Email Injection in Axum

Email injection in Axum applications typically occurs when user input is incorporated into email headers or body without proper validation. Since Axum is an async web framework for Rust, these vulnerabilities often appear in route handlers that process form submissions or API requests intended to trigger email notifications.

The most common manifestation involves newline character injection ( ) that allows attackers to add arbitrary email headers. For example, a contact form endpoint might accept parameters like name, email, and message, then construct an email where these values are inserted directly into the message body or headers.

use axum::{routing::post, Json, Router};
use serde::Deserialize;

#[derive(Deserialize)]
struct ContactForm {
    name: String,
    email: String,
    message: String,
}

async fn contact_handler(form: Json<ContactForm>) -> String {
    let email_body = format!("Name: {}\nEmail: {}\nMessage: {}",
        form.name, form.email, form.message);
    
    // Send email using email library...
    "Thank you for your message!".to_string()
}

let app = Router::new().route("/contact", post(contact_handler));

The vulnerability appears when form.name or form.message contains newline characters. An attacker could submit:

name: Alice
Subject: Urgent

Important message here
message: This is the real message

This would create an email with modified headers, potentially changing the subject line or adding new headers like Bcc: to exfiltrate sensitive information.

Another Axum-specific scenario involves email templates rendered with user input. When using template engines like Askama or Handlebars with Axum, unescaped user input in email templates can lead to injection if the template engine doesn't properly handle special characters in email contexts.

Rate limiting in Axum can also interact with email injection. Without proper rate limiting on email-sending endpoints, attackers can flood your email infrastructure or use your server as a spam relay, amplifying the impact of any injection vulnerabilities.

Detecting email injection in Axum applications requires both static analysis and runtime scanning. Static analysis involves reviewing route handlers that process user input for email-related operations.

middleBrick's black-box scanning approach is particularly effective for Axum applications since it tests the running API without requiring source code access. The scanner examines unauthenticated endpoints for email injection vulnerabilities by sending payloads containing newline characters and observing how the application processes them.

For Axum applications, middleBrick tests specific patterns relevant to Rust/axum codebases:

• Newline character injection in form fields that become email content
• Header injection attempts in query parameters that might affect email generation
• Rate limiting bypass attempts on email-sending endpoints
• Template injection patterns in endpoints using Rust template engines
• Cross-site request forgery (CSRF) vulnerabilities that could automate email injection attacks

The scanner's LLM/AI security module also checks for AI-powered email generation endpoints, testing for prompt injection that could manipulate AI-generated email content.

To manually test Axum email endpoints, you can use curl or similar tools to send payloads with newline characters:

curl -X POST http://localhost:3000/contact \
  -H "Content-Type: application/json" \
  -d '{
    "name": "Test\r\nSubject: Hacked\r\nBcc: attacker@example.com",
    "email": "user@example.com",
    "message": "This is a test message"
  }'

Monitor your email server logs to see if the injected headers were processed. Additionally, implement logging in your Axum handlers to detect unusual patterns in email submissions.

Remediating email injection in Axum requires input validation and sanitization specific to email contexts. The most effective approach is to sanitize user input before incorporating it into emails.

Here's an Axum-specific remediation using Rust's sanitization libraries:

use axum::{routing::post, Json, Router};
use serde::Deserialize;
use sanitize::sanitize;

#[derive(Deserialize)]
struct ContactForm {
    name: String,
    email: String,
    message: String,
}

async fn contact_handler(form: Json<ContactForm>) -> String {
    // Sanitize input to remove newline characters and other dangerous content
    let sanitized_name = sanitize(&form.name);
    let sanitized_message = sanitize(&form.message);
    
    let email_body = format!("Name: {}\nEmail: {}\nMessage: {}",
        sanitized_name, form.email, sanitized_message);
    
    // Send email...
    "Thank you for your message!".to_string()
}

let app = Router::new().route("/contact", post(contact_handler));

The sanitize function from the sanitize crate removes newline characters and other potentially dangerous content. You can also implement custom sanitization:

fn sanitize_email_input(input: &str) -> String {
    input
        .replace('\r', "")
        .replace('\n', " ")
        .replace(":", " ")
        .trim()
        .to_string()
}

// In your handler:
let sanitized_name = sanitize_email_input(&form.name);
let sanitized_message = sanitize_email_input(&form.message);

For Axum applications using email libraries like lettre, ensure you're using the library's built-in validation features:

use lettre::{message::Mailbox, Message};

// Validate email format
let email_addr = form.email.parse::<Mailbox>()
    .map_err(|_| /* handle invalid email format */)?;

Implement rate limiting in Axum to prevent abuse of email endpoints:

use axum_extra::rate_limit::{RateLimit, RateLimitLayer};

let app = Router::new()
    .route("/contact", post(contact_handler))
    .layer(RateLimitLayer::new(RateLimit::by_key(
        |request: &axum::http::Request| {
            // Rate limit by IP address
            request.remote_addr().unwrap_or_else(|| "unknown".parse().unwrap())
        },
        100, // 100 requests
        std::time::Duration::from_secs(3600), // per hour
    )));

For template-based emails, use template engines that automatically escape content appropriately for email contexts, and always validate user input against expected formats before rendering.