Email Injection in Buffalo with Dynamodb

Email Injection occurs when user-controlled input is improperly sanitized before being used in email-related commands or headers, allowing an attacker to inject additional headers or commands. In a Buffalo application using Amazon DynamoDB as a persistence layer, the risk arises when data stored in DynamoDB is later used in email workflows without strict validation or encoding. For example, if a user registration flow stores an email address in DynamoDB and that value is later interpolated into mail headers or passed to an external mailer without sanitization, an attacker can inject newline characters (e.g., %0a or %0d) to add extra headers such as Cc:, Bcc:, or even inject SMTP commands.

DynamoDB itself does not execute or interpret email headers, so the injection surface is not in the database query but in how retrieved data is used downstream. A typical pattern in Buffalo is to load a user record by email from DynamoDB and then use that email value in a call to a mailer. If the stored email contains injected header content, the mailer may interpret it as part of the message envelope, leading to unintended recipients or header smuggling. This becomes particularly dangerous when the application uses unauthenticated or loosely configured mail servers that accept injected headers without enforcement.

Consider a scenario where a Buffalo handler retrieves a user profile from DynamoDB using the AWS SDK for Go, then passes the email to a mailer function. If the email field in DynamoDB was previously set via an unvalidated input channel (such as an API endpoint or form submission), it may contain sequences like \r\nSubject: Urgent Reset. When the mailer uses this value in a gomail.Message, the injected lines can alter the message routing or recipients. Because DynamoDB returns the data as a plain string, the application must treat it as untrusted input. The framework does not automatically sanitize data retrieved from DynamoDB, so developers must implement context-specific output encoding and header validation.

The OWASP API Top 10 category API1:2023 Broken Object Level Authorization intersects here when email values retrieved from DynamoDB are used without verifying that the requesting user is authorized to view or act on that data. Additionally, API9:2023 Improper Assets Management can manifest if logs or error messages inadvertently expose injected content, aiding reconnaissance. While the scan checks for BOLA/IDOR and Data Exposure across endpoints, developers must also ensure that any data flowing from DynamoDB into email contexts is validated against a strict allowlist of characters for email addresses, rejecting newlines, control characters, and special headers.

Remediation focuses on input validation at ingestion, strict type handling when reading from DynamoDB, and safe encoding before using data in email contexts. Always validate email addresses against a robust allowlist using a well-maintained library, and avoid direct interpolation of stored values into mail headers.

1. Validate and normalize email on write

When storing user emails in DynamoDB, validate and normalize the value in the application layer before persistence. Use a dedicated email parsing library to reject malformed or suspicious input.

import (
    "github.com/emicklei/go-restful/v3"
    "github.com/markbates/goth/gothic"
    "github.com/satori/go.uuid"
    "github.com/asaskevich/govalidator"
)

func (u User) ValidateAndStoreEmail(request *restful.Request, db dynamoClient) error {
    email := request.QueryParameter("email")
    if !govalidator.IsEmail(email) {
        return fmt.Errorf("invalid email format")
    }
    // Normalize per RFC 6854: lowercase and trim whitespace
    normalized := strings.ToLower(strings.TrimSpace(email))
    item := map[string]attribute.Value{
        "user_id": attribute.ValueOf(uuid.NewV4().String()),
        "email":   attribute.ValueOf(normalized),
    }
    _, err := db.PutItem(context.TODO(), &dynamodb.PutItemInput{
        TableName: aws.String("users"),
        Item:      item,
    })
    return err
}

2. Sanitize data when reading for email workflows

When retrieving user data from DynamoDB, treat the email field as untrusted. Re-validate and encode before passing to mailers. Do not rely on stored values to be clean.

import (
    "context"
    "github.com/aws/aws-sdk-go-v2/service/dynamodb"
    "github.com/aws/aws-sdk-go-vendor/aws"
    "github.com/gomarkdown/markdown/parser"
    "net/mail"
)

func GetUserEmailForSending(ctx context.Context, db dynamoClient, userID string) (string, error) {
    out, err := db.GetItem(ctx, &dynamodb.GetItemInput{
        TableName: aws.String("users"),
        Key: map[string]attribute.Value{
            "user_id": attribute.ValueOf(userID),
        },
    })
    if err != nil {
        return "", err
    }
    emailAttr, ok := out.Item["email"]
    if !ok {
        return "", fmt.Errorf("email attribute missing")
    }
    // Convert to string safely
    email := emailAttr.ToString()
    // Re-validate and parse to reject any injection artifacts
    parsedEmail, err := mail.ParseAddress(email)
    if err != nil {
        return "", fmt.Errorf("invalid email in record: %w", err)
    }
    // Use parsedEmail.Address for mail headers, never raw user input
    return parsedEmail.Address, nil
}

3. Use context-aware encoding in mailers

When constructing email messages, use a mail library that separates headers from body and does not allow raw newline injection. Configure the mailer to add recipients via structured methods rather than header concatenation.

import (
    "github.com/gomarkdown/markdown/renderer"
    "github.com/microcosm-cc/bluemonday"
    "gopkg.in/gomail.v2"
)

func SendWelcomeEmail(toEmail string) error {
    m := gomail.NewMessage()
    h := gomail.SetHeader
    // Use SetHeader which safely replaces existing headers
    m.SetHeader("From", "noreply@example.com")
    m.SetHeader("To", toEmail) // gomail handles proper header formatting
    m.SetHeader("Subject", "Welcome")
    m.SetBody("text/plain", "Welcome to our platform.")
    d := gomail.NewDialer("smtp.example.com", 587, "user", "password")
    return d.DialAndSend(m)
}

These steps ensure that data flowing from DynamoDB into email contexts is validated, normalized, and encoded, reducing the risk of injection through stored user data. They align with the scan checks for Input Validation and Data Exposure, and they complement the framework’s broader security posture without relying on automatic fixes.