The DOM location interface exposes the HTML document's URI parsed into its properties. However, it is ancient and has problems that bug me but otherwise rarely show up in the real world. Complaining about mostly theoretical issues is why blogging exists, so here goes:

• The location object's search, hash, and protocol properties are all misnomers that lead to confusion about the correct terms:
  • The 'search' property returns the URI's query property. The query property isn't limited to containing search terms.
  • The 'hash' property returns the URI's fragment property. This one is just named after its delimiter. It should be called the fragment.
  • The 'protocol' property returns the URI's scheme property. A URI's scheme isn't necessarily a protocol. The http URI scheme of course uses the HTTP protocol, but the https URI scheme is the HTTP protocol over SSL/TLS - there is no HTTPS protocol. Similarly for something like mailto - there is no mailto wire protocol.
• The 'hash' and 'search' location properties both return null in the case that their corresponding URI property doesn't exist or if its the empty string. A URI with no query property and a URI with an empty string query property that are otherwise the same, are not equal URIs and are allowed by HTTP to return different content. Similarly for the fragment. Unless the specific URI scheme defines otherwise, an empty query or hash isn't the same as no query or hash.

But like complaining about the number of minutes in an hour none of this can ever change without huge compat issues on the web. Accordingly I can only give my thanks to Anne van Kesteren and the awesome work on the URL standard moving towards a more sane (but still working practically within the constraints of compat) location object and URI parsing in the browser.

I just got a FitBit One from my wife. Unfortunately I had issues running their app on my Windows 8.1 Preview machine. But I recognized the errors as IE compatibility issues, for instance an IE dialog popup from the FitBit app telling me about an error in the app's JavaScript. Given my previous post on WebOC versioning you may guess what I tried next. I went into the registry and tried out different browser mode and document mode versions until I got the FitBit software running without error. Ultimately I found the following registry value to work well ('FitBit connect.exe' set to DWORD decimal 8888).

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Internet Explorer\Main\FeatureControl\FEATURE_BROWSER_EMULATION]
"Fitbit Connect.exe"=dword:000022b8

For those familiar with the Windows registry the above should be enough. For those not familiar, copy and paste the above into notepad, save as a file named "fitbit.reg", and then double click the reg file and say 'Yes' to the prompt. Hopefully in the final release of Windows 8.1 this won't be an issue.

When writing a JavaScript library that uses postMessage and the message event, I must be considerate of other JS code that will be running along side my library. I shouldn't assume I'm the only sender and receiver on a caller provided MessagePort object. This means obviously I should use addEventListener("message" rather than the onmessage property (see related What if two programs did this?). But considering the actual messages traveling over the message channel I have the issue of accidentally processing another libraries messages and having another library accidentally process my own message. I have a few options for playing nice in this regard:

Require a caller provided unique MessagePort

This solves the problem but puts a lot of work on the caller who may not notice nor follow this requirement.

Uniquely mark my messages

To ensure I'm acting upon my own messages and not messages that happen to have similar properties as my own, I place a 'type' property on my postMessage data with a value of a URN unique to me and my JS library. Usually because its easy I use a UUID URN. There's no way someone will coincidentally produce this same URN. With this I can be sure I'm not processing someone else's messages. Of course there's no way to modify my postMessage data to prevent another library from accidentally processing my messages as their own. I can only hope they take similar steps as this and see that my messages are not their own.

Use caller provided MessagePort only to upgrade to new unique MessagePort

I can also make my own unique MessagePort for which only my library will have the end points. This does still require the caller to provide an initial message channel over which I can communicate my new unique MessagePort which means I still have the problems above. However it clearly reduces the surface area of the problem since I only need once message to communicate the new MessagePort.

The best solution is likely all of the above.
Photo is Sharing by leezie5. Two squirrels sharing food hanging from a bird feeder. Used under Creative Commons license Attribution-NonCommercial-NoDerivs 2.0 Generic.

My first app for Windows 8 was Shout Text. You type into Shout Text, and your text is scaled up as large as possible while still fitting on the screen, as you type. It is the closest thing to a Hello World app as you'll find on the Windows Store that doesn't contain that phrase (by default) and I approached it as the simplest app I could make to learn about Windows modern app development and Windows Store app submission.

