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Proper input validation sanitization can prevent insertion of malicious data into the system. However, such validation fails to provide the assurance that validated data remains consistent throughout its lifetime. For instance, if an insider is allowed to insert data into a database without validation, he can glean unauthorized information or execute arbitrary code on the client side by means of attacks such as Cross Site Scripting (XSS). Output filtering a subsystem such as a database. However, different subsystems require different types of sanitization. Fortunately, it is usually obvious which subsystems will eventually receive which inputs, and consequently what type of sanitization is required.

Several subsystems exist for the purpose of outputting data. An HTML renderer is one common subsystem for displaying output. Data sent to an output subsystem may appear to originate from a trusted source. However, it is dangerous to assume that output sanitization is 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, HTML renderers are prone to HTML injection and cross-site scripting (XSS) attacks [OWASP 2011]. Output sanitization to prevent such attacks is as important vital as input validationsanitization.

As with input validation, data should be normalized data before filtering sanitizing it for malicious characters. To avoid vulnerabilities caused data that may bypass validation, we recommended that Properly encode all output characters other than those known to be safe should be encodedto 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 displays input obtained from a database directly uses the model-view-controller (MVC) concept of the Java EE–based Spring Framework to display data to the user without performing any output validation or encodingencoding 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 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 inputModelAndView 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.getNotificationsForUserId(userDetails.getPersonId());
           
    for (Notification notification: notificationList) {
      Map<String,Object> 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) {
    // DisplayLog to the user or pass descriptionfile and inputhandle
 to another}
 system
  }return mv;
}

Compliant Solution

...

This compliant solution defines a {{ValidateOutput}} class that normalizes the output to a known character set, performs output validation sanitization 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 2008|AA. Bibliography#OWASP 08]\a whitelist, and encodes any unspecified data values to enforce a double-checking mechanism. Note that the required whitelisting patterns can vary according to the specific needs of different fields [OWASP 2013].

Code Block
bgColor #ccccff
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 nonnonvalid valid 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) { ValidateOutput //vo 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);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) { Map<String,Object> map = new HashMap<String,Object>(); map.put("id", vo.validate("id", notification.getId())); map.put("message", vo.validate("message", notification.getMessage())); list.add(map); } mv.addObject("Notifications", list); } catch(Throwable t) { // PassLog to anotherfile systemand orhandle display to} the user return }mv; }

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 in the other system.

Related Vulnerabilities

GERONIMO-1474

Bibliography

\[[MITRE 2009|AA. Bibliography#MITRE 09]\] [CWE ID 116|http://cwe.mitre.org/data/definitions/116.html] "Improper Encoding or Escaping of Output"
\[[OWASP 2008|AA. Bibliography#OWASP 08]\] [How to add validation logic to HttpServletRequest|http://www.owasp.org/index.php/How_to_add_validation_logic_to_HttpServletRequest], [How to perform HTML entity encoding in Java|http://www.owasp.org/index.php/How_to_perform_HTML_entity_encoding_in_Java], [XSS (Cross Site Scripting) Prevention Cheat Sheet|http://www.owasp.org/index.php/XSS_%28Cross_Site_Scripting%29_Prevention_Cheat_Sheet#Escaping_.28aka_Output_Encoding.29]

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.

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 failed to sanitize the data it forwarded to the administrator console, thereby enabling a classic XSS attack.

Bibliography

...

Image Added Image Added Image AddedIDS03-J. Do not delete non-character code points      13. Input Validation and Data Sanitization (IDS)      IDS05-J. Library methods should validate their parameters