GitHub has been hammered by a continuous DDoS attack for three days. It's the "largest DDoS attack in github.com's history." The attack is aimed at anti-censorship GreatFire and CN-NYTimes projects, but affected all of GitHub. The traffic is reportedly coming from China, as attackers are using the Chinese search engine Baidu for the purpose of "HTTP hijacking."

According to tweeted GitHub status messages, GitHub has been the victim of a Distributed Denial of Service (DDoS) attack since Thursday March 26. 24 hours later, GitHub said it had "all hands on deck" working to mitigate the continuous attack. After GitHub later deployed "volumetric attack defenses," the attack morphed to include GitHub pages and then "pages and assets." Today, GitHub said it was 71 hours into defending against the attack.

Seven hours later, GitHub said the attackers changed DDoS tactics and it was experiencing outages again.

Four hours later, GitHub said its services were stable although it was continuing to mitigate against the large-scale DDoS attack. Four hours after that, GitHub said the ongoing attack was affecting connectivity to GitHub Gist.

The attack is aimed at GreatFire and CN-NYTimes projects that help Chinese users get around their government's online censorship, but affected all of GitHub. The traffic is coming from China.

According to analysis by Insight Labs, by a Chinese security researcher living outside of China, "My first thought was someone naughty XSSed the page, so I opened developer tools to find the source of the XSS," Anthr@X wrote. "Almost instantly I saw it was keep trying [sic] to load these two URLs: github.com/greatefire/ and github.com/cn-nytimes/ every a [sic] few seconds."

Baidu is like China's Google, and many Chinese websites use tracking and advertising code from Baidu. The "HTTP hijacking" was explained as "a certain device at the border of China's inner network, and the Internet has hijacked the HTTP connections" and "replaced some JavaScript files from Baidu with malicious ones" that would load GitHub's GreatFire and CN-NYTimes projects "every two seconds."

CN-NYTimes gets around censorship by mirroring the New York Times, and GreatFire has long fought against Chinese censorship.

Ten days ago, GreatFire said it was "under attack" and asked for help. Last week, GreatFire wrote about having "concrete proof from Google that CNNIC (and by extension CAC) is indeed complicit in MITM attacks." That post pointed to Google Security Engineer Adam Langley saying Google was aware of "unauthorized digital certificates for several Google domains." The certificates were issued by CNNIC (China Internet Network Information Center).

While GreatFire was reportedly "delighted that Google, Microsoft, and Mozilla have taken steps to blacklist the intermediate certificate used in the attack," the online censorship watchdog called on "Google, Mozilla, Microsoft and Apple to revoke trust for CNNIC immediately in order to protect Chinese user data and user data worldwide."

Back in January, after GreatFire uncovered another 'devious' man-in-the-middle attack by Chinese authorities against Microsoft's Outlook, the anticensorship group urged Microsoft to revoke trust for CNNIC certificate authority.

After announcing it was under the largest DDoS attack in its history, GitHub said it "involves a wide combination of attack vectors. These include every vector we've seen in previous attacks as well as some sophisticated new techniques that use the web browsers of unsuspecting, uninvolved people to flood github.com with high levels of traffic. Based on reports we've received, we believe the intent of this attack is to convince us to remove a specific class of content."

Put another way by Anthr@X at Insight Labs:

In other words, even people outside China are being weaponized to target things the Chinese government does not like, for example, freedom of speech.

Baidu denied any involvement in the attack and released a statement claiming, "We've notified other security organizations, and are working together to get to the bottom of this."