Colin Mc Hugo

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Colin Mc Hugo
Security Engineer Manager & CEO at Quantum Infinite Solutions Group Ltd.

New TsuNAME Flaw Could Let Attackers Take Down Authoritative DNS Servers

May 7, 2021

Safety researchers Thursday disclosed a brand new important vulnerability affecting Area Title System (DNS) resolvers that may very well be exploited by adversaries to hold out reflection-based denial-of-service assaults in opposition to authoritative nameservers.

The flaw, referred to as ‘TsuNAME,’ was found by researchers from SIDN Labs and InternetNZ, which handle the nationwide top-level web domains ‘.nl’ and ‘.nz’ for the Netherlands and New Zealand, respectively.

“TsuNAME happens when domains are misconfigured with cyclic dependent DNS information, and when weak resolvers entry these misconfigurations, they start looping and ship DNS queries quickly to authoritative servers and different resolvers,” the researchers mentioned.

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A recursive DNS resolver is among the core elements concerned in DNS resolution, i.e., changing a hostname reminiscent of right into a computer-friendly IP handle like To attain this, it responds to a consumer’s request for an online web page by making a sequence of requests till it reaches the authoritative DNS nameserver for the requested DNS document. The authoritative DNS server is akin to a dictionary that holds the precise IP handle for the area that is being seemed up.

However with TsuNAME, the concept is that misconfigurations throughout area registration can create a cyclic dependency such that nameserver information for 2 zones level to one another, main weak resolvers to “merely bounce again from zone to zone, sending continuous queries to the authoritative servers of each father or mother zones,” thereby overwhelming their father or mother zone authoritative servers.

As to how this occurs, all of it boils all the way down to recursive resolvers being oblivious to the cycle and never caching cyclically dependent title information.

Information gathered from the .nz area discovered that two misconfigured domains alone led to a 50% improve in general visitors quantity for the .nz’s authoritative servers. Google Public DNS (GDNS) and Cisco OpenDNS — which had been abused to focus on .nz and .nl domains in 2020 — have since addressed the problem of their DNS resolver software program.

To mitigate the impression of TsuNAME within the wild, the researchers have printed an open-source instrument referred to as CycleHunter that enables for authoritative DNS server operators to detect cyclic dependencies. The examine additionally analyzed 184 million domains spanning seven massive top-level domains and three.6 million distinct nameserver information, uncovering 44 cyclic dependencies utilized by 1,435 domains.

“Provided that [nameserver] information can change at any time, there is no such thing as a everlasting resolution,” the researchers cautioned. “In different phrases, if a DNS zone has no cyclically dependent NS information at time t, it implies that this zone shouldn’t be weak at solely that specific time t. We due to this fact additionally suggest that registrars run CycleHunter regularly, for example, as a part of their area title registration course of.”

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