Security Externalities and DDOS Attacks

Ed Felten has a really good blog post about the externalities that the recent Spamhaus DDOS attack exploited:

The attackers’ goal was to flood Spamhaus or its network providers with Internet traffic, to overwhelm their capacity to handle incoming network packets. The main technical problem faced by a DoS attacker is how to amplify the attacker’s traffic-sending capacity, so that the amount of traffic arriving at the target is much greater than the attacker can send himself. To do this, the attacker typically tries to induce many computers around the Internet to send large amounts of traffic to the target.

The first stage of the attack involved the use of a botnet, consisting of a large number of software agents surreptitiously installed on the computers of ordinary users. These bots were commanded to send attack traffic. Notice how this amplifies the attacker’s traffic-sending capability: by sending a few commands to the botnet, the attacker can induce the botnet to send large amounts of attack traffic. This step exploits our first externality: the owners of the bot-infected computers might have done more to prevent the infection, but the harm from this kind of attack activity falls onto strangers, so the computer owners had a reduced incentive to prevent it.

Rather than having the bots send traffic directly to Spamhaus, the attackers used another step to further amplify the volume of traffic. They had the bots send queries to DNS proxies across the Internet (which answer questions about how machine names like related to IP addresses like This amplifies traffic because the bots can send a small query that elicits a large response message from the proxy.

Here is our second externality: the existence of open DNS proxies that will respond to requests from anywhere on the Internet. Many organizations run DNS proxies for use by their own people. A well-managed DNS proxy is supposed to check that requests are coming from within the same organization; but many proxies fail to check this — they’re “open” and will respond to requests from anywhere. This can lead to trouble, but the resulting harm falls mostly on people outside the organization (e.g. Spamhaus) so there isn’t much incentive to take even simple steps to prevent it.

To complete the attack, the DNS requests were sent with false return addresses, saying that the queries had come from Spamhaus — which causes the DNS proxies to direct their large response messages to Spamhaus.

Here is our third externality: the failure to detect packets with forged return addresses. When a packet with a false return address is injected, it’s fairly easy for the originating network to detect this: if a packet comes from inside your organization, but it has a return address that is outside your organization, then the return address must be forged and the packet should be discarded. But many networks fail to check this. This causes harm but — you guessed it — the harm falls outside the organization, so there isn’t much incentive to check. And indeed, this kind of packet filtering has long been considered a best practice but many networks still fail to do it.

I’ve been writing about security externalities for years. They’re often much harder to solve than technical problems.

By the way, a lot of the hype surrounding this attack was media manipulation.

via Schneier on Security

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