Email Injection in Actix with Bearer Tokens

Email injection occurs when user-controlled data is reflected into email headers or body content without proper sanitization, enabling attackers to inject additional headers (e.g., CC, BCC, or custom headers) or break out of the email format. In Actix web applications, this typically arises when request parameters or JSON payloads are used to compose email messages, and developer-supplied validation is limited to format checks like ensuring an @ symbol is present.

When Bearer Tokens are involved, the risk pattern shifts from simple header injection to authorization bypass and data exposure. For example, an endpoint that accepts a Bearer Token for authentication might also accept an email address as a user-controlled parameter. If the API uses that email to construct notification emails and does not validate or encode special characters (such as newline sequences or control characters), an attacker can inject malicious headers. A crafted payload like email=attacker%40example.com%0D%0AX-Custom:%20injected can introduce unintended headers when the email is rendered by an insecure mailer, potentially altering routing or exposing internal details.

In an Actix-based service, this becomes an issue when middleware or handler logic trusts the email field after token validation, rather than applying strict canonicalization. Bearer Tokens often gate access to user-specific endpoints; if the token is valid but the email input is unchecked, the authenticated context is abused to trigger injection. This can lead to unintended recipients, header smuggling, or injection into logs and monitoring systems. In a broader scan, such patterns are flagged by checks like Input Validation and Property Authorization, which correlate authenticated contexts with risky data flows.

Consider an endpoint that builds a confirmation email using format strings without sanitization:

let email = params.get("email").unwrap_or("");
let body = format!("Confirm your account at {}", email);

If email contains newline characters or additional header-like syntax, the composed message can be corrupted. When combined with Bearer Token–based authentication, the request appears legitimate, which may suppress generic anomaly detection while still enabling injection. This is why middleBrick’s checks for Input Validation and Data Exposure are particularly relevant: they identify places where authenticated email data reaches downstream systems without canonicalization.

Remediation focuses on strict input validation, canonicalization, and avoiding direct concatenation of user data into email structures. Treat email addresses as opaque strings and normalize them before use. In Actix, use extractor patterns that enforce validation via libraries such as validator or custom guards, and ensure that any data used in email composition is sanitized for SMTP-safe content.

Do not rely on simple substring checks (e.g., presence of @). Instead, use a two-step approach: (1) validate format with a robust email parser or regex that rejects control characters and whitespace, and (2) encode or remove characters that could alter message structure (e.g., carriage return and line feed). Below is a secure Actix handler example that demonstrates this approach:

use actix_web::{post, web, HttpResponse};
use validator::validate_email;

#[post("/notify")]
async fn notify_email(
    token: actix_web::HttpRequest,
    payload: web::Json<EmailRequest>,
) -> HttpResponse {
    // Bearer Token presence and scope checks would occur earlier in your auth middleware
    let email = payload.email.trim();

    // Strict validation: reject newlines, tabs, and invalid formats
    if !validate_email(email) {
        return HttpResponse::BadRequest().json(serde_json::json!({ "error": "invalid_email" }));
    }

    // Safe composition: no direct interpolation of raw user input
    let body = format!("Confirm your account at {}", sanitize_email(email));
    // sendmail or SMTP client usage here

    HttpResponse::Ok().finish()
}

fn sanitize_email(email: &str) -> String {
    email
        .replace('\r', "")
        .replace('\n', "")
        .replace(',', "")
}

#[derive(serde::Deserialize)]
struct EmailRequest {
    email: String,
}

This handler ensures that the email is trimmed, format-validated, and stripped of characters that could alter SMTP semantics. By removing carriage returns, line feeds, and commas, it prevents both header injection and multi-line payloads that could bypass naive filters.

For Bearer Token–specific concerns, couple this with middleware that verifies token scope and audience before the handler runs. If your authorization layer already confirms the token is valid and maps to a user context, the handler can trust the authenticated identity while still treating email as untrusted input. This separation of authentication and input validation aligns with middleBrick’s findings for BOLA/IDOR and Property Authorization checks, which look for scenarios where valid tokens do not guarantee safe data handling.

Additionally, when using OpenAPI specs, explicitly define email as a string with pattern constraints and mark it as required. Tools that generate clients from specs benefit from these constraints, reducing the likelihood of malformed or injection-prone values reaching your Actix routes.