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Researcher Takes Over .IO Domains by Registering Name Servers

A security researcher could have taken over thousands of .IO domains after being able to register four domain names of the top level domain (TLD)’s authoritative name servers.

A security researcher could have taken over thousands of .IO domains after being able to register four domain names of the top level domain (TLD)’s authoritative name servers.

The incident happened last month, when researcher Matthew Bryant was “graphing out the DNS delegation paths of various TLDs.” During his project, he discovered a name server domain that was available for registration and was able to purchase it.

“.IO” is the country code top level domain (ccTLD) assigned to the British Indian Ocean Territory.

TLDs have authoritative name servers at arbitrary domain names but, by exploiting errors such as misconfiguration, expiration, or other issues, it is possible to “register a name server domain name and use it to serve new DNS records for the entire TLD zone,” the security researcher explains.

For that, one would have to enumerate all name server hostnames for a given extension and then check for base-domains that expired and are available for registration. In some instances, however, the expired domains would not be available for purchase even if not marked as reserved.

Using this method, the researcher stumbled upon the name server domain of, which appeared as available for the registration price of 90.00 USD. After successfully purchasing it, Bryant attempted to contact the .io TLD to get the issue fixed, but failed.

As a result, he decided to look for other similar name server domains and found,, and domains available for purchase as well. All four domains are listed as authoritative name servers for the .io TLD, and anyone controlling them could potentially “poison/redirect the DNS for all .io domain names registered,” the researcher explains.

Bryant was eventually able to send an email to the appropriate security contact and was informed the next day that the issue was resolved. The researcher verified that he was not able to re-register these domains, showing that the error was remediated.

“Given the fact that we were able to take over four of the seven authoritative name servers for the .io TLD we would be able to poison/redirect the DNS for all .io domain names registered. Not only that, but since we have control over a majority of the name servers it’s actually more likely that clients will randomly select our hijacked name servers over any of the legitimate name servers even before employing tricks like long TTL responses, etc to further tilt the odds in our favor,” the researcher explains.

He also notes that, because the .io TLD has Domain Name System Security Extensions (DNSSEC) enabled, which adds security by enabling DNS responses to be validated, users should be defended from attackers able to send bad/forged DNS data. However, “DNSSEC support is pretty abysmal and I rarely encounter any support for it unless I specifically set a resolver up that supports it myself,” the researcher also points out.

According to Matt Pounsett, however, while the Backend Registry Operator for the .io TLD clearly made a big mistake by allowing a third-party to register the name servers, the issue “definitely does not constitute the catastrophe implied.” He explains that “the name servers for the .io TLD don’t respond with their own NS set in their response,” meaning that attack won’t work as suggested.

The issue with the authoritative name servers was that the .io TLD apparently transitioned last month from the operators of the registry to a third-party already in charge with the backend for other top-level domains. The third-party, Afilias, got hold of three domain name servers, but left the other four available.

Written By

Ionut Arghire is an international correspondent for SecurityWeek.

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