Weaponizing d0z.me: Improved HTML5 DDoS
Well, here were are, about three months since I initially released d0z.me, and I've finally gotten away from school and life for a bit this week and updated it. However, I think it was definitely worth the wait. You can grab the code over at d0z.me's new Google Code repository, and see it in action here.
Beyond making the backend code a little bit less of a disaster than it was originally, I have also made the attack itself significantly more effective. For the impatient among you, I will summarize the changes here:
- More efficient web worker implementation for making the requests.
- Some cosmetic changes that make it less obvious that an attack is occurring.
- Switched to POST requests by default, which allow us to hold server threads longer and exhaust a target's bandwidth.
- Lots of updates to the backend code.
Before I go on though, I'd like to send another big THANK YOU to Lavakumar Kuppan over at andlabs.org for his research, feedback, and suggestions. His research was what originally inspired d0z.me, and he has helped give me a few very useful suggestions on how to improve it. Go follow the andlabs blog. Also, thank you to everyone else who sent in bug reports, suggestions, etc since I released it. You rock.
Web Worker Changes
My original implementation of the HTML5 DDoS attack did its job well, but was not exactly polished. I had some ideas for speed improvements even at the time, but hadn't spent much time optimizing. As it was, it opened four webworkers, and only made one request at a time. This produced good results, but was very processor intensive for Firefox users, and wasted valuable time waiting for a response from the server at times. I also was unable to recreate the results from Lava's original presentation (although this later turned out to be a flaw in my testing procedure).
After I released d0z.me, Lava contacted me and suggested instead that I run one web worker and launch many simultaneous requests. Obviously, running multiple requests at a time is much more efficient. With some slight modifications to the pseudocode he provided (to ensure a full request queue is maintained), I was able to achieve slightly better speeds, using only two web workers instead of four.
Cosmetic Improvements
Originally, d0z.me also implemented an attack almost identical to that of JSLOIC, meaning that an image constantly reloaded in the background. While it added a few extra requests per second, it was rather insignificant compared to its HTML5 counterpart, could only perform GET requests, and had the serious downside of displaying a progress bar in some browsers. Because of this, it has now been removed. In addition, d0z.me now attempts to pull the embedded site's favicon as it's own, so as to appear more legitimate. With these two changes, the URL becomes the only way to tell the embedded site and d0z.me apart in most browsers.
Using POST Requests for Attack Amplification
Advantages to POST Attack
One limitation of the original d0z.me implementation was that it could do little in regards to consuming bandwidth. In addition, while it was able to overwhelm servers with the sheer number of requests, server threads were not held for a very sizable amount of time. This meant that it required a decent number of users to significantly affect performance (either by consuming all available threads, crashing the database, etc). Bandwidth and thread exhaustion are both commonly used DDoS techniques, so why can't we do the same with HTML5 DDoS? Well, turns out, we can!
While the original version of d0z.me used GET requests, we can also make POST requests via CORS. Of course, we can simply issue the same number of requests/second as we can with GET, meaning that in most situations, even without a payload, the effect will be similar. However, there are a number of advantages that POST gives us that GET does not that should be obvious.
Unlike the previous version, however, attackers don't need to find large files on the host to overwhelm the hosts' bandwidth. Given that the default maximum request size is 2GB on Apache, we can send quite sizable requests safely. Most configurations do, in fact, override this default, but we can still send decently large requests regardless. To ensure that it works on most hosts, d0z.me's attack is set to use a 1MB request body. In practice, this is more than sufficient to generate excessive amounts of traffic.
Beyond the bandwidth advantages, we also tie up the server threads for a much longer amount of time, as the host must receive the request before responding. While this doesn't use a "slow POST" style attack like Slowloris, it has a similar effect: tying up processing threads that must receive the overly large requests, and thereby slowing down response times drastically.
F*ckin' CORS, How Does It Work?
So what hosts does this affect, you might ask? Just CORS enabled hosts, right? Wrong.
The CORS working draft defines a series of steps that a browser should go through when attempting to make a cross origin request. First, it should check its cache to see if it has previously connected to this URL within the cache timeout period, and if it has, whether or not cross origin requests were allowed on that URL. If it was allowed, it can go ahead and make the request; if not, the request process should stop there. If, however, the URL is not in the cache, then the draft states that the browser should make a "pre-flight request", which is essentially an empty request that seeks to get the headers for that particular URL (see OPTIONS request). The exception to this rule, however, is if the request is a "simple request", i.e. a GET, HEAD, or POST request.