I rely on WinJS's default layout to use CSS transforms to scale up the user's text as they type. And they are typing into a simple content editable div.

The app was too simple for me to even consider using ads or charging for it which I learned more about in future apps.

The first interesting issue I ran into was that copying from and then pasting into the content editable div resulted in duplicates of the containing div with copied CSS appearing recursively inside of the content editable div. To fix this I had to catch the paste operation and remove the HTML data from the clipboard to ensure only the plain text data is pasted:

        function onPaste() {
            var text;

            if (window.clipboardData) {
                text = window.clipboardData.getData("Text").toString();
                window.clipboardData.clearData("Html");
                window.clipboardData.setData("Text", util.normalizeContentEditableText(text));
            }
        }
        shoutText.addEventListener("beforepaste", function () { return false; }, false);
        shoutText.addEventListener("paste", onPaste, false);

I additionally found an issue in IE in which applying a CSS transform to a content editable div that has focus doesn't move the screen position of the user input caret - the text is scaled up or down but the caret remains the same size and in the same place on the screen. To fix this I made the following hack to reapply the current cursor position and text selection which resets the screen position of the user input caret.

        function resetCaret() {
            setTimeout(function () {
                var cursorPos = document.selection.createRange().duplicate();
                cursorPos.select();
            }, 200);
        }

        shoutText.attachEvent("onresize", function () { resetCaret(); }, true);

The recent short story Twitter API returning results that do not respect arrow of time by Tim May written as a Twitter bug report reminded me of a few other short sci-fi stories written in the style of some sort of Internet document:

• comp.basalisk FAQ by David Langford written as a FAQ.
• Wikihistory by Desmond Warzel presented as a forum thread.
• The Riddle of the Universe and Its Solution by Christopher Cherian written as a scientific report.

Level 7 of the Stripe CTF involved running a length extension attack on the level 7 server's custom crypto code.

Code

@app.route('/logs/')
@require_authentication
def logs(id): 
    rows = get_logs(id) 
    return render_template('logs.html', logs=rows) 

...

def verify_signature(user_id, sig, raw_params):
    # get secret token for user_id
    try:
        row = g.db.select_one('users', {'id': user_id})
    except db.NotFound:
        raise BadSignature('no such user_id')
    secret = str(row['secret'])

    h = hashlib.sha1()
    h.update(secret + raw_params)
    print 'computed signature', h.hexdigest(), 'for body', repr(raw_params)
    if h.hexdigest() != sig:
        raise BadSignature('signature does not match')
    return True

Issue

The level 7 web app is a web API in which clients submit signed RESTful requests and some actions are restricted to particular clients. The goal is to view the response to one of the restricted actions. The first issue is that there is a logs path to display the previous requests for a user and although the logs path requires the client to be authenticatd, it doesn't restrict the logs you view to be for the user for which you are authenticated. So you can manually change the number in the '/logs/[#]' to '/logs/1' to view the logs for the user ID 1 who can make restricted requests. The level 7 web app can be exploited with replay attacks but you won't find in the logs any of the restricted requests we need to run for our goal. And we can't just modify the requests because they are signed.

However they are signed using their own custom signing code which can be exploited by a length extension attack. All Merkle–Damgård hash algorithms (which includes MD5, and SHA) have the property that if you hash data of the form (secret + data) where data is known and the length but not content of secret is known you can construct the hash for a new message (secret + data + padding + newdata) where newdata is whatever you like and padding is determined using newdata, data, and the length of secret. You can find a sha-padding.py script on VNSecurity blog that will tell you the new hash and padding per the above. With that I produced my new restricted request based on another user's previous request. The original request was the following.

count=10&lat=37.351&user_id=1&long=%2D119.827&waffle=eggo|sig:8dbd9dfa60ef3964b1ee0785a68760af8658048c

The new request with padding and my new content was the following.

count=10&lat=37.351&user_id=1&long=%2D119.827&waffle=eggo%80%02%28&waffle=liege|sig:8dbd9dfa60ef3964b1ee0785a68760af8658048c

My new data in the new request is able to overwrite the waffle parameter because their parser fills in a map without checking if the parameter existed previously.

