In recent months, attackers have increased their use of programs that run on Internet-facing servers as a way to remotely control compromised systems and networks, according to security firms and government security agencies.
Known as Web shells, the attack tools allow their operators to retain flexible access to compromised assets while avoiding detection by firewalls, intrusion detection systems, and host-based anti-malware tools. While their use is more than decade old, Web shells remain a stealthy way for attackers to continue to issue commands to compromised networks because the technique utilizes ubiquitous Web traffic. They have become the de facto approach to establishing a beachhead for many cybercriminal and cyber-espionage groups, says Wade Woolwine, principal security researcher at security-management firm Rapid7.
"Putting Web shells on a Web server is a really easy way to evade traditional detections that we see on the endpoint," he says. "Attackers were using [Web shells] pretty regularly as a secondary, tertiary, or redundant backdoor, and we still see those today because it is a way of evading detection, and a number of the modern Web frameworks allow you a direct pipeline to the operating system for scripting."
Last week the US National Security Agency and the Australian Signal Directorate stated in a joint advisory that Web shell use has increased in recent months and poses an ongoing risk to government systems.
In February Microsoft warned that an undisclosed public-sector organization had, through misconfiguring a Web server, "allowed attackers to upload a web shell, which let the adversaries gain a foothold for further compromise." The North Korea-linked Lazarus Group — which Microsoft refers to as ZINC, CrowdStrike as Hidden Cobra, and FireEye as APT38 — have used Web shells in their campaigns, as have Russian and Chinese groups.
Roughly every month, Microsoft detects an average of 77,000 Web shell and related artifacts on some 46,000 machines, the company said.
"Web shell attacks have affected a wide range of industries," Microsoft stated in the advisory issued by its Detection and Response Team (DART). "The organization in the public sector [that the company warned about] represents one of the most common targeted sectors."
While some attack techniques peak and then fade away, Web shells have become a staple of attackers. For example, first appearing almost a decade ago, China Chopper is a Web shell that works on both Linux and Windows servers and remains in active use. In 2018, attackers targeting telecommunications firms in Operation Soft Cell used China Chopper for persistance and control of the target network. The campaign was linked to Chinese-affiliated actors, dubbed Gallium by Microsoft, and lasted until mid-2019.
Yet China Chopper is not just used by China-linked attack groups but by many other threat actors as well, according to Cisco's Talos intelligence group.
"I see adversaries of all different types and all different skill levels using Web shells," says Nick Biasini, a threat researcher at Cisco Talos. "Even with China Chopper, it was used for everything from Web defacement to cyber espionage."
The increase in Web shell use appears to be slight but noticeable. The number of Web shells ecountered by Microsoft doubled last summer but ended the year with a modest increase, according to its advisory. Rapid7 estimates that about 10% of the attacks it investigates use a Web shell for access and persistance.
More recently, the use of Web shells for compromising and controlling internal servers has increased, says Tod Beardsley, director of research at Rapid7.
"The Web is not just on the Internet," he says. "All this stuff that is applicable to your edge, it is also applicable to your internal network. Any Web app is just as vulnerable on the inside of your network, and often it has less security around it."
For that reason, companies should not just focus on Web servers. The term "Web shell" is also generically used for any remote access program that allows commands over HTTP or HTTPS. Both Outlook and Sharepoint servers have Web-based interfaces and so are targeted by attackers with Web shell installation.
Companies should use network segmentation to prevent Internet-facing Web servers from being used as a beachhead into the rest of the network, Cisco's Biasini says. "If you have a Web server with access to the Internet, it should not have internal access, he says.
In addition, regular scans should be done for common vulnerability types.
"From a prevention perspective, businesses should be scanning their Web applications from the outside, and anything that is touching the Internet should be patched," Biasini says.
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