Header Injection in Fiber with Dynamodb

Header Injection in a Fiber application that interacts with DynamoDB occurs when untrusted input from an HTTP request is used to construct DynamoDB API parameters or to influence response headers without proper validation or encoding. This can lead to unintended behavior such as header manipulation, injection of additional requests, or exposure of sensitive information in responses. The risk is heightened when user-controlled data is directly passed into DynamoDB operations like GetItem, Query, or Scan, and the results are used to form HTTP headers.

For example, an endpoint that accepts a query parameter userId and uses it as a key in a DynamoDB request can become vulnerable if the response from DynamoDB is used to set headers such as x-user-id without sanitization. An attacker could supply a crafted value like userId=../../../etc/passwd in an attempt to manipulate downstream processing or leak data if the response includes sensitive payloads. While DynamoDB itself does not interpret header-like syntax, the application layer does, and unsafe mapping between DynamoDB results and HTTP headers creates the injection surface.

In a typical Fiber handler, the following pattern is risky:

app.Get('/user/:id', func(c *fiber.Ctx) error {
    id := c.Params('id')
    params := &dynamodb.GetItemInput{
        TableName: aws.String("Users"),
        Key: map[string]types.AttributeValue{
            "UserId": &types.AttributeValueMemberS{Value: id},
        },
    }
    result, err := svc.GetItem(context.TODO(), params)
    if err != nil {
        return c.SendStatus(fiber.StatusInternalServerError)
    }
    if result.Item != nil {
        if name, ok := result.Item["Name"]; ok {
            c.Set("x-user-name", name.ToString()) // Unsafe if name contains newline or header-like content
        }
    }
    return c.SendString("OK")
})

Here, the DynamoDB GetItem call retrieves an item, and the application directly sets an HTTP header using a value from the item. If the Name attribute contains newline characters or header-like sequences, it may corrupt the HTTP response and enable injection. This specific combination of Fiber routing, DynamoDB as the data source, and unsafe header assignment is where the vulnerability manifests.

Additionally, if the DynamoDB response is used to construct redirection URLs or other header-sensitive contexts, further risks emerge. For instance, an attacker might manipulate input to cause a redirect to a malicious site if the response includes a URL fragment that is not validated. The key issue is not DynamoDB itself but how its output is interpreted and assigned at the HTTP layer in Fiber.

To mitigate, always validate and sanitize any data from DynamoDB before using it in headers, and prefer using a strict allowlist for header values. Treat DynamoDB outputs as untrusted data, especially when they originate from user-supplied keys or filters.

Remediation focuses on strict input validation, safe mapping of DynamoDB results, and avoiding direct use of raw DynamoDB values in HTTP headers. Below are concrete, safe patterns for a Fiber application using the AWS SDK for DynamoDB.

First, validate and sanitize any user input used as DynamoDB keys:

func isValidUserID(id string) bool {
    matched, _ := regexp.MatchString(`^[a-zA-Z0-9_-]{1,64}$`, id)
    return matched
}

app.Get('/user/:id', func(c *fiber.Ctx) error {
    id := c.Params('id')
    if !isValidUserID(id) {
        return c.SendStatus(fiber.StatusBadRequest)
    }
    params := &dynamodb.GetItemInput{
        TableName: aws.String("Users"),
        Key: map[string]types.AttributeValue{
            "UserId": &types.AttributeValueMemberS{Value: id},
        },
    }
    result, err := svc.GetItem(context.TODO(), params)
    if err != nil {
        return c.SendStatus(fiber.StatusInternalServerError)
    }
    if result.Item != nil {
        if name, ok := result.Item["Name"]; ok {
            safeName := strings.ReplaceAll(name.ToString(), "\n", "")
            safeName = strings.ReplaceAll(safeName, "\r", "")
            c.Set("X-User-Name", safeName) // Safe header assignment
        }
    }
    return c.SendString("OK")
})

Second, when constructing values that may be used in headers or redirects, use explicit encoding and allowlists:

func buildSafeHeader(value types.AttributeValue) string {
    raw := value.ToString()
    // Remove characters that can break header formatting
    cleaned := strings.ReplaceAll(raw, "\n", "")
    cleaned = strings.ReplaceAll(cleaned, "\r", "")
    // Optionally, enforce a format, e.g., alphanumeric + limited symbols
    cleaned = regexp.MustCompile(`[^a-zA-Z0-9\-_.]`).ReplaceAllString(cleaned, "")
    return cleaned
}

// Usage in handler
if url, ok := result.Item["RedirectUrl"]; ok {
    safeURL := buildSafeHeader(url)
    if strings.HasPrefix(safeURL, "https://trusted.example.com/") {
        c.Redirect(fiber.StatusTemporaryRedirect, safeURL)
    }
}

Third, consider using structured responses instead of dynamic header injection. For example, return user metadata in the response body rather than in custom headers, which avoids the need to sanitize for header context entirely. If headers are necessary, use predefined values or mappings that do not depend on raw DynamoDB content.

Finally, leverage middleware to enforce header safety across routes. A simple logging or validation middleware can inspect outgoing headers and reject those containing unexpected newlines or patterns before they reach the client.