and_then(|v| v.to_str().ok());
if let Some(val) = raw_header {
    Command::new("sh")
        .arg("-c")
        .arg(format!("echo {}", val)) // Injection risk if val is untrusted
        .output();
}

Instead, validate and restrict header values, and avoid shell command construction with external input. Use structured logging that escapes special characters.

For LLM-related endpoints in Axum, apply the same discipline: validate and constrain all inputs, and do not assume mTLS prevents prompt injection or data exfiltration through authenticated channels. middleBrick’s LLM security checks can help identify whether Axum services with mTLS remain exposed to prompt injection or output leakage.