Sanitizing user input before adding it to the DOM in Javascript

Question

I m writing the JS for a chat application I m working on in my free time  and I need to have HTML identifiers that change according to user submitted data  This is usually something conceptually shaky enough that I would not even attempt it  but I don t see myself having much of a choice this time  What I need to do then is to escape the HTML id to make sure it won t allow for XSS or breaking HTML   Here s the code   var user id   escape id  var txt     lt div class  chut  gt                  lt div class  log  id  chut   user id    gt  lt  div gt                  lt textarea id  chut   user id   msg  gt  lt  textarea gt                  lt label for  chut   user id   to  gt To  lt  label gt                  lt input type  text  id  chut   user id   to  value   user id   readonly  readonly    gt                  lt input type  submit  id  chut   user id   send  value  Message   gt                lt  div gt      What would be the best way to escape id to avoid any kind of problem mentioned above  As you can see  right now I m using the built-in escape   function  but I m not sure of how good this is supposed to be compared to other alternatives  I m mostly used to sanitizing input before it goes in a text node  not an id itself

User · Answer

You could use a simple regular expression to assert that the id only contains allowed characters, like so:

if(id.match(/^[0-9a-zA-Z]{1,16}$/)){
    //The id is fine
}
else{
    //The id is illegal
}

My example allows only alphanumerical characters, and strings of length 1 to 16, you should change it to match the type of ids that you use.

By the way, at line 6, the value property is missing a pair of quotes, an easy mistake to make when you quote on two levels.

I can't see your actual data flow, depending on context this check may not at all be needed, or it may not be enough. In order to make a proper security review we would need more information.

In general, about built in escape or sanitize functions, don't trust them blindly. You need to know exactly what they do, and you need to establish that that is actually what you need. If it is not what you need, the code your own, most of the time a simple whitelisting regex like the one I gave you works just fine.

User · Answer

You need to take extra precautions when using user supplied data in HTML attributes  Because  attributes has many more attack vectors than output inside HTML tags   The only way to avoid XSS attacks is to encode everything except alphanumeric characters  Escape all characters with ASCII values less than 256 with the  amp  xHH  format  Which unfortunately may cause problems in your scenario  if you are using CSS classes and javascript to fetch those elements   OWASP has a good description of how to mitigate HTML attribute XSS   http   www owasp org index php XSS  Cross Site Scripting  Prevention Cheat Sheet RULE  233 - JavaScript Escape Before Inserting Untrusted Data into HTML JavaScript Data Values

User · Answer

Never use escape    It s nothing to do with HTML-encoding  It s more like URL-encoding  but it s not even properly that  It s a bizarre non-standard encoding available only in JavaScript   If you want an HTML encoder  you ll have to write it yourself as JavaScript doesn t give you one  For example   function encodeHTML s        return s replace   amp  g    amp amp    replace   lt  g    amp lt    replace    g    amp quot         However whilst this is enough to put your user id in places like the input value  it s not enough for id because IDs can only use a limited selection of characters   And   isn t among them  so escape   or even encodeURIComponent   is no good    You could invent your own encoding scheme to put any characters in an ID  for example   function encodeID s        if  s       return          return s replace    a-zA-Z0-9 -  g  function match            return     match 0  charCodeAt 0  toString 16                   But you ve still got a problem if the same user id occurs twice  And to be honest  the whole thing with throwing around HTML strings is usually a bad idea  Use DOM methods instead  and retain JavaScript references to each element  so you don t have to keep calling getElementById  or worrying about how arbitrary strings are inserted into IDs   eg    function addChut user id        var log  document createElement  div        log className   log       var textarea  document createElement  textarea        var input  document createElement  input        input value  user id      input readonly  True      var button  document createElement  input        button type   button       button value   Message        var chut  document createElement  div        chut className   chut       chut appendChild log       chut appendChild textarea       chut appendChild input       chut appendChild button       document getElementById  chuts   appendChild chut        button onclick  function             alert  Send   textarea value   to   user id               return chut      You could also use a convenience function or JS framework to cut down on the lengthiness of the create-set-appends calls there   ETA      I m using jQuery at the moment as a framework   OK  then consider the jQuery 1 4 creation shortcuts  eg    var log      lt div gt     className   log     var input      lt input gt     readOnly  true  val  user id             The problem I have right now is that I use JSONP to add elements and events to a page  and so I can not know whether the elements already exist or not before showing a message    You can keep a lookup of user id to element nodes  or wrapper objects  in JavaScript  to save putting that information in the DOM itself  where the characters that can go in an id are restricted   var chut lookup           function getChut user id        var key    map   user id      if  key in chut lookup          return chut lookup key       return chut lookup key   addChut user id        The  map  prefix is because JavaScript objects don t quite work as a mapping of arbitrary strings  The empty string and  in IE  some Object member names  confuse it

User · Answer

You can use this   function sanitize string      const map             amp      amp amp            lt      amp lt            gt      amp gt                 amp quot                 amp  x27                 amp  x2F           const reg      amp  lt  gt      ig    return string replace reg   match   gt  map match         Also see OWASP XSS Prevention Cheat Sheet

User · Answer

Since the text that you are escaping will appear in an HTML attribute  you must be sure to escape not only HTML entities but also HTML attributes   var ESC MAP           amp      amp amp          lt      amp lt          gt      amp gt               amp quot               amp  39       function escapeHTML s  forAttribute        return s replace forAttribute      amp  lt  gt     g      amp  lt  gt   g  function c            return ESC MAP c               Then  your escaping code becomes var user id   escapeHTML id  true    For more information  see Foolproof HTML escaping in Javascript

[javascript] Sanitizing user input before adding it to the DOM in Javascript

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