Notes

Code review red flags included custom crypto looking code. However I am not a crypto expert and it was difficult for me to find the solution to this level.

Level 5 of the Stripe CTF revolved around a design issue in an OpenID like protocol.

Code

    def authenticated?(body)
      body =~ /[^\w]AUTHENTICATED[^\w]*$/
    end

...

      if authenticated?(body)
        session[:auth_user] = username
        session[:auth_host] = host
        return "Remote server responded with: #{body}." \
               " Authenticated as #{username}@#{host}!"

Issue

This level is an implementation of a federated identity protocol. You give it an endpoint URI and a username and password, it posts the username and password to the endpoint URI, and if the response is 'AUTHENTICATED' then access is allowed. It is easy to be authenticated on a server you control, but this level requires you to authenticate from the server running the level. This level only talks to stripe CTF servers so the first step is to upload a document to the level 2 server containing the text 'AUTHENTICATED' and we can now authenticate on a level 2 server. Notice that the level 5 server will dump out the content of the endpoint URI and that the regexp it uses to detect the text 'AUTHENTICATED' can match on that dump. Accordingly I uploaded an authenticated file to

https://level02-2.stripe-ctf.com/user-ajvivlehdt/uploads/authenticated

Using that as my endpoint URI means authenticating as level 2. I can then choose the following endpoint URI to authenticate as level 5.

https://level05-1.stripe-ctf.com/user-qtoyekwrod/?pingback=https%3A%2F%2Flevel02-2.stripe-ctf.com%2Fuser-ajvivlehdt%2Fuploads%2Fauthenticated&username=a&password=a

Navigating to that URI results in the level 5 server telling me I'm authenticated as level 2 and lists the text of the level 2 file 'AUTHENTICATED'. Feeding this back into the level 5 server as my endpoint URI means level 5 seeing 'AUTHENTICATED' coming back from a level 5 URI.

Notes

I didn't see any particular code review red flags, really the issue here is that the regular expression testing for 'AUTHENTICATED' is too permisive and the protocol itself doesn't do enough. The protocol requires only a set piece of common literal text to be returned which makes it easy for a server to accidentally fall into authenticating. Having the endpoint URI have to return variable text based on the input would make it much harder for a server to accidentally authenticate.

Level 4 and level 6 of the Stripe CTF had solutions around XSS.

Level 4

Code

> Registered Users 

<%@registered_users.each do |user| %>
<%last_active = user[:last_active].strftime('%H:%M:%S UTC') %>
<%if @trusts_me.include?(user[:username]) %>

• 
<%= user[:username] %>
(password: <%= user[:password] %>, last active <%= last_active %>)


Issue

The level 4 web application lets you transfer karma to another user and in doing so you are also forced to expose your password to that user. The main user page displays a list of users who have transfered karma to you along with their password. The password is not HTML encoded so we can inject HTML into that user's browser. For instance, we could create an account with the following HTML as the password which will result in XSS with that HTML:

This HTML runs script that uses jQuery to post to the transfer URI resulting in a transfer of karma from the attacked user to the attacker user, and also the attacked user's password.

Notes

Code review red flags in this case included lack of encoding when using user controlled content to create HTML content, storing passwords in plain text in the database, and displaying passwords generally. By design the web app shows users passwords which is a very bad idea.

Level 6

Code

...

    def self.safe_insert(table, key_values)
      key_values.each do |key, value|
        # Just in case people try to exfiltrate
        # level07-password-holder's password
        if value.kind_of?(String) &&
            (value.include?('"') || value.include?("'"))
          raise "Value has unsafe characters"
        end
      end

      conn[table].insert(key_values)
    end

Issue

This web app does a much better job than the level 4 app with HTML injection. They use encoding whenever creating HTML using user controlled data, however they don't use encoding when injecting JSON data into script (see post_data initialization above). This JSON data is the last five most recent messages sent on the app so we get to inject script directly. However, the system also ensures that no strings we write contains single or double quotes so we can't get out of the string in the JSON data directly. As it turns out, HTML lets you jump out of a script block using no matter where you are in script. For instance, in the middle of a value in some JSON data we can jump out of script. But we still want to run script, so we can jump right back in. So the frame so far for the message we're going to post is the following: