A Monero cryptocurrency-mining campaign has emerged that exploits a known vulnerability in public-facing web applications built on the ASP.NET open-source web framework.
The campaign has been dubbed Blue Mockingbird by the analysts at Red Canary that discovered the activity. Research uncovered that the cybercriminal gang is exploiting a deserialization vulnerability, CVE-2019-18935, which can allow remote code execution. The bug is found in the Progress Telerik UI front-end offering for ASP.NET AJAX.
AJAX stands for Asynchronous JavaScript and XML; It’s used to add script to a webpage which is executed and processed by the browser. Progress Telerik UI is an overlay for controlling it on ASP.NET implementations.
The vulnerability lies specifically in the RadAsyncUpload function, according to the writeup on the bug in the National Vulnerability Database. This is exploitable when the encryption keys are known (via another exploit or other attack), meaning that any campaign relies on a chaining of exploits.
In the current attacks, Blue Mockingbird attackers are uncovering unpatched versions of Telerik UI for ASP.NET, deploying the XMRig Monero-mining payload in dynamic-link library (DLL) form on Windows systems, then executing it and establishing persistence using multiple techniques. From there, the infection propagates laterally through the network.
The activity appears to stretch back to December, according to the analysis, and continued through April at least.
XMRig is open-source and can be compiled into custom tooling, according to the analysis. Red Canary has observed three distinct execution paths: Execution with rundll32.exe explicitly calling the DLL export fackaaxv; execution using regsvr32.exe using the /s command-line option; and execution with the payload configured as a Windows Service DLL.
“Each payload comes compiled with a standard list of commonly used Monero-mining domains alongside a Monero wallet address,” explained researchers at Red Canary, in a Thursday writeup. “So far, we’ve identified two wallet addresses used by Blue Mockingbird that are in active circulation. Due to the private nature of Monero, we cannot see the balance of these wallets to estimate their success.”
To establish persistence, Blue Mockingbird actors must first elevate their privileges, which they do using various techniques; for instance, researchers observed them using a JuicyPotato exploit to escalate privileges from an IIS Application Pool Identity virtual account to the NT Authority\SYSTEM account. In another instance, the Mimikatz tool (the official signed version) was used to access credentials for logon.
Armed with the proper privileges, Blue Mockingbird leveraged multiple persistence techniques, including the use of a COR_PROFILER COM hijack to execute a malicious DLL and restore items removed by defenders, according to Red Canary.
“To use COR_PROFILER, they used wmic.exe and Windows Registry modifications to set environment variables and specify a DLL payload,” the writeup explained.
Blue Mockingbird likes to move laterally to distribute mining payloads across an enterprise, added researchers. The attackers do this by using their elevated privileges and Remote Desktop Protocol (RDP) to access privileged systems, and then Windows Explorer to then distribute payloads to remote systems.
Although Blue Mockingbird has been making noticeable waves, the toolkit is a work in progress.
“In at least one engagement, we observed Blue Mockingbird seemingly experimenting with different tools to create SOCKS proxies for pivoting,” said the researchers. “These tools included a fast reverse proxy (FRP), Secure Socket Funneling (SSF) and Venom. In one instance, the adversary also tinkered with PowerShell reverse TCP shells and a reverse shell in DLL form.”
In terms of preventing the threat, patching web servers, web applications and dependencies of the applications to inhibit initial access is the best bet, according to Red Canary.
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