"The Chinese government is a staunch defender of the Internet's security, and resolutely opposes any form of cyber attack," the Cyberspace Administration of China said on Thursday.
On Saturday, Microsoft's email service faced a brief "man-in-the-middle" style attack in China, according to the watchdog and anti-censorship group GreatFire.org. Microsoft later confirmed the attack took place, stating that a "small number of customers" were being routed to a server impersonating Outlook.com
Although the hacking attempt lasted only a day, it was just the latest man-in-the-middle style attack to hit a foreign Internet service in China. In October, Apple's iCloud faced a similar attack, and weeks before that, Yahoo as well.
GreatFire.org and other security analysts suspect that Chinese authorities could be behind the attacks, given that the hackers would need access to China's state-owned Internet service providers to launch them.
China has always rejected claims that the country sponsors hacking attempts, so Thursday's denial was no surprise. But the country's cyber authority also blasted GreatFire.org for alleging that the government could have a role behind the Outlook.com attack.
"This is unprovoked speculation, and purely amounts to disinformation and slander from anti-China forces based abroad," the government office said.
In addition to monitoring cyber attacks in China, GreatFire.org is also notable for its activism to undermine the country's online censorship. It's done this by creating functioning mirror sites to foreign internet services that have been blocked in China.
The country's government, however, isn't pleased. In the same posting, the cyber authority's spokesman Jiang Jun accused GreatFire.org of launching "unprovoked attacks" against the Chinese government.
He also alleged GreatFire.org of using the Outlook.com hacking attempt as a way to "incite discontent", after China had recently shut down a swath of illegal Internet sites and social media accounts.
In response, GreatFire.org on Friday questioned why Chinese authorities weren't doing more to investigate the recent man-in-the-middle attacks.
"If CAC (Cyberspace Administration of China) claims they are not responsible, how could someone get into the backbone of the Chinese Internet and implement nation-wide attacks for six times over the course of two years without being noticed" the group said in an email.
"All of our writings are backed up by hard data," the group added. "How is it groundless"
GreatFire.org has said it was founded by three members, without the aid of any other groups. The members have yet to reveal their identities for security reasons. But one of them said back in 2012 that he was a foreigner living in China, and previously worked as a Web developer.