Experts explain how to bypass recent improvement of China’s Great Firewall

Experts from Great Firewall Report analyzed recent upgrades to China’s Great Firewall and revealed that it can be circumvented.

Members of the Great Firewall Report group have analyzed the recent improvement implemented for China’s Great Firewall censorship system and revealed that it is possible to bypass it.

Last year, the group published a detailed analysis on how the Chinese government has improved its surveillance system to detect and block the popular circumvention tools Shadowsocks and its variants.

“Using measurement experiments, we find that the GFW uses the length and entropy of the first data packet in each connection to identify probable Shadowsocks traffic, then sends seven different types of active probes, in different stages, to the corresponding servers to test whether its guess is correct.” reads the paper published by the experts. “Based on our gained understanding, we present a temporary workaround that successfully mitigates the traffic analysis attack by the GFW”

Shadowsocks leverages SOCKS5 proxies outside China to avoid government censorship.

Shadowsocks is a free and open-source encryption protocol project, widely used in China to circumvent Internet censorship. It was created in 2012 by a Chinese programmer named “clowwindy“, and multiple implementations of the protocol have been made available since. Shadowsocks software allows to connect to a third party socks5 proxy, speaking the shadowsocks language on the machine it is running on, which internet traffic can then be directed towards, similarly to a Secure tunnel(SSH tunnel). Unlike an SSH tunnel,

In 2019, the Chinese authorities implemented the ability to detect Shadowsocks through traffic analysis and network probing, and block its connections.

Great Firewall Report experts revealed that recent versions of Shadowsocks (3.3.1 and earlier) could bypass the firewall.

“In this short post, we provide practical suggestions for non-technical users and circumvention tool developers to prevent their circumvention servers from being detected and blocked. We also introduce the mitigation to partitioning oracle attacks newly demonstrated by Len et al.. ” reads the post published by Great Firewall Report.

The post is a practical guide to defend against the GFW’s latest active probing, it includes a list of precautions that need to be taken to avoid censorship. The post also includes detailed instructions to build a Shadowsocks-compatible proxy server.

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Pierluigi Paganini

(SecurityAffairs – hacking, China)

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