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Defending Against Application Denial of Service Attacks [New Series]

By Mike Rothman

As we discussed last year in Defending Against Denial of Service Attacks, attackers increasingly leverage availability-impacting attacks both to cause downtime (which costs site owners money) and to mask other kinds of attacks. These availability-impacting attacks are better known as Denial of Service (DoS) attacks. Our research identified a number of adversaries who increasingly use DoS attacks, including:

  • Protection Racketeers: These criminals use DoS threats to demand ransom money. Attackers hold a site hostage by threatening to knock it down, and sometimes follow through. They get paid. They move on to the next target. The only thing missing is the GoodFellas theme music.
  • Hacktivists: DoS has become a favored approach of hacktivists seeking to make a statement and shine a spotlight on their cause, whatever it may be. Hacktivists care less about the target than their cause. The target is usually collateral damage, though they are happy hit two birds with one stone by attacking an organization that opposes their cause when they can. You cannot negotiate with these folks, and starting public discourse is one of their goals.
  • ‘CyberWar’: We don’t like the term – no one has been killed by browsing online (yet), but we can expect to see online attacks as a precursor to warplanes, ships, bombing, and ground forces. By knocking out power grids, defense radar, major media, and other critical technology infrastructure, the impact of an attack can be magnified.
  • Exfiltrators: These folks use DoS to divert attention from the real attack: stealing data they can monetize. This could be an intellectual property theft or a financial attack such as stealing credit cards. Either way, they figure that if they blow in your front door you will be too distracted to notice your TV scooting out through the garage. They are generally right.
  • Competitors: They say all’s fair in love and business. Some folks take that one a bit too far, and actively knock down competitor sites for an advantage. Maybe it’s during the holiday season. Maybe it happens after a competitor resists an acquisition or merger offer. It could be locking folks out from bidding on an auction. Your competition might screen scrape your online store to make sure they beat your pricing, causing a flood of traffic on a very regular and predictable basis. A competitor might try to ruin your hard-earned (and expensive) search rankings. Regardless of the reason, don’t assume an attacker is a nameless, faceless crime syndicate in a remote nation. It could be the dude down the street trying to get any advantage he can – legalities be damned.

Given the varied adversaries, it is essential to understand that two totally different types of attacks are commonly lumped under the generic ‘DoS’ label. The first involves the network, blasting a site with enough traffic (sometimes over 300gbps) to flood the pipes and overwhelm security and network devices, as well as application infrastructure. This volumetric attack basically is the ‘cyber’ version of hitting something a billion times with a rock. This brute force attack typically demands a scrubbing service and/or CDN (Content Delivery Network) to deal with the onslaught of traffic and keep sites available.

The second type of DoS attack targets weaknesses in applications. In Defending Against DoS we described an application attack as follows:

Application-based attacks are different – they target weaknesses in web application components to consume all the resources of a web, application, or database server to effectively disable it. These attacks can target either vulnerabilities or ‘features’ of an application stack to overwhelm servers and prevent legitimate traffic from accessing web pages or completing transactions.

These attacks require knowledge of the application and how to break or game it. They can be far more efficient than just blasting traffic at a network, and in many cases take advantage of legitimate features of the application, making defense all the harder.

We are pleased to launch the next chapter in our Denial of Service research, entitled “Defending Against Application Denial of Service Attacks” (yep, we are thinking way out of the box for titles). In this series we will dig far more deeply into application DoS attacks and provide both an overview of the tactic and possible mitigations for defense. Here is a preliminary list of what we intend to cover:

  • Application Server Attacks: The first group of AppDoS attacks targets the server and infrastructure stack. We will profile attacks such as Slowloris, Slow HTTP Post, RUDY, Slow read, and XerXes, discussing mitigations for each attack. We will also talk about brute force attacks on SSL (overwhelming servers with SSL handshake requests) and loading common pages – such as login, password reset, and store locators – millions of times.
  • Attacking the Stack: Targeting Databases and Programming Languages: In this post we will talk about the next layers in the application stack – including the database and languages used to build the application. Regarding database DoS, we will highlight some of our recent research in Dealing with Database Denial of Service.
  • Abusing Application Logic: As we continue to climb the application stack, we will talk about how applications are targeted directly with GET floods and variants. By profiling applications and learning which pages are most resource intensive, attackers can focus their efforts on the most demanding pages. To mitigate these attacks, we will discuss the roles of rate controls and input validation, as well as WAF and CDN based approaches to filter out attack requests before the application needs to deal with them.
  • Billions of Results Served: We will profile the common attacks which overwhelm applications by overflowing memory with billions of results from either search results or shopping carts. We will touch on unfriendly scrapers, including search engines and other catalog aggregators that perform ‘legitimate’ searching but can be gamed by attackers. These attacks can only be remediated within the application, so we will discuss mechanisms for doing that (without alienating the developers).
  • Building DoS Protections in: We will wrap up the series by talking about how to implement a productive process for working with developers to build in AppDoS protections. We will also discuss strategies for finding the balance between defense tactics implemented at the server, within the cloud, and directly in the application.

If we missed something please do let us know in the comments, and we’ll turn over some rocks and make sure to include any other AppDoS attack vectors.

By the end of this series you should have a guide and an understanding of how your applications will be attacked, and what you can do about it. As always, we will produce our research using our Totally Transparent Research methodology. And we would like to thank Akamai for potentially licensing this content at the end.

We will dive in later this week, looking at directly attacking application servers.

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