The hijacking occurred at the DNS (Domain Name System) level, with attackers modifying the DNS records for google.ro, yahoo.ro, microsoft.ro, hotmail.ro, windows.ro, kaspersky.ro and paypal.ro, according to Costin Raiu, director of the global research and analysis team at security vendor Kaspersky Lab.
This led to the websites displaying an attacker-supplied page instead of their regular content - an attack commonly known as a website defacement. The rogue page displayed in this case attributed the attack to an Algerian hacker using the alias MCA-CRB. The hacker also posted screen shots of the defaced websites on the Zone-H.org website, a Web defacement archive.
The hacker pointed the domains to a server in the Netherlands - server1.joomlapartner.nl - that also appears to have been hacked, said Bogdan Botezatu, a senior e-threat analyst at Romanian antivirus vendor Bitdefender.
Botezatu believes that the DNS records were modified as a result of a security breach at the RoTLD domain registry, which manages the authoritative DNS servers for the entire .ro domain space.
The Romanian National Institute of Informatics Research and Development, the organization that runs the RoTLD registry, did not respond to a request for comment.
A compromise of the RoTLD Web system used by .ro domain name owners to administer their domains, or the registry's DNS servers is one of the possibilities, Raiu said.
Kaspersky Lab's RoTLD account that was used to administer kaspersky.ro - one of the affected domain names - did not display any alerts or other obvious signs of compromise, Raiu said. However, this does not exclude the possibility of hackers gaining access to the account of a RoTLD administrator directly, he said.
Kaspersky is in the process of filing an official complaint with RoTLD, Raiu said.
Another scenario involves attackers launching a so-called DNS poisoning attack, that resulted in rogue DNS records being inserted in Google's public DNS resolver servers - 220.127.116.11 and 18.104.22.168 - Kaspersky researchers said Wednesday in a blog post.
Not all Romanian users were affected by the attack. In fact, the DNS resolver servers of many Romanian ISPs did not report the poisoned records, Raiu said.
However, this might be caused by differences in caching times. Google's public DNS servers might be configured to refresh DNS records by interrogating authoritative DNS servers, like those operated by RoTLD, faster than the DNS resolvers of some ISPs.
"Google services in Romania were not hacked," a Google representative said Wednesday via email. "For a short period, some users visiting www.google.ro and a few other web addresses were redirected to a different website. We are in contact with the organisation responsible for managing domain names in Romania."
"We are aware that Yahoo.ro was inaccessible to some users in Romania," a Yahoo spokeswoman said via email. "This issue is resolved and we apologize for any inconvenience this may have caused."
"On 27 November, Microsoft.ro was impacted by a third-party DNS issue," Microsoft said in an emailed statement. "The site has since been fully restored and we can confirm that no customer information was compromised. We are working with our third-party partners to evaluate their security practices."
It's not clear whether the paypal.ro domain name is actually owned by PayPal. PayPal did not immediately respond to a request for comment seeking clarification.
The attack in Romania follows a similar one that occurred last week in Pakistan and affected the .pk domains of Google, Microsoft, Yahoo, PayPal and other companies. The security breach was traced back to PKNIC, the .pk domain registry.
"PKNIC became aware of a vulnerability in one of its systems which caused a total of four user accounts to be breached on Friday evening 23rd November, impacting nine DNS records, out of a total of around fifty thousand," the registry said in a statement published on its website this week. "That led to several website addresses to be redirected to a message page, with a defaced message in Turkish language for a few hours. Almost all of these websites were mirrors of global sites such as google.pk, microsoft.pk, or place-holders for International brand names who do not actually do business in Pakistan such as paypal.pk, etc."
Botezatu believes that the hackers who hijacked the DNS of the Romanian domains Wednesday might be the same ones responsible for the attack in Pakistan last week.
The attacks against country-code top-level domain (ccTLD) registry organizations seem to be increasing. In October, attackers managed to change the NS records of several Irish domain names including Google.ie and Yahoo.ie.
On 9 November, the .IE Domain Registry (IEDR) issued a statement saying that the incident was the result of hackers exploiting a vulnerability in the registry's website.