Part II in a two-part series. Read Part I.

Introduction

In Part I of this series, I described China Chopper’s easy-to-use interface and advanced features — all the more remarkable considering the Web shell’s tiny size: 73 bytes for the aspx version, 4 kilobytes on disk. In this post, I’ll explain China Chopper’s platform versatility, delivery mechanisms, traffic patterns, and detection. My hope is that armed with this information, you can eradicate this pest from your environment.

Platform

So what platform can China Chopper run on? Any Web server that is capable of running JSP, ASP, ASPX, PHP, or CFM. That’s the majority of the Web application languages out there. What about operating systems? China Chopper is flexible enough to run transparently on both Windows and Linux. This OS and application flexibility makes this an even more dangerous Web shell. Continue reading »

Part I in a two-part series.

China Chopper: The Little Malware That Could

China Chopper is a slick little web shell that does not get enough exposure and credit for its stealth. Other than a good blog post from security researcher Keith Tyler, we could find little useful information on China Chopper when we ran across it during an incident response engagement. So to contribute something new to the public knowledge base — especially for those who happen to find the China Chopper server-side payload on one of their Web servers — we studied the components, capabilities, payload attributes, and the detection rate of this 4 kilobyte menace.

Components

China Chopper is a fairly simple backdoor in terms of components. It has two key components:the Web shell command-and-control (CnC) client binary and a text-based Web shell payload (server component). The text-based payload is so simple and short that an attacker could type it by hand right on the target server — no file transfer needed.

Continue reading »