This means that rather than respecting all that silly "pre-flight request" nonsense when Javascript attempts to make a simple method cross-origin request, the browser can decide to simply forward the request along with whatever data was attached to it. That's right! We can send arbitrary POST data to arbitrary hosts as fast as the network allows. Clearly, this attack could pretty quickly inundate a host or cause its owner significant bandwidth charges. This is not at all out of reach of even small Twitter spam campaigns, and a hacked ad network could rival the power of the largest botnets. Judging from some of the traffic spikes I've seen in my few months with d0z.me (~300,000 hits one week), one could fairly easy gather the amount of traffic necessary to bring down sizable websites.
Final Notes
I considered adding HTTP referrer / origin obfuscation support, which I previously demonstrated was possible. However, as my goal is not to make d0z.me impossible to detect and block, and I still wanted people to be able to find the site if abuse occurs, I decided against doing so. I think it is sufficient that I have warned multiple times against using that to block attacks. It's a band-aid, not a fix. I also considered adding IE support, but the attack is significantly slower without the benefit of web workers. When IE adds support for web workers, I will attempt to add support for it.
I have left GET requests in as an option, although I believe that it is usually a less effective one. However, it may be better to use such an attack if a host disallows all POST requests, or if a.) CORS is enabled on the URL and b.) responding to that request causes the host to do a significant amount of processing. The GET attack also uses significantly less memory on machines viewing the link, which might be a consideration in some instances.
As I said earlier, it's been three months since I released d0z.me. As far as I can tell, all it has achieved is a GTFO message from Dreamhost and a decent number of complaint emails. I do like to think that it has raised awareness of some of the problems with URL shortnerers and HTML5, but no browsers have attempted to limit the number of XHRs that can be made in a given time period (except *maybe* Safari?), and no changes have been made to the CORS working draft. This needs to be fixed.
While I do find a lot of the issues involved here interesting, my main reason in making this new release is to again encourage browser developers and those working on the CORS draft to fix this problem, and do it quickly. I hope it will also be useful for administrators to gauge their systems' susceptibility to these attacks, as well as to come up with defenses against them.
As always, I certainly welcome any constructive criticisms or advice. PHP/Javascript is not my forte, as I'm positive is obvious in the code, so any tips from you gurus out there are much appreciated.
d0z.me Taken Down
This was not what was supposed to get posted this week, but sadly, this is what my time got spent on. From the d0z.me main page:
Hey all,
Dreamhost informed me today that they received complaints regarding d0z.me, which was not wholly unexpected. I would certainly have appreciated it if the complaints had been forwarded to me, so that I could take appropriate action; however, this did not occur. Dreamhost also proceeded to notify me that d0z.me, as-is, violates their terms of service. Unless I was willing to "ensure that (my) site can't be used to actually DoS anyone", I was told that I needed to remove the offending content altogether or risk having the account shut down permanently. While I do appreciate them contacting me before completely disabling my account, I still think that this stance is unwarranted.
d0z.me was never intended to be used as an attack tool. Rather, it was meant as a proof-of-concept that served to both illustrate the dangers posed by URL shorteners and HTML5, as well as to give concerned parties an easy way to test detection/mitigation techniques for the attack. I have quickly and consistently responded to all abuse requests I received, and ensured that offending links were removed. Of course, this could not prevent all abuse, and some certainly occurred. However, I still believe that d0z.me was and is simply a tool, one that could be used for positive ends or malicious ones, and should not be banned simply because it can be misused.
Given the situation, I have decided to temporarily take down the site while I search for a host that is more willing to stand up for its customers. As such, don't expect it to reliably be up over the next few days. I don't believe that any kind of artificial limitation on d0z.me's abilities will help prevent these kinds of attacks; rather, they will encourage small, lesser known sites to join the fray, making for a nearly impossible game of whack-a-d0z.me that would put users at more risk. While of course this will most likely happen with the site down as well, I at least will not have to waste my time crippling my code just for that to occur.
