Hackers inject new malware called "Frebniss" into Microsoft Internet Information Services (IIS). The program secretly executes commands sent via web requests.
Frebniis was discovered by the Symantec Threat Hunter Team, who reported that unknown attackers are currently using the malware against a number of Taiwanese targets.
In the attacks seen by Symantec, the hackers used an IIS feature called Failed Request Event Buffering (FREB) that collects request metadata (IP address, HTTP headers, cookies). Its purpose is to help server administrators troubleshoot unexpected HTTP status codes or request processing issues.
The malware injects malicious code into a specific function of the DLL file that manages FREB (“iisfreb.dll”) so that attackers can intercept and monitor all HTTP POST requests sent to the ISS server. When the malware detects certain HTTP requests that an attacker sends, it parses the request to determine what commands to execute on the server.
Symantec said the attackers first need to break into the IIS server in order to compromise the FREB module. However, experts were unable to determine the method that was used to gain access in the actual attack.
The injected code is a .NET backdoor that supports proxying and executing C# code without writing files to the local drive. This makes the malware completely invisible. It looks for requests to logon.aspx or default.aspx pages with a specific password parameter.
The second HTTP parameter, which is a base64 encoded string, allows commands to be executed on other systems through a compromised IIS. This potentially allows Frebniis to reach secure internal systems that are not available on the Internet.
"If an HTTP call to logon.aspx or default.aspx is received without a password parameter but with a Base64 string, the Base64 string is assumed to be C# code that will be executed directly in memory," Symantec explains in the report.
The main benefit of using the FREB component for the purposes described is to avoid being detected by security tools. This unique HTTP backdoor leaves no traces or files and does not create suspicious processes on the system.
While the initial compromise path is unknown, experts recommend updating the software first to minimize the chances of hackers exploiting known vulnerabilities. Advanced network traffic monitoring tools can also help detect unusual activity from malware like Frebniis.
In October 2022, Symantec discovered another similar malware used by the Cranefly hacker group. The malware secretly used the ISS logs to exchange commands with the C2 server.