As thousands of people swarm the streets of Hong Kong asking the Chinese government to fulfill its promise of real democracy in the autonomous region, Chinese censors seems to be tightening their control over the Internet.

In what's almost unprecedented, China appears to be targeting Yahoo with what's called a "man-in-the-middle attack." With such an attack, connections to Yahoo.com, which are normally encrypted, would be vulnerable to snooping, and Chinese censors could also block search terms or specific Yahoo links with the goal of preventing Chinese netizens from accessing information about the protests in Hong Kong.

The attack was first reported on Tuesday by GreatFire, a group of anti-Chinese censorship activists.

Yahoo appears to under Man-in-the-middle attack in China. 3rd case of country-wide MITM, after Google, Github pic.twitter.com/xWQN6W9Q0c — GreatFire.org (@GreatFireChina) September 30, 2014

The reasons behind this attack, as well as the attacker, are unclear at this point. Adam Segal, a Council on Foreign Relations expert on cybersecurity and China, speculated the goal of the attack could be to block certain keyword searches on Yahoo, something China would be interested in since "the Chinese Communist Party is clearly sensitive about Hong Kong right now," he told Mashable.

But given that Yahoo's site is encrypted, China's so-called Great Firewall can't block individual searches or Yahoo links without blocking the whole website, experts said. Hence, a man-in-the middle attack might be the only way to do selective censorship.

"We think the Chinese authorities feel Yahoo is too big to block all together," Percy Alpha, one of the members of GreatFire, told Mashable. "But the Hong Kong protest means that some articles have to be censored."

The attack impacts all users connecting to Yahoo within China, Alpha said. His assessment seemed to be confirmed by other users complaining about the attack on Twitter.

@YahooCare It seems Yahoo has been being MitMed for 10 hours by using Chinese ip Address, Times on pics are at GMT+9. pic.twitter.com/dLNB8kl7Zu — ベラミ (@belleepoque20) September 30, 2014

The activists also believe the Chinese government is behind this attack.

"For an attack at this large of a scale, the attacker can only be the Chinese authorities," Charlie Smith, another GreatFire member, told Mashable.

Both Smith and Alpha use pseudonyms to protect their identity and prevent retaliation by Chinese authorities.

Michael Carbone, a technologist at human rights organization Access, agreed that such an attack could only be done by an organization like the Chinese government.

"If it's happening, it would certainly be the government," Carbone told Mashable.

In this case, it appears that the connection between Chinese Internet users and Yahoo.com is being manipulated along the way between the user and the website, experts said. This could happen in many ways, but in this case, it seems like whoever is messing with Yahoo is using a fake certificate to break the encryption. Certificates are used to ensure that an encrypted connection really goes to the intended target (in this case Yahoo.com). By using a fake certificate, an attacker can virtually put himself in the middle and intercept or mess with the connection.

Something like this would prompt a warning for users with Chrome or Firefox, but most people click through those without thinking about it, according to Steven Bellovin, a computer science professor at Columbia University.

Yahoo declined to comment for this story, while the Chinese embassy in the United States did not respond to Mashable's requests for comment.

This is not the first time China has been accused of a man-in-the-middle-attack against a major website. Earlier this year, local authorities allegedly launched a man-in-the-middle attack against Google within the country's nationwide education and research network, one of the few places on the Chinese net where Google is still accessible. And in early 2013, it was GitHub that became victim of a similar disruption.

But it's not just Yahoo which has gotten in Chinese censors' crosshairs.

On Sunday, authorities ordered Chinese media websites to "clear away" all information related to the protests in Hong Kong. Terms, including "police," "Hong Kong" or "Umbrella Revolution," have also been blocked on the Chinese Twitter-like microblogging platform Weibo. And on Tuesday, reporters from the South China Morning Post said that their Weibo accounts had been suspended, along with those of many other Hong Kong-based users.

BREAKING: Many Hong Kong users of Weibo reported their accounts were suspended by Nasdaq-listed Sina after posting HK news, incl @zhengwei75 — George Chen (@george_chen) September 30, 2014

China also blocked Instagram altogether on Monday to prevent Chinese users from seeing pictures of the protests, which some have dubbed the "Umbrella Revolution."

UPDATE, Oct. 2, 2:36 p.m. ET: Erik Hjelmvik, a network forensics expert at Netresec confirmed that as of Wednesday, there was an ongoing man-in-the-middle (MITM) attack against Yahoo in China.

Given this attack's characteristics and similarities with the two previous attacks in China, against GitHub and Google, all signs point to the Chinese government as the culprit, Hjelmvik said.

"Provided the scale of this attack, combined with the fact that the MITM is performed within the borders of China," Hjelmvik told Mashable. "I can assert that the MITM attack is performed or sponsored by the Chinese government."

UPDATE, Oct. 3, 12:46 p.m. ET: Yahoo confirmed to Mashable that the company has detected an attack against some of its sites in China.

"We have been monitoring the situation very closely since receiving reports of blocking activities in China," a Yahoo spokesperson said in a statement. "We can confirm that some Yahoo sites were blocked but have found no evidence that any users have had their credentials or personal information compromised."