If you or someone you know would be willing to host d0z.me permanently, please let me know at supernothing 4T spareclockcycles D0T org. Feel free to grab the code here and start your own version of d0z.me as well, and help demonstrate the futility of censoring this site. Whether or not I find a new host, I will continue making updates to the d0z.me code, which will be posted on my blog. I am currently sidetracked a bit by this issue, but I have been working on definite improvements that I will be releasing soon.
Until then, follow me on Twitter for updates.
Peace,
Ben Schmidt (supernothing)
http://spareclockcycles.org
EDIT: After looking through some hosting providers, I have temporarily moved d0z.me to nearlyfreespeech.net (thanks to piecritic for pointing it out). Hopefully they live up to their namesake and let it be. We'll see how things go there, and I will move again if necessary. If you can't get to it yet, it's because the DNS entries are still propagating.
Follow-up On d0z.me: Some Thoughts
Security/Bug Fixes
I'm a big believer in being up front and loud about security vulnerabilities and software bugs that are found, so I wanted to first and foremost tell anyone using the d0z.me source code to grab an updated version. I have to apologize, as I really had not expected this site to get anywhere near as popular as quickly as it did, and so I had not spent a ton of time testing it before I put in production. This is a sadly common mistake that the worst of us tend to fall into :-/ . As such, one of the regexs I was using in a sanitize routine did not function correctly, allowing for XSS and SQL injection with a specially crafted URL. The hole was patched quickly, and no significant user data was taken (as I keep none), but I wanted to make sure everyone knew in case they're putting up their own versions. Props to the person on Reddit who pointed out the flaw to me. I'm not sure who it was, as it has since been deleted, but props all the same.
Secondly, a minor bug, but a reader named Max pointed out that I had mistyped a couple characters in my charset. Should have used Python -> string.letters+string.digits, but oh well. Thanks for the report!
Mitigation Techniques
Now, onto happier / more interesting topics!
DNS Blocking
An oldie, but goodie. Used to fight malware all the time, this would mitigate malicious shorteners by simply by redirecting the user to a warning page by changing the DNS entry. It was interesting to see that, within a day of releasing d0z.me, it was already being blocked by OpenDNS. Impressive response time, but given that it was posted all over Slashdot and Reddit before it got blocked, I think the response time was better than it usually is.
Conficker and IE 0-days? I don't think it's THAT dangerous...
I'm not going to spend much space discussing this one because, rather obviously, this is not a feasible technique in the long run, as it hasn't been particularly effective for malware either. Attackers can easily register domains more quickly than they're being blocked, and not all DNS providers provide such a defense for their users in the first place. However, it is a decent first line of defense on the user's end, and will at least block more popular and older malicious URL shorteners.
HTTP Referrer + mod_rewrite
Some smart people have been discussing the use of mod_rewrite (or similar) to redirect based on HTTP referrers to block these attacks (interested readers can find an excellent write-up at jaymill.net). Although it's an interesting approach that might deter the casual attacker, I see a couple big problems with this.
Firstly, it is absolutely trivial to make this attack function with a whole host of different HTTP referrers, simply by hosting a couple static files on more trusted domains. The HTTP referrer header and the Origin header for HTML5 cross-origin requests is based off the the source domain of the script location. To circumvent HTTP referrer protections, one needs only to upload a small html file and a Javascript file to Google Pages, Google App Engine, Amazon, etc, etc to give the requests a new domain. I don't believe most sites would be OK with blocking refers from a large number of these kinds of domains just to avoid this attack, unless the attack was already underway and serious. Also, the fact that such a large number of domains can be used means that whoever is managing the site would have to keep track and enter these rules for every single domain, which could become quite the herculean task.
I took the liberty of writing up a PoC demonstrating that, using a hidden iframe, one can make attacks like d0z.me appear to have a different referrer address (spareclockcycles.org). You can view it at http://d0z.me/poc_refer.html (note: this link attacks example.com, which should be local). If you look at the requests in Wireshark, you can see that the Origin and HTTP Referrer headers are set to spareclockcycles.org and spareclockcycles.org/evil.html respectively.
Requests with a different referrer
This was tested with both Firefox and Chrome, and this behavior conforms with what the working draft has to say on the topic. Some browsers might be non-compliant, but the big two apparently aren't.
