IDS04-J. Properly encode or escape output

Proper input validation can ensure that malicious data is not inserted into the system. However, it fails to provide the assurance that validated data will remain consistent throughout its lifetime. For instance, if an insider is allowed to insert data into a database without validation, it is possible to glean unauthorized information or execute arbitrary code on the client side by means of attacks such as Cross Site Scripting (XSS). Consequently, output filtering is as important as input validation.

As with input validation, data must be normalized before it is filtered for malicious characters. To ensure that any data that bypasses the validation does not cause vulnerabilities, it is highly recommended that output characters be encoded, except those that are known to be safe.

Noncompliant Code Example

This noncompliant code example displays input obtained from a database directly to the user without performing any output validation or encoding.

public class BadOutput {
  // description and input are String variables containing values obtained from a database
  // description = "description" and input = "<script> executable code </script>"
  public static void display(String description, String input) {
    // Display to the user or pass description and input to another system
  }
}

Compliant Solution

This compliant solution defines a ValidateOutput class that normalizes the output to a known character set, performs output validation using a white-list and encodes any non-specified data values to enforce a double checking mechanism. Different fields may require different white-listing patterns. [[OWASP 08]]

public class ValidateOutput {
  // Allows only alphanumeric characters and spaces
  private Pattern pattern = Pattern.compile("^[a-zA-Z0-9\\s]{0,20}$");

  // Validates and encodes the input field based on a whitelist
  private String validate(String name, String input) throws ValidationException {
    String canonical = normalize(input);

    if(!pattern.matcher(canonical).matches()) {
      throw new ValidationException("Improper format in " + name + " field");
    }
    
    // Performs output encoding for non valid characters 
    canonical = HTMLEntityEncode(canonical);
    return canonical;
  }

  // Normalizes to known instances 	
  private String normalize(String input) {
    String canonical = java.text.Normalizer.normalize(input, Normalizer.Form.NFKC);
    return canonical;
  }

  // Encodes non valid data
  public static String HTMLEntityEncode(String input) {
    StringBuffer sb = new StringBuffer();

    for (int i = 0; i < input.length(); i++) {
      char ch = input.charAt(i);
        if (Character.isLetterOrDigit(ch) || Character.isWhitespace(ch)) {
          sb.append(ch);
        } else {
          sb.append("&#" + (int)ch + ";");
        }
    }
    return sb.toString();
  }

  // description and input are String variables containing values obtained from a database
  // description = "description" and input = "2 items available"
  public static void display(String description, String input) throws ValidationException {
    ValidateOutput vo = new ValidateOutput();
    vo.validate(description, input);
    // Pass to another system or display to the user
  }
}

Risk Assessment

Failure to encode or escape output before it is displayed or passed to another system can result in the execution of arbitrary code on the client's side.