‘Frebniis’ Malware Hijacks Microsoft IIS Function to Deploy Backdoor

A recently identified malware family is abusing Microsoft Internet Information Services (IIS) to deploy a backdoor and monitor all HTTP traffic to the infected system, Symantec reports.

Dubbed Frebniis, the malware injects code into a DLL that an IIS feature called Failed Request Event Buffering (FREB) uses when troubleshooting failed requests.

FREB collects data about the tracked requests, including HTTP headers with cookies, originating IP address and port, and more.

As part of the observed Frebniis attacks, the malware first ensures that FREB is in use, after which it accesses the IIS process to retrieve information on where the targeted FREB DLL (iisfreb.dll) is loaded.

According to Symantec, “the authors of Frebniis have determined that a particular function pointer within iisfreb.dll is called by iiscore.dll whenever any HTTP request is made to IIS from a web client.”

The malware then proceeds to inject code into the IIS process to hijack the function by replacing its pointer with its own malicious code.

“This hijack point allows Frebniis to stealthily receive and inspect every HTTP request to the IIS server before returning to the original function,” Symantec notes.

By hijacking the IIS function, the HTTP backdoor remains completely stealthy on the system, while being able to inspect all HTTP requests to identify specially formatted ones.

Frebniis parses all requests for /logon.aspx or /default.aspx with a specific parameter password, which allows it to decrypt and execute .NET code when a password match is found.

The code provides proxying and remote code execution capabilities, which allows the malware operators to communicate with internal resources typically blocked from accessing the internet, as well as to execute code directly in memory using crafted HTTP requests.

“These requests allow remote code execution and proxying to internal systems in a stealthy manner. No files or suspicious processes will be running on the system, making Frebniis a relatively unique and rare type of HTTP backdoor seen in the wild,” Symantec concludes.

The cybersecurity company says that Frebniis has been used by an unknown threat actor in attacks targeting entities in Taiwan.

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