Friday, October 5, 2012

Defending Against DoS Attacks: The Attacks

Defending Against DoS Attacks: The Attacks:
Our first post built a case for considering availability as an aspect of security context, rather than only confidentiality and integrity. This has been driven by Denial of Service (DoS) attacks, which are used by attackers in many different ways, including extortion (using the threat of an attack), obfuscation (to hide exfiltration), hacktivism (to draw attention to a particular cause), or even friendly fire (when a promotion goes a little too well).

Understanding the adversary and their motivation is one part of the puzzle. Now let’s look at the types of DoS attacks you may face – attackers have many arrows in their quivers, and use them all depending on their objectives and targets.

Flooding the Pipes

The first kind of Denial of Service attack is really a blunt force object. It’s basically about trying to oversubscribe the bandwidth and computing resources of network (and increasingly server) devices to impact resource availability. These attacks aren’t very sophisticated, but as evidenced by the ongoing popularity of volume-based attacks, fairly effective effective. These tactics have been in use since before the Internet bubble, leveraging largely the same approach. But they have gotten easier with bots to do the heavy lifting. Of course, this kind of blasting must be done somewhat carefully to maintain the usefulness of the bot, so bot masters have developed sophisticated approaches to ensure their bots avoid ISPs penalty boxes. So you will see limited bursts of traffic from each bot and a bunch of IP address spoofing to make it harder to track down where the traffic is coming from, but even short bursts from 100,000+ bots can flood a pipe.

Quite a few specific techniques have been developed for volumetric attacks, but most look like some kind of flood. In a network context, the attackers focus on overfilling the pipes. Floods target specific protocols (SYN, ICMP, UDP, etc.), and work by sending requests to a target using the chosen protocol, but not acknowledging the response. Enough of these outstanding requests limit the target’s ability to communicate. But attackers need to stay ahead of Moore’s Law, because targets’ ability to handle floods has improved with processing power. So network-based attacks may include encrypted traffic, forcing the target to devote additional computational resources to process massive amounts of SSL traffic. Given the resource-intensive nature of encryption, this type of attack can melt firewalls and even IPS devices unless they are configured specifically for large-scale SSL support. We also see some malformed protocol attacks, but these aren’t as effective nowadays, as even unsophisticated network security perimeter devices drop bad packets at wire speed.

These volume-based attacks are climbing the stack as well, targeting web servers by actually completing connection requests and then making simple GET request and resetting the connection over and over again, with approximately the same impact as a volumetric attack – over-consumption of resources effectively knocking down servers. These attacks may also include a large payload to further consume bandwidth. The now famous Low Orbit Ion Cannon, a favorite tool of the hacktivist crowd, has undertaken a similar evolution, first targeting network resources and proceeding to now target web servers as well. It gets even better – these attacks can be magnified to increase their impact by simultaneously spoofing the target’s IP address and requesting sessions from thousands of other sites, which then bury the target in a deluge of misdirected replies, further consuming bandwidth and resources.

Fortunately defending against these network-based tactics isn’t overly complicated, as we will discuss in the next post, but without a sufficiently large network device at the perimeter to block these attacks or an upstream service provider/traffic scrubber to dump offending traffic, devices fall over in short order.

Overwhelming the Application

But attackers don’t only attack the network – they increasingly attack the applications as well, following the rest of attackers up the stack. Your typical n-tier web application will have some termination point (usually a web server), an application server to handle application logic, and then a database to store the data. Attackers can target all tiers of the stack to impact application availability. So let’s dig into each layer to see how these attacks work.

The termination point is usually the first target in application DoS attacks. They started with simple GET floods as described above, but quickly evolved to additional attack vectors. The best known application DoS attack is probably RSnake’s Slowloris, which consumes web server resources by sending partial HTTP requests, effectively opening connections and then leaving the sessions open by sending additional headers at regular intervals. This approach is far more efficient than the GET flood, requiring only hundreds of requests at regular intervals rather than constant thousands, and only requires one device to knock down a large site. These application attacks have evolved over time and now send complete HTTP requests to evade IDS and WAF devices looking for incomplete HTTP requests, but they tamper with payloads to confuse applications and consume resources. As defenders learn the attack vectors and deploy defenses, attackers evolve their attacks. The cycle continues.

Web server based attacks can also target weaknesses in the web server platform. For example the Apache Killer attack sends a malformed HTTP range request to take advantage of an Apache vulnerability. The Apache folks quickly patched the code to address this issue, but it shows how attackers target weaknesses in the underlying application stack to knock the server over. And of course unpatched Apache servers are still vulnerable today at many organizations. Similarly, the RefRef attack leverages SQL injection to inject a rogue .js file onto a server, which then hammers a backend database into submission with seemingly legitimate traffic originating from an application server. Again, application and database server patches are available for the underlying infrastructure, but vulnerability remains if either patch is missing.

Attackers can also target legitimate application functionality. One example of such an attack targets the search capability within a web site. If an attacker scripts a series of overly broad searches, the application can waste a large amount of time polling the database and presenting results. Likewise, attackers can game shopping carts by opening many shopping sessions, adding thousands of items to each cart, constantly refreshing the carts, and then abandoning them. Unless the application is architected to handle these use cases efficiently, such attacks accomplish their goals. For most sites, failing to return search results or track shopping carts is a complete failure with severe business ramifications. Which is the success scenario for a DoS attack.

The advantage to application attacks is their ability to evade many of the defenses put in place to stop DoS. Our Managing WAF series discussed WAF evasion, and IDS evasion is a similarly mature and effective attack discipline. These network security devices are little more than speed bumps to knowledgeable attackers, so developers need to ensure their applications can deal with the attacks. That’s another point we made in the Application Lifecycle Integration post.

Targeting the Defenses

Attackers can also perform some reconnaissance to learn about targets’ defenses in order to game them. For instance, financial institutions tend to do a lot of security monitoring due to severe regulatory oversight requirements, so if an attacker constantly loads web pages or enters dummy transactions they may manage to overwhelm the monitoring system. Does this impact application availability? Probably not, but it at least hampers the security team’s efforts to figure out what’s going on, providing an opportunity for another attack to exfiltrate data undetected.

Impacting the Wallet

An emerging DoS attack works economically even when it does not impair service availability. The so-called EDoS (economic denial of service) attack involves a focused attempt to increase the cost of the target’s technology infrastructure. In a network attack, bad guys take advantage of excessive bandwidth charges. So even if a target successfully defends against an attack, their bandwidth to mitigate a multi-gigabyte attack could cost as much or more than an outage. Similarly, if a target leverages public cloud infrastructure to auto-provision new instances as utilization thresholds are met, an application attack can have a substantial financial cost without ever threatening availability. One potential endgame for cloud-based attacks is reaching the target’s credit limit with their cloud provider, triggering an outage when the provider caps their usage.

Attackers can mix and match a variety of different DoS attacks to achieve their goal of adversely impacting availability of an application or service. Next we will move on to tactics and approaches to defend against DoS, starting with network attacks.

- Mike Rothman
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