security, Security / Cyberattacks and Hacks

Web Giants Scrambled to Head Off a Dangerous DDoS Technique

In October 2016, a botnet of hacked security cameras and internet routers called Mirai aimed a gargantuan flood of junk traffic at the servers of Dyn, one of the companies that provides the global directory for the web known as the Domain Name System or DNS. The attack took down Amazon, Reddit, Spotify, and Slack temporarily for users along the East Coast of the US. Now one group of researchers says that a vulnerability in DNS could allow a similar scale of attack, but requiring far fewer hacked computers. For months, the companies responsible for the internet’s phone book have been rushing to fix it.

Today researchers from Tel Aviv University and the Interdisciplinary Center of Herzliya in Israel released new details of a technique they say could allow a relatively small number of computers to carry out distributed denial of service attacks on a massive scale, overwhelming targets with fraudulent requests for information until they’re knocked offline. The DDoS technique, which the researchers called NXNSAttack, takes advantage of vulnerabilities in common DNS software. DNS converts the domain names you click or type into the address bar of your browser into IP addresses. But the NXNSAttack can cause an unwitting DNS server to perform hundreds of thousands of requests every time a hacker’s machine sends just one.

That multiplicative effect means that an attacker could use just a handful of hacked machines, or even their own devices, to carry out powerful DDoS attacks on DNS servers, potentially causing Mirai-scale disruption. “Mirai had like 100,000 IoT devices, and here I think you can have the same impact with only a few hundred devices,” says Lior Shafir, one of the Tel Aviv University researchers. “It’s a very serious amplification,” Shafir adds. “You could use this to knock down critical parts of the internet.”

Or at least, you could have a few months ago. Since February, the researchers have alerted a broad collection of companies responsible for the internet’s infrastructure to their findings. The researchers say those firms, including Google, Microsoft, Cloudflare, Amazon, Dyn (now owned by Oracle), Verisign, and Quad9 have all updated their software to address the problem, as have several makers of the DNS software those companies use.

While DNS amplification attacks aren’t new, NXNSAttack represents a particularly explosive one. In some cases, the researchers say, it’s capable of multiplying the bandwidth of just a few machines as much as 1,600-fold. And even after months of coordinated patching, corners of the internet may still remain vulnerable to the technique, says Dan Kaminsky, the chief scientist at security firm White Ops and a well-known DNS researcher. In 2008, Kaminsky found a fundamental flaw in DNS that threatened to allow hackers to redirect users trying to visit a website to a fraudulent site of their choosing, and similarly launched a coordinated fix across major DNS providers. Even then, it took months for Kaminsky’s flaw—one that was far more serious than NXNSAttack—to be close to fully patched.

“There are a million of these things, and even if some of them are patched, there will always be one that hasn’t gotten an update,” Kaminsky says of the DNS servers distributed around the internet. “This is very good work about how DNS can fail.”

To grasp how the NXNSAttack works, it helps to understand the larger hierarchical structure of DNS across the internet. When a browser reaches out for a domain like google.com, it checks a DNS server to find out that domain’s IP address, a number like 64.233.191.255. Typically those requests are answered by DNS “resolver” servers, controlled by DNS providers and internet service providers. But if those resolvers don’t have the right IP address on hand, they ask an “authoritative” server associated with specific domains for an answer.

The NXNSAttack abuses the trusted communications between those different layers of the DNS hierarchy. It requires not only access to a collection of PCs or other devices—anything from a single computer to a botnet—but also the creation of DNS servers for a domain; call it “attacker.com.” (The researchers argue anyone can put that set-up together for just a few dollars.) Then the attacker would send a barrage of requests from their devices for the domain they control, or more specifically a series of fake subdomains like 123.attacker.com, 456.attacker.com and so on, using strings of random numbers to constantly vary the subdomain requests.

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