ISP fail in Spamhaus DDoS puts ‘open DNS resolvers’ on EU regulator agenda

The cyber-attacks on an anti-spam group that rattled Europe’s internet last month could have been countered by ISPs, according to the EU’s security agency, which is taking new mitigation recommendations to operators and telecoms regulators.

The 300 Gbps distributed denial of service (DDoS) attacks on Swiss and UK based anti-spam group Spamhaus may not have threatened the internet globally, but its local effects were “rather noticeable”, the European Network and Information Security Agency (ENISA) says in its analysis of the attacks released Friday.

“The attack on Spamhaus lasted more than one week. In its final phase, the enormous amount of traffic generated caused problems at the London Internet Exchange (LINX),” the agency noted in a report looking into the relationship between network interconnections and the resilience of internet infrastructure in Europe.

LINX, whose members include ISPs, provides peering services that allow carriers to interconnect at exchange points.

“Operators of internet exchange points should consider that their infrastructure might be attacked directly and make sure that they have appropriate security measures in place,” ENISA said.

ENISA also notes internet users in the UK, Germany and other parts of Western Europe were affected by noticeable delays.

According to agency, the attacks could have been countered if ISPs had implemented best practices recommended by the Internet Engineering Task Force (IETF) in 2000.

“Internet Service Providers (ISPs) have failed to apply well-known security measures which have been available for over a decade,” it said.

The Spamhaus attackers relied on open Domain Name Service (DNS) resolvers to amplify their assault. A DNS resolver should, according to best practice, only respond to queries from IP addresses from a fixed range while ignoring requests outside that it. Open DNS resolvers respond to requests from any address block, including forged ones.

The two IETF documents ENISA points to are Best Current Practice 38 (BCP38), which, in 2000, urged network operators to filter ingress traffic to prevent IP source address spoofing in DDoS attacks; and BCP140 from 2008, which details how networks can prevent recursive namerservers from being used in “reflector attacks”.

To achieve the rate of 300 Gbps, the attackers relied on the “small query-large response” feature of the DNS system to amplify attack traffic. But, as BCP140 notes, the attack first takes advantage of the fact that non-BCP38 networks do not restrict transit traffic from downstream networks to “known, and intentionally advertised” IP addresses. This permits an attacker to forge a DNS query using the source address of their target and send it to an open recursive DNS server.

“Network Operators that have yet to implement BCP38 and BCP140 should seriously consider doing so without delay, failing which their customers, and hence their reputations, will suffer,” said ENISA’s executive director, Professor Udo Helmbrecht.

“Prevention is key to effectively countering cyber-attacks. We therefore welcome the EU’s Cyber Security Strategy, which is proposing a strengthened role for ENISA, with adequate resources, to help protect Europe’s digital society and economy,” he added.

ENISA is aiming to provide policy makers, telecoms regulators, cyber security agencies and network operators with recommendations on how to address issues related to the resilience of Europe’s internet infrastructure, it notes.

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