client - Dave's Blog


WPAD Server Fiddler Extension Source

2016 Aug 5, 3:18

I've put my WPAD Fiddler extension source and the installer on GitHub.

Six years ago I made a WPAD DHCP server Fiddler extension (described previously and previously). The extension runs a WPAD DHCP server telling any clients that connect to connect to the running Fiddler instance. I've finally got around to putting the source on GitHub. I haven't touched it in five or so years so this is either for posterity or education or something.

PermalinkCommentsdhcp fiddler network security wpad

Let's Encrypt Setup

2016 Feb 4, 2:48

2016-Nov-5: Updated post on using Let's Encrypt with

I use for my webhosting for my personal website and I've just finished setting up TLS via Let's Encrypt. The process was slightly more complicated than what you'd like from Let's Encrypt. So for those interested in doing the same on, I've taken the following notes.

The standard Let's Encrypt client requires su/sudo access which is not available on's servers. Additionally NFSN's webserver doesn't have any Let's Encrypt plugins installed. So I used the Let's Encrypt Without Sudo client. I followed the instructions listed on the tool's page with the addition of providing the "--file-based" parameter to

One thing the script doesn't produce is the chain file. But this topic "Let's Encrypt - Quick HOWTO for NSFN" covers how to obtain that:

curl -o domain.chn

Now that you have all the required files, on your NFSN server make the directory /home/protected/ssl and copy your files into it. This is described in the NFSN topic provide certificates to NFSN. After copying the files and setting their permissions as described in the previous link you submit an assistance request. For me it was only 15 minutes later that everything was setup.

After enabling HTTPS I wanted to have all HTTP requests redirect to HTTPS. The normal Apache documentation on how to do this doesn't work on NFSN servers. Instead the NFSN FAQ describes it in "redirect http to https and HSTS". You use the X-Forwarded-Proto instead of the HTTPS variable because of how NFSN's virtual hosting is setup.

RewriteEngine on
RewriteCond %{HTTP:X-Forwarded-Proto} !https
RewriteRule ^.*$ https://%{SERVER_NAME}%{REQUEST_URI} [L,R=301]

Turning on HSTS is as simple as adding the HSTS HTTP header. However, the description in the above link didn't work because my site's NFSN realm isn't on the latest Apache yet. Instead I added the following to my .htaccess. After I'm comfortable with everything working well for a few days I'll start turning up the max-age to the recommended minimum value of 180 days.

Header set Strict-Transport-Security "max-age=3600;" 

Finally, to turn on CSP I started up Fiddler with my CSP Fiddler extension. It allows me to determine the most restrictive CSP rules I could apply and still have all resources on my page load. From there I found and removed inline script and some content loaded via http and otherwise continued tweaking my site and CSP rules.

After I was done I checked out my site on SSL Lab's SSL Test to see what I might have done wrong or needed improving. The first time I went through these steps I hadn't included the chain file which the SSL Test told me about. I was able to add that file to the same files I had already previously generated from the Let's Encrypt client and do another NFSN assistance request and 15 minutes later the SSL Test had upgraded me from 'B' to 'A'.

PermalinkCommentscertificate csp hsts https lets-encrypt

Retweet of grittygrease

2015 Jul 2, 9:53
TLS Client Puzzles Extension 

From Inside Edward Snowden’s Life as a Robot: Wizner had...

2014 Jun 23, 7:04

From Inside Edward Snowden’s Life as a Robot:

Wizner had to jump on a phone call during a meeting with his whistleblower client. When he got off the phone, he found that Snowden had rolled the bot into civil liberties lawyer Jameel Jaffer’s office and was discussing the 702 provision of the Foreign Intelligence Surveillance Act. “It was kind of cool,” Wizner says.

It is neat but they’re marketing video is at times strangely terrifying. Put different music on when the Susan-bot comes up behind the unknowing Mark and this could be a horror movie trailer.

PermalinkCommentsedward-snowden beam robot telepresence

Stripe CTF - Level 7

2012 Sep 13, 5:00

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


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
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


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 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.

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


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.

PermalinkCommentshash internet length-extension security sha1 stripe-ctf technical web

A New HTTP Status Code for Legally-restricted Resources

2012 Jun 11, 2:00

451 Unavailable for Legal Reasons: The 451 status code is optional; clients cannot rely upon its use. It is imaginable that certain legal authorities may wish to avoid transparency, and not only forbid access to certain resources, but also disclosure that the restriction exists.

That was fast.

PermalinkCommentshttp internet web 451 law legal rfc ietf censorship technical

Code: Flickr Developer Blog » Parsing Exif client-side using JavaScript

2012 Jun 1, 2:51

Flickr parses the exif out of images using Web Workers, and Blob (File API)!

PermalinkCommentsjavascript blob exif image technical programming dom webworker web-browser

Client Side Cross Domain Data YQL Hack

2012 Feb 27, 2:28

One of the more limiting issues of writing client side script in the browser is the same origin limitations of XMLHttpRequest. The latest version of all browsers support a subset of CORS to allow servers to opt-in particular resources for cross-domain access. Since IE8 there's XDomainRequest and in all other browsers (including IE10) there's XHR L2's cross-origin request features. But the vast majority of resources out on the web do not opt-in using CORS headers and so client side only web apps like a podcast player or a feed reader aren't doable.