Secondly, while this mitigates the attack somewhat, it simply raises the bar of how many users an attacker needs to recruit through clicks on malicious links. Enough traffic will still overwhelm the server, as valid connections are still being fully established, and some level of processing still has to be done on their requests.
I believe that Jeremy mentioned both of these issues, to some degree, in his post, and seemed to believe that both were of small enough concern that it doesn't really matter. I have to respectfully disagree on that point, however. I think that yes, it raises the bar slightly for an attacker, but in the long run proves very little hindrance to one with any level of technical ability. It is certainly a good recommendation for those currently under an attack; however, I think a more solid approach is needed to dealing with this issue in the long run, and that we need to eventually address it with a complete solution rather than with temporary band-aids.
Changing Cross-Origin Requests Standards
It is clear to me that this problem is going to have to be fixed with modifications on both ends, servers and browsers alike, if we want a real fix.
The main problem seems to be that browsers, when making HTML5 cross-origin requests, do not cache the fact that they have been denied access on a certain server. Most servers have no reason to allow cross-origin requests, and so rightly deny them across the board, but do so (per official specifications) by replying without the domain name of the requesting server in an Access-Control-Allow-Origin header. The browsers, in turn, cache the denial for that particular piece of content, but will willingly try to grab another piece of content immediately. This is particularly obvious if you watch d0z.me work in Chrome: the Javascript console fills up with denial after denial error, but the browser continues to pound the server with requests.
Chrome CORS Errors
This seems silly at first glance, but is actually a side-effect of the fact that different files/directories on the same site can have different cross-origin policies (i.e., you can make specific files cross-origin accessible by only adding the proper header when you respond to those requests). The browser doesn't want to cache a denial for the entire site when it's possible that some of the content could be allowed. So here is our problem: how do we remember cross origin denials for sites that have no such content, but still allow for sites to have differing policies for different sections of their site?
My first thought was to modify the standards to require any site that services cross-origin requests in any capacity to put an "Allows-Cross-Origin" header in all of their responses to cross-origin response. This way, if a browser tries to access the server once with a cross origin request and doesn't see this header, it can cache the denied response for a certain period of time before allowing any other requests to be sent, and be assured that there is no cross-origin content available on the server. This will mitigate the attack for a large portion of servers, as most have little reason to allow cross-origin requests in the first place. For those that do need to provide this service, it might be advisable for browsers to begin rate-limiting cross-origin requests, so as to minimize the potential effectiveness of this attack on those sites as well. There is no valid reason that I can think of that a browser should need to make 10,000 cross-origin requests a minute, so why let it?
This is by no means the only solution to this problem, however, and I would love to hear what other ideas people have on the matter. I very well could have overlooked a much more simple and effective fix, or missed problems with my own.
d0z.me Improvements/Updates
As I mentioned earlier, d0z.me was simply something I hacked together in a few hours for research purposes (and, of course, fun). This, unfortunately, led to code that is really not up to anyone's standards, including mine, as I had been more concerned with testing the attack than writing good stuff. I also did not experiment too much with the best ways to exploit the HTML5 cross-origin approach, as I was planning on doing this more later after I released the tool. I have been in discussion with the researcher who originally reported on this problem, Lavakumar Kuppan, who very kindly helped identify a number of places where my code could be improved upon.
Because of these things, I am working on releasing a more secure, reliable, and effective PoC, written for Django and the Google App Engine. The strain of keeping both spareclockcycles.org and d0z.me running has not been easy on my server, so it will be nice to get half the load into the cloud. This release will also hopefully include some significant refinements to the stealth and speed of the DoS. It will probably be delayed until after the holidays though, as I want to give concerned parties some time to come up with fixes for this issue before releasing a more potent tool, as well as give myself some time to make sure I get things right.
Statistics Analysis
I know I promised @sanitybit that I would have some stats on d0z.me today, but sadly, this post has already gotten way too long, and I'd like to spend some quality time poking around with the numbers. However, quick stats: at the time of this writing, I've had ~30,000 page views (14,000 unique visitors) on the d0z.me domain and ~19,000 pageviews (16,000 unique visitors) on spareclockcycles.org since Sunday. Definitely not bad, especially for someone who is used to having visitor counts in the tens...
But yeah, welcome to all my readers! More interesting statistics are still coming, so come back soon.
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