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Proper input sanitization can prevent insertion of malicious data into a subsystem such as a database. However, different subsystems require different types of sanitization. Fortunately, it is usually obvious which subsystems will eventually receive input which inputs, and consequently what type of sanitization is required.

Several subsystems exist for the purpose of showing outputoutputting data. An HTML renderer , as part of a web browser, is one common subsystem for displaying output. Since data that is Data sent to an output subsystem might not come directly from an untrusted sourcemay appear to originate from a trusted source. However, it is tempting dangerous to assume that no output sanitization is required. Data that is not properly sanitized for these subsystems can enable unnecessary because such data may indirectly originate from an untrusted source and may include malicious content. Failure to properly sanitize data passed to an output subsystem can allow several types of attacks. For example, an HTML renderer can be renderers are prone to HTML injection and Crosscross-Site Scripting site scripting (XSS) attacks [OWASP 2011].  (Note that, although this is not necessarily an attack from one site to another, the term Cross-Site Scripting attack is still applied to such attacks.) Therefore, output Output sanitization to prevent such attacks is as vital as input sanitization.

As with input validation, normalize data should be normalized before sanitizing it for malicious characters. All Properly encode all output characters other than those known to be safe should be encoded to avoid vulnerabilities caused by data that bypasses validation. See IDS01-J. Normalize strings before validating them for more information.

Noncompliant Code Example

This noncompliant code example uses the model-view-controller (MVC) concept of the Java EE based EE–based Spring Framework to display data to the user without encoding or escaping it. Because the data is sent to a web browser, the code is subject to both HTML injection and XSS attacks.

@RequestMapping("/getnotifications.htm")
public ModelAndView getNotifications(
  HttpServletRequest request, HttpServletResponse response) {
  ModelAndView mv = new ModelAndView();
  try {
    UserInfo userDetails = getUserInfo();
    List<Map<String,Object>> list = new ArrayList<Map<String, Object>>();
    List<Notification> notificationList = notificationService 
        NotificationService.getNotificationsForUserId(userDetails.getPersonId());
           
    for (Notification notification: notificationList) {
      Map<String,Object>mapObject> map = new HashMap<String, Object>();
      map.put("id", notification.getId());
      map.put("message", notification.getMessage());
      list.add(map);
    }
            
     mv.addObject("Notifications", list);
  } 
    catch(Throwable t) {
    // logLog to file and handle
  }
 
  return mv;
}

Compliant Solution

This compliant solution defines a ValidateOutput class that normalizes the output to a known character set, performs output sanitization using a white-list whitelist, and encodes any non-specified unspecified data values to enforce a double-checking mechanism. Note that the required white-listing whitelisting patterns may can vary according to the specific needs of different fields [OWASP 20082013].

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

  // Validates and encodes the input field based on a whitelist
  privatepublic 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 nonnonvalid 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 validnonvalid data
  publicprivate 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();
  }
}
 
// ...
 
@RequestMapping("/getnotifications.htm")
public ModelAndView getNotifications(HttpServletRequest request, HttpServletResponse response) {
  // 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 ValidationExceptionValidateOutput vo = new ValidateOutput();

  ModelAndView mv = new ModelAndView();
  try {
    UserInfo userDetails = getUserInfo();
    List<Map<String,Object>> list = new ArrayList<Map<String,Object>>();
    List<Notification> notificationList = 
        NotificationService.getNotificationsForUserId(userDetails.getPersonId());
           
    for (Notification notification: notificationList) {
    ValidateOutput vo  Map<String,Object> map = new ValidateOutput() HashMap<String,Object>();
      map.put("id", vo.validate("id", notification.getId()));
      map.put("message", vo.validate(description, input("message", notification.getMessage()));
      list.add(map);
    }
            
     mv.addObject("Notifications", list);
  }
  catch(Throwable t) {
    // PassLog to another system or display to the user
  }
}
 file and handle
  }
 
  return mv;
}

Output encoding and escaping is mandatory when accepting dangerous characters such as double quotes and angle braces. Even when input is whitelisted to disallow such characters, output escaping is recommended because it provides a second level of defense. Note that the exact escape sequence can vary depending on where the output is embedded. For example, untrusted output may occur in an HTML value attribute, CSS, URL, or script; output encoding routine will be different in each case. It is also impossible to securely use untrusted data in some contexts. Consult the OWASP XSS (Cross-Site Scripting) Prevention Cheat Sheet for more information on preventing XSS attacks.

Noncompliant Code Example

This noncompliant code example takes a user input query string and build a URL. Because the URL is not properly encoded, the URL returned may not be valid if it contains non-URL-safe characters, as per RFC 1738.

String buildUrl(String q) {
  String url = "https://example.com?query=" + q;
 
  return url;
}

For example, if the user supplies the input string "<#catgifs>", the url returned is "https://example.com?query=<#catgifs>" which is not a valid URL.

Compliant Solution (Java 8)

Use java.util.Base64 to encode and decode data when transferring binary data over mediums that only allow printable characters like URLs, filenames, and MIME.

String buildEncodedUrl(String q) {
    String encodedUrl = "https://example.com?query=" + Base64.getUrlEncoder().encodeToString(q.getBytes());
 
    return encodedUrl;
}

If the user supplies the input string "<#catgifs>", the url returned is "https://example.com?query=%3C%23catgifs%3E" which is a valid URL.See, also, the method weblogic.servlet.security.Utils.encodeXSS().

Applicability

Failure to encode or escape output before it is displayed or passed across a trust boundary can result in the execution of arbitrary code.

Automated Detection

Related Vulnerabilities

The Apache GERONIMO-1474 vulnerability, reported in January 2006, allowed attackers to submit URLs containing JavaScript.   The Web - Access -Log viewer did not Log Viewer failed to sanitize the data it forwarded to the administrator console, thereby enabling a classic Cross-Site Scripting XSS attack.

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