Email Injection in Actix with Mutual Tls

Email injection in Actix typically arises when user-controlled data is concatenated into email headers without validation. Actix-web applications that build messages via string formatting (e.g., adding a user-supplied name or reply-to address) can allow an attacker to inject additional headers such as CC, BCC, or newlines that alter the message routing or recipients. This is a classic input validation issue, mapped as one of the 12 parallel checks in middleBrick, and it remains relevant even when Mutual TLS is used to authenticate and encrypt transport.

Mutual TLS (mTLS) in Actix ensures that both client and server present valid certificates, which provides strong identity assurance and protects against on-path tampering. However, mTLS does not constrain how the application processes request content. If an endpoint accepts a JSON body like {"email": "[email protected]" } and uses that value directly in an email header, an authenticated client with a valid certificate can still supply newline characters (e.g., alice%40example.com%0d%0aCc:%[email protected]) to inject extra headers. middleBrick’s Input Validation and Unsafe Consumption checks are designed to surface these risks by analyzing the unauthenticated attack surface and identifying places where untrusted input reaches messaging logic.

In practice, the combination of mTLS and email injection creates a false sense of security: transport-layer authentication and encryption prevent network-level interception and client impersonation, but they do not stop a legitimate, authenticated client from sending malicious header content. The server must therefore treat all user-supplied data as untrusted, regardless of the strength of the TLS mutual authentication. Attack patterns like CRLF injection (CVE-2019-11071-related techniques in mail libraries) demonstrate how injected sequences can break header parsing, leading to spoofed recipients or unintended message disclosure. middleBrick’s LLM/AI Security checks do not apply here because this is a traditional input validation flaw, yet the scanner’s cross-referencing of OpenAPI specs with runtime findings helps pinpoint endpoints where header-building logic may be vulnerable.

Concrete risk example: an endpoint POST /notify with mTLS that reads {"to": "[email protected]" } and builds a mailto header by simple concatenation. If the to field is not sanitized, an attacker can supply user%40example.com%0d%0aBcc:%[email protected]. The server may interpret the newline and forward a copy to an unintended recipient while the mTLS session appears legitimate. This illustrates why input validation must be enforced independently of transport security, and why findings from middleBrick’s parallel checks remain essential even for services using Mutual TLS.

Remediation focuses on strict input validation and safe header construction, independent of the TLS layer. In Actix, you should parse and sanitize user data before using it in any email header, and avoid building headers via string concatenation. Use dedicated header types that prevent injection, and apply allow-lists for permitted characters in email addresses.

Below are concrete, syntactically correct Actix examples that demonstrate how to implement mTLS alongside safe email handling. The first snippet shows a minimal Actix server configured for mTLS using rustls, with proper certificate and key loading. This configuration ensures client certificates are required and validated by the server.

use actix_web::{web, App, HttpServer, Responder};
use actix_web::http::header::{HeaderValue, CONTENT_TYPE};
use actix_web::middleware::Logger;
use std::sync::Arc;
use rustls::{ServerConfig, Certificate, PrivateKey};
use rustls_pemfile::{certs, pkcs8_private_keys};
use std::fs::File;
use std::io::BufReader;

async fn notify(web::Json(payload): web::Json) -> impl Responder {
    // Safe: validate and construct headers using actix_web::http::HeaderMap
    let mut headers = actix_web::http::HeaderMap::new();
    headers.insert(CONTENT_TYPE, HeaderValue::from_static("text/plain"));

    // Validate email with a simple allow-list (alphanum, @, dot, hyphen, underscore)
    let email_regex = regex::Regex::new(r"^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$").unwrap();
    if !email_regex.is_match(&payload.to) {
        return actix_web::HttpResponse::BadRequest().body("invalid email");
    }

    // Use a safe header value; HeaderValue::from_str will reject newlines
    let to_header = HeaderValue::from_str(&payload.to).unwrap();
    headers.insert("X-To", to_header);

    // Proceed to send email using a library that expects validated data
    actix_web::HttpResponse::Ok().headers(headers).body("queued")
}

#[derive(serde::Deserialize)]
struct NotifyPayload {
    to: String,
}

#[actix_web::main]
async fn main() -> std::io::Result<()> {
    // Load server cert and key
    let cert_file = &mut BufReader::new(File::open("server-cert.pem").unwrap());
    let key_file = &mut BufReader::new(File::open("server-key.pem").unwrap());
    let cert_chain: Vec = certs(cert_file).unwrap().into_iter().map(Certificate).collect();
    let mut keys: Vec = pkcs8_private_keys(key_file).unwrap().into_iter().map(PrivateKey).collect();

    let mut server_config = ServerConfig::builder()
        .with_safe_defaults()
        .with_no_client_auth() // we will require client auth via verify_cert later
        .with_single_cert(cert_chain, keys.remove(0))
        .unwrap();

    // Require client authentication
    server_config.client_auth_root_subjects = vec![/* trusted CA subject */ /* omitted for brevity */];
    server_config.client_auth_mode = rustls::server::ClientAuthMode::Required;

    let config = Arc::new(server_config);

    HttpServer::new(move || {
        App::new()
            .wrap(Logger::default())
            .route("/notify", web::post().to(notify))
    })
    .bind_rustls("127.0.0.1:8443", config.clone())?
    .run()
    .await
}

The second snippet focuses on the handler where email headers are constructed. It demonstrates strict validation and the use of HeaderValue to prevent newline injection. Even with mTLS ensuring the client is authenticated, user input is never directly placed into headers. The example also shows how to return a 400 for malformed email formats, which aligns with the scanner’s findings and remediation guidance.

Additionally, you can integrate middleBrick’s CLI to verify that such endpoints are included in scans: use middlebrick scan <url> to test the unauthenticated surface and, if you have authenticated flows, extend tests with authenticated crawls. For pipelines, the GitHub Action can enforce a threshold so that builds fail if risky patterns are detected, while the Web Dashboard and MCP Server in your IDE help track scores and findings over time.