One hack-y way around this I've found is to use YQL as a CORS proxy. YQL applies the CORS header to all its responses and among its features it allows a caller to request an arbitrary XML, HTML, or JSON resource. So my network helper script first attempts to access a URI directly using XDomainRequest if that exists and XMLHttpRequest otherwise. If that fails it then tries to use XDR or XHR to access the URI via YQL. I wrap my URIs in the following manner, where type is either "html", "xml", or "json":

        yqlRequest = function(uri, method, type, onComplete, onError) {
var yqlUri = "" +
encodeURIComponent("SELECT * FROM " + type + ' where url="' + encodeURIComponent(uri) + '"');

if (type == "html") {
yqlUri += encodeURIComponent(" and xpath='/*'");
else if (type == "json") {
yqlUri += "&callback=&format=json";

This also means I can get JSON data itself without having to go through JSONP.
PermalinkCommentsxhr javascript yql client-side technical yahoo xdr cors

Prime HTTP Status Codes

2012 Feb 22, 4:00
These are the prime HTTP status codes:
PermalinkCommentshttp prime technical useless

Mothereffing animated gif

2012 Feb 17, 7:31

Client side animated GIF creator web app.

PermalinkCommentstechnical javascript gif animated compression html5

Why I Like Glitch

2012 Feb 17, 4:00

Sarah and I have been enjoying Glitch for a while now. Reviews are usually positive although occasionally biting (but mostly accurate).

I enjoy Glitch as a game of exploration: exploring the game's lands with hidden and secret rooms, and exploring the games skills and game mechanics. The issue with my enjoyment coming from exploration is that after I've explored all streets and learned all skills I've got nothing left to do. But I've found that even after that I can have fun writing client side JavaScript against Glitch's web APIs making tools (I work on the Glitch Helperator) for use in Glitch. And on a semi-regular basis they add new features reviving my interest in the game itself.

PermalinkCommentsvideo-game glitch glitch-helperator me project game


2011 Jul 18, 2:38Neat idea: "When the user wants to visit a blacklisted site, the client establishes an encrypted HTTPS connection to a non-blacklisted web server outside the censor’s network, which could be a normal site that the user regularly visits... The client secretly marks the connection as a Telex request by inserting a cryptographic tag into the headers. We construct this tag using a mechanism called public-key steganography... As the connection travels over the Internet en route to the non-blacklisted site, it passes through routers at various ISPs in the core of the network. We envision that some of these ISPs would deploy equipment we call Telex stations."PermalinkCommentsinternet security tools censorship technical

Using client-side storage, today. ✩ Mozilla Hacks – the Web developer blog

2011 Apr 5, 5:14A JS wrapper script that lets you use storage in IE6/7 via userData or localStorage every where else.PermalinkCommentsjavascript html web webbrowser storage technical userdata localstorage

draft-nottingham-http-portal - The Network Authentication Required HTTP Status Code

2010 Oct 4, 2:05Proposed 428 HTTP error code for hijacking proxies to indicate to the client the user needs to login to the network etc. Glad to see this one's finally happening.PermalinkCommentshttp http-status captive-portal hijack proxy authentication technical rfc reference

What every programmer needs to know about game networking « Gaffer on Games

2010 Jul 5, 8:38"This way the player appears to control their own character without any latency, and provided that the client and server character simulation code is deterministic – giving exactly the same result for the same inputs on the client and server – it is rarely corrected."PermalinkCommentsnetwork programming game technical quake history

RFC 5854 - The Metalink Download Description Format

2010 Jun 1, 6:46"Metalink describes download locations (mirrors), cryptographic hashes, and other information. Clients can transparently use this information to reliably transfer files."PermalinkCommentstechnical internet download web url xml metalink

Installable Web Apps - Google Code

2010 May 24, 6:29Installable web apps makes total sense given the Google Chrome OS: "An installable web app is a normal web site with a bit of extra metadata. You build and deploy this app exactly as you would build and deploy any web app, using any server-side or client-side technologies you like. The only thing that is different about an installable web app is how the app is packaged."PermalinkCommentstechnical web browser webapp google chrome

The Emperor’s New APIs: On the (In)Secure Usage of New Client-side Primitives

2010 May 6, 7:43Covers case studies of insecure usage of HTML5 cross-document messaging and web storage.PermalinkCommentshtml html5 web browser security technical webstorage research facebook google system:filetype:pdf system:media:document

RFC 5849 - The OAuth 1.0 Protocol

2010 Apr 21, 6:49"OAuth provides a method for clients to access server resources on behalf of a resource owner (such as a different client or an end-user). It also provides a process for end-users to authorize third-party access to their server resources without sharing their credentials (typically, a username and password pair), using user-agent redirections."PermalinkCommentsoauth authorization security privacy internet web rfc standard technical

People's Republic of Interactive Fiction

2010 Apr 1, 2:42Its like a better version of what I was doing with my Web Frotz Interpreter. Its all client side javascript, HTML, & CSS to play Z-machine based interactive fiction games. They even do the saved game in a URL piece.PermalinkCommentsif interactive-fiction game web browser webapp
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