Interface Encoder

  • All Known Implementing Classes:
    DefaultEncoder

    public interface Encoder
    The Encoder interface contains a number of methods for decoding input and encoding output so that it will be safe for a variety of interpreters. To prevent double-encoding, callers should make sure input does not already contain encoded characters by calling canonicalize. Validator implementations should call canonicalize on user input before validating to prevent encoded attacks.

    All of the methods must use a "whitelist" or "positive" security model. For the encoding methods, this means that all characters should be encoded, except for a specific list of "immune" characters that are known to be safe.

    The Encoder performs two key functions, encoding and decoding. These functions rely on a set of codecs that can be found in the org.owasp.esapi.codecs package. These include:

    • CSS Escaping
    • HTMLEntity Encoding
    • JavaScript Escaping
    • MySQL Escaping
    • Oracle Escaping
    • Percent Encoding (aka URL Encoding)
    • Unix Escaping
    • VBScript Escaping
    • Windows Encoding

    Since:
    June 1, 2007
    Author:
    Jeff Williams (jeff.williams .at. aspectsecurity.com) Aspect Security
    • Method Detail

      • canonicalize

        java.lang.String canonicalize​(java.lang.String input)
        This method is equivalent to calling
        Encoder.canonicalize(input, restrictMultiple, restrictMixed);
        The default values for restrictMultiple and restrictMixed come from ESAPI.properties
         Encoder.AllowMultipleEncoding=false
         Encoder.AllowMixedEncoding=false
         
        Parameters:
        input - the text to canonicalize
        Returns:
        a String containing the canonicalized text
        See Also:
        canonicalize, W3C specifications
      • canonicalize

        java.lang.String canonicalize​(java.lang.String input,
                                      boolean strict)
        This method is the equivalent to calling
        Encoder.canonicalize(input, strict, strict);
        Parameters:
        input - the text to canonicalize
        strict - true if checking for multiple and mixed encoding is desired, false otherwise
        Returns:
        a String containing the canonicalized text
        See Also:
        canonicalize, W3C specifications
      • canonicalize

        java.lang.String canonicalize​(java.lang.String input,
                                      boolean restrictMultiple,
                                      boolean restrictMixed)
        Canonicalization is simply the operation of reducing a possibly encoded string down to its simplest form. This is important, because attackers frequently use encoding to change their input in a way that will bypass validation filters, but still be interpreted properly by the target of the attack. Note that data encoded more than once is not something that a normal user would generate and should be regarded as an attack.

        Everyone says you shouldn't do validation without canonicalizing the data first. This is easier said than done. The canonicalize method can be used to simplify just about any input down to its most basic form. Note that canonicalize doesn't handle Unicode issues, it focuses on higher level encoding and escaping schemes. In addition to simple decoding, canonicalize also handles:

        • Perverse but legal variants of escaping schemes
        • Multiple escaping (%2526 or <)
        • Mixed escaping (%26lt;)
        • Nested escaping (%%316 or &%6ct;)
        • All combinations of multiple, mixed, and nested encoding/escaping (%253c or ┦gt;)

        Using canonicalize is simple. The default is just...

             String clean = ESAPI.encoder().canonicalize( request.getParameter("input"));
         
        You need to decode untrusted data so that it's safe for ANY downstream interpreter or decoder. For example, if your data goes into a Windows command shell, then into a database, and then to a browser, you're going to need to decode for all of those systems. You can build a custom encoder to canonicalize for your application like this...
             ArrayList list = new ArrayList();
             list.add( new WindowsCodec() );
             list.add( new MySQLCodec() );
             list.add( new PercentCodec() );
             Encoder encoder = new DefaultEncoder( list );
             String clean = encoder.canonicalize( request.getParameter( "input" ));
         
        In ESAPI, the Validator uses the canonicalize method before it does validation. So all you need to do is to validate as normal and you'll be protected against a host of encoded attacks.
             String input = request.getParameter( "name" );
             String name = ESAPI.validator().isValidInput( "test", input, "FirstName", 20, false);
         
        However, the default canonicalize() method only decodes HTMLEntity, percent (URL) encoding, and JavaScript encoding. If you'd like to use a custom canonicalizer with your validator, that's pretty easy too.
             ... setup custom encoder as above
             Validator validator = new DefaultValidator( encoder );
             String input = request.getParameter( "name" );
             String name = validator.isValidInput( "test", input, "name", 20, false);
         
        Although ESAPI is able to canonicalize multiple, mixed, or nested encoding, it's safer to not accept this stuff in the first place. In ESAPI, the default is "strict" mode that throws an IntrusionException if it receives anything not single-encoded with a single scheme. This is configurable in ESAPI.properties using the properties:
         Encoder.AllowMultipleEncoding=false
         Encoder.AllowMixedEncoding=false
         
        This method allows you to override the default behavior by directly specifying whether to restrict multiple or mixed encoding. Even if you disable restrictions, you'll still get warning messages in the log about each multiple encoding and mixed encoding received.
             // disabling strict mode to allow mixed encoding
             String url = ESAPI.encoder().canonicalize( request.getParameter("url"), false, false);
         
        Parameters:
        input - the text to canonicalize
        restrictMultiple - true if checking for multiple encoding is desired, false otherwise
        restrictMixed - true if checking for mixed encoding is desired, false otherwise
        Returns:
        a String containing the canonicalized text
        See Also:
        W3C specifications
      • encodeForCSS

        java.lang.String encodeForCSS​(java.lang.String input)
        Encode data for use in Cascading Style Sheets (CSS) content.
        Parameters:
        input - the text to encode for CSS
        Returns:
        input encoded for CSS
        See Also:
        CSS Syntax [w3.org]
      • decodeForHTML

        java.lang.String decodeForHTML​(java.lang.String input)
        Decodes HTML entities.
        Parameters:
        input - the String to decode
        Returns:
        the newly decoded String
      • encodeForHTMLAttribute

        java.lang.String encodeForHTMLAttribute​(java.lang.String input)
        Encode data for use in HTML attributes.
        Parameters:
        input - the text to encode for an HTML attribute
        Returns:
        input encoded for use as an HTML attribute
      • encodeForJavaScript

        java.lang.String encodeForJavaScript​(java.lang.String input)
        Encode data for insertion inside a data value or function argument in JavaScript. Including user data directly inside a script is quite dangerous. Great care must be taken to prevent including user data directly into script code itself, as no amount of encoding will prevent attacks there. Please note there are some JavaScript functions that can never safely receive untrusted data as input – even if the user input is encoded. For example:
          <script>
              window.setInterval('<%= EVEN IF YOU ENCODE UNTRUSTED DATA YOU ARE XSSED HERE %>');
          </script>
         
        Parameters:
        input - the text to encode for JavaScript
        Returns:
        input encoded for use in JavaScript
      • encodeForVBScript

        java.lang.String encodeForVBScript​(java.lang.String input)
        Encode data for insertion inside a data value in a Visual Basic script. Putting user data directly inside a script is quite dangerous. Great care must be taken to prevent putting user data directly into script code itself, as no amount of encoding will prevent attacks there. This method is not recommended as VBScript is only supported by Internet Explorer
        Parameters:
        input - the text to encode for VBScript
        Returns:
        input encoded for use in VBScript
      • encodeForSQL

        java.lang.String encodeForSQL​(Codec codec,
                                      java.lang.String input)
        Encode input for use in a SQL query, according to the selected codec (appropriate codecs include the MySQLCodec and OracleCodec). This method is not recommended. The use of the PreparedStatement interface is the preferred approach. However, if for some reason this is impossible, then this method is provided as a weaker alternative. The best approach is to make sure any single-quotes are double-quoted. Another possible approach is to use the {escape} syntax described in the JDBC specification in section 1.5.6. However, this syntax does not work with all drivers, and requires modification of all queries.
        Parameters:
        codec - a Codec that declares which database 'input' is being encoded for (ie. MySQL, Oracle, etc.)
        input - the text to encode for SQL
        Returns:
        input encoded for use in SQL
        See Also:
        JDBC Specification
      • encodeForOS

        java.lang.String encodeForOS​(Codec codec,
                                     java.lang.String input)
        Encode for an operating system command shell according to the selected codec (appropriate codecs include the WindowsCodec and UnixCodec). Please note the following recommendations before choosing to use this method: 1) It is strongly recommended that applications avoid making direct OS system calls if possible as such calls are not portable, and they are potentially unsafe. Please use language provided features if at all possible, rather than native OS calls to implement the desired feature. 2) If an OS call cannot be avoided, then it is recommended that the program to be invoked be invoked directly (e.g., System.exec("nameofcommand" + "parameterstocommand");) as this avoids the use of the command shell. The "parameterstocommand" should of course be validated before passing them to the OS command. 3) If you must use this method, then we recommend validating all user supplied input passed to the command shell as well, in addition to using this method in order to make the command shell invocation safe. An example use of this method would be: System.exec("dir " + ESAPI.encodeForOS(WindowsCodec, "parameter(s)tocommandwithuserinput");
        Parameters:
        codec - a Codec that declares which operating system 'input' is being encoded for (ie. Windows, Unix, etc.)
        input - the text to encode for the command shell
        Returns:
        input encoded for use in command shell
      • encodeForLDAP

        java.lang.String encodeForLDAP​(java.lang.String input)
        Encode data for use in LDAP queries.
        Parameters:
        input - the text to encode for LDAP
        Returns:
        input encoded for use in LDAP
      • encodeForDN

        java.lang.String encodeForDN​(java.lang.String input)
        Encode data for use in an LDAP distinguished name.
        Parameters:
        input - the text to encode for an LDAP distinguished name
        Returns:
        input encoded for use in an LDAP distinguished name
      • encodeForXPath

        java.lang.String encodeForXPath​(java.lang.String input)
        Encode data for use in an XPath query. NB: The reference implementation encodes almost everything and may over-encode. The difficulty with XPath encoding is that XPath has no built in mechanism for escaping characters. It is possible to use XQuery in a parameterized way to prevent injection. For more information, refer to this article which specifies the following list of characters as the most dangerous: ^&"*';<>(). This paper suggests disallowing ' and " in queries.
        Parameters:
        input - the text to encode for XPath
        Returns:
        input encoded for use in XPath
        See Also:
        XPath Injection [ibm.com], Blind XPath Injection [packetstormsecurity.org]
      • encodeForXML

        java.lang.String encodeForXML​(java.lang.String input)
        Encode data for use in an XML element. The implementation should follow the XML Encoding Standard from the W3C.

        The use of a real XML parser is strongly encouraged. However, in the hopefully rare case that you need to make sure that data is safe for inclusion in an XML document and cannot use a parse, this method provides a safe mechanism to do so.

        Parameters:
        input - the text to encode for XML
        Returns:
        input encoded for use in XML
        See Also:
        XML Encoding Standard
      • encodeForXMLAttribute

        java.lang.String encodeForXMLAttribute​(java.lang.String input)
        Encode data for use in an XML attribute. The implementation should follow the XML Encoding Standard from the W3C.

        The use of a real XML parser is highly encouraged. However, in the hopefully rare case that you need to make sure that data is safe for inclusion in an XML document and cannot use a parse, this method provides a safe mechanism to do so.

        Parameters:
        input - the text to encode for use as an XML attribute
        Returns:
        input encoded for use in an XML attribute
        See Also:
        XML Encoding Standard
      • encodeForURL

        java.lang.String encodeForURL​(java.lang.String input)
                               throws EncodingException
        Encode for use in a URL. This method performs URL encoding on the entire string.
        Parameters:
        input - the text to encode for use in a URL
        Returns:
        input encoded for use in a URL
        Throws:
        EncodingException - if encoding fails
        See Also:
        URL encoding
      • decodeFromURL

        java.lang.String decodeFromURL​(java.lang.String input)
                                throws EncodingException
        Decode from URL. Implementations should first canonicalize and detect any double-encoding. If this check passes, then the data is decoded using URL decoding.
        Parameters:
        input - the text to decode from an encoded URL
        Returns:
        the decoded URL value
        Throws:
        EncodingException - if decoding fails
      • encodeForBase64

        java.lang.String encodeForBase64​(byte[] input,
                                         boolean wrap)
        Encode for Base64.
        Parameters:
        input - the text to encode for Base64
        wrap - the encoder will wrap lines every 64 characters of output
        Returns:
        input encoded for Base64
      • decodeFromBase64

        byte[] decodeFromBase64​(java.lang.String input)
                         throws java.io.IOException
        Decode data encoded with BASE-64 encoding.
        Parameters:
        input - the Base64 text to decode
        Returns:
        input decoded from Base64
        Throws:
        java.io.IOException