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Proper input validation can ensure that 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 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 vital as input sanitization.

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 should be normalized before sanitizing 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 Properly encode all output characters other than those known to be safe 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 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 ModelAndView getNotifications(
  HttpServletRequest request, HttpServletResponse response) {
  publicModelAndView mv static= voidnew displayModelAndView();
  try {
    UserInfo userDetails = getUserInfo();
    List<Map<String,Object>> list = new ArrayList<Map<String, Object>>();
    List<Notification> notificationList = 
        // description and input are String variables containing values obtained from a database
    // description = "description" and input = "<script> XSS </script>"
    
    // display to the user or pass description and input to another system
  }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) {
    // Log to file and handle
  }
 
  return mv;
}

Compliant Solution

Wiki MarkupThis compliant solution defines a {{ValidateOutput}} class that normalizes the output to a known character set, performs output validation sanitization using a whitelist , and encodes any non-specified data to enforce a double checking mechanism. Different fields may require different whitelisting patterns. \[[OWASP 08|AA. Java References#OWASP 08]\]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].

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

  // validatesValidates 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");
    }
    
    // performsPerforms output encoding for nonnonvalid valid characters 
    canonical = HTMLEntityEncode(canonical);
    return canonical;
  }

  // normalizesNormalizes 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++i) {
      char ch = input.charAt(i);
        if (Character.isLetterOrDigit(ch) || Character.isWhitespace(ch)) {
          sb.append(ch);
        } else {
          sb.append("&amp;amp;#" + (int)ch + ";");
        }
    }
    return sb.toString();
  }
}
 
// ...
 
@RequestMapping("/getnotifications.htm")
public ModelAndView getNotifications(HttpServletRequest request, HttpServletResponse response) {
  ValidateOutput vo = new ValidateOutput();

  ModelAndView publicmv static= voidnew displayModelAndView();
 throws ValidationExceptiontry {
    UserInfo userDetails =  // description and input are String variables containing values obtained from a database
    // description = "description" and input = "2 items available"
    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.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[OWASP 08|AA. Java References#OWASP 08]\] [How to add validation logic to HttpServletRequest|http://www.owasp.org/index.php/How_to_add_validation_logic_to_HttpServletRequest] and [How to perform HTML entity encoding in Java|http://www.owasp.org/index.php/How_to_perform_HTML_entity_encoding_in_Java]
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 116|http://cwe.mitre.org/data/definitions/116.html] "Improper Encoding or Escaping of Output"

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) {
    // Log to 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.

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

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Image Added Image Added Image AddedIDS12-J. Prevent XML external entity attacks      10. Input Validation and Data Sanitization (IDS)      IDS14-J. Do not use locale dependent methods on locale insensitive data