Email Injection in Axum with Dynamodb

Email injection in an Axum application that uses DynamoDB typically arises when user-controlled data is incorporated into email headers or body content without validation or encoding, and then the data is stored in or retrieved from DynamoDB. Axum is a Rust web framework, and while it does not directly handle email, patterns that build email messages from request inputs can become injection vectors if the data is later used by downstream services or libraries to construct emails.

DynamoDB itself is a NoSQL database and does not process email, but it can store user-submitted fields such as email addresses, display names, or message templates. If an attacker can control a field stored in DynamoDB and that field is later interpolated into an email header (e.g., To, From, Reply-To, Subject) or body without sanitization, the attacker can inject additional headers or content. For example, a newline character (%0A or \r\n) in a name field can cause header splitting, enabling the injection of arbitrary headers such as CC or BCC, or even additional email content after the stored data is read and used by an email-sending component.

In this stack, the risk is not in DynamoDB or Axum directly causing injection; it is in the handling and formatting of data retrieved from DynamoDB when composing emails. A common scenario: an Axum handler stores a user profile in DynamoDB, including a display_name field. Later, a background job or handler reads the profile from DynamoDB and builds an email using string concatenation or formatting, placing display_name into a header. If display_name contains sequences like \r\nAttacker: attacker@example.com, and the email library does not enforce strict header validation, injected headers may be interpreted by the receiving mail server or client.

DynamoDB’s attribute type system and conditional writes do not prevent this class of issue because injection is contextual to how retrieved data is used, not how it is stored. The scan checks for places where data from DynamoDB is used in email composition without encoding or validation. For Axum services, this typically manifests in handler code that builds emails using format! or similar macros with data sourced from database rows, and where newline or control characters are not stripped or encoded.

Remediation focuses on validating and sanitizing data at the point of email composition, not at storage. Ensure that any data read from DynamoDB intended for email headers is stripped of control characters and encoded appropriately. Below are concrete Axum handler examples with DynamoDB interactions using the official AWS SDK for Rust (aws-sdk-dynamodb).

First, define a safe email header helper that removes or replaces dangerous characters:

/// Sanitize a string for safe use in email headers (e.g., To, From, Subject).
/// Removes carriage return and line feed characters and trims whitespace.
fn sanitize_email_header(value: &str) -> String {
    value.replace("\r", "").replace("\n", "").trim().to_string()
}

Example Axum handler storing user input into DynamoDB and later using it safely in email composition:

use aws_sdk_dynamodb::Client as DynamoClient;
use axum::{routing::post, Router};
use serde::{Deserialize, Serialize};

#[derive(Debug, Deserialize, Serialize)]
struct UserProfile {
    user_id: String,
    display_name: String,
    email: String,
}

/// Handler that stores a profile in DynamoDB.
async fn store_profile(
    db: DynamoClient,
    profile: axum::extract::Json<UserProfile>,
) -> Result<axum::response::Response, (axum::http::StatusCode, String)> {
    let profile = profile.into_inner();
    // Store raw input in DynamoDB (acceptable; store as provided).
    db.put_item()
        .table_name("profiles")
        .item(
            "user_id",
            aws_sdk_dynamodb::types::AttributeValue::S(profile.user_id.clone()),
        )
        .item(
            "display_name",
            aws_sdk_dynamodb::types::AttributeValue::S(profile.display_name.clone()),
        )
        .item(
            "email",
            aws_sdk_dynamodb::types::AttributeValue::S(profile.email.clone()),
        )
        .send()
        .await
        .map_err(|e| (axum::http::StatusCode::INTERNAL_SERVER_ERROR, e.to_string()))?;

    Ok(axum::response::Response::new(axum::http::StatusCode::CREATED))
}

/// Handler that reads from DynamoDB and builds an email safely.
async fn build_email_for_user(
    db: DynamoClient,
    user_id: String,
) -> Result<String, (axum::http::StatusCode, String)> {
    let resp = db
        .get_item()
        .table_name("profiles")
        .key(
            "user_id",
            aws_sdk_dynamodb::types::AttributeValue::S(user_id),
        )
        .send()
        .await
        .map_err(|e| (axum::http::StatusCode::INTERNAL_SERVER_ERROR, e.to_string()))?;

    let item = resp.item().ok_or((axum::http::StatusCode::NOT_FOUND, "User not found".to_string()))?;
    let display_name = item.get("display_name")
        .and_then(|v| v.as_s().ok())
        .map(|s| sanitize_email_header(s))
        .unwrap_or_default();
    let email = item.get("email")
        .and_then(|v| v.as_s().ok())
        .map(|s| sanitize_email_header(s))
        .unwrap_or_default();

    // Safe composition: sanitized values used in headers/body.
    let subject = format!("Welcome, {}", display_name);
    let body = format!("Hello {}\nemail:{}", display_name, email);

    // In practice, pass subject and body to your email library.
    Ok(format!("To: {}\nSubject: {}\n\n{}", email, subject, body))
}

Key points:

• Store data in DynamoDB as provided; do not attempt to "clean" it for storage, as this can alter user intent and complicate audits.
• When reading data for email composition, sanitize header-specific fields by removing \r and \n and trimming whitespace. This prevents header splitting regardless of what was stored in DynamoDB.
• Use parameterized email libraries where possible, and avoid directly interpolating untrusted strings into header lines.

These patterns ensure that even if malicious input is stored in DynamoDB, it cannot compromise email header integrity when the data is later used in Axum-generated emails.