Security Experts:

Stealth Backdoor Abused NSA Exploit Before WannaCrypt

In the aftermath the WannaCry ransomware outbreak, security researchers discovered numerous attacks that have been abusing the same EternalBlue exploit for malware delivery over the past several weeks.

Targeting a Server Message Block (SMB) vulnerability on TCP port 445, the exploit was made public in April by the group of hackers calling themselves “The Shadow Brokers” and is said to have been stolen from the National Security Agency-linked Equation Group. The targeted flaw was patched in March.

The fast spreading WannaCry brought EternalBlue to everyone’s attention, yet other malware families have been using it for infection long before the ransomware started using it. One of them was the Adylkuzz botnet, active since April 24, researchers revealed.

Now, Cyphort says that evidence on a honeypot server suggests attacks on SMB were active in early May, and they were dropping a stealth Remote Access Trojan (RAT) instead of ransomware. The malware didn’t have the worm component and didn’t spread like WannaCry.

The malware appears to have been distributed from an IP (182.18.23.38) located in China. Following successful exploitation, an encrypted payload is sent as a shellcode, and the security researchers found a DLL embedded in the shellcode, which they say “is basically a Trojan which downloads additional malware and receives commands from its controller.”

One of the files downloaded by this malware is meant to close port 445, thus preventing other malware from abusing the same flaw. Another file is believed to be a second-stage payload. The RAT sets a series of Registry Run entries to download and execute additional malware, the researchers say.

The malware attempts to delete a number of users and terminate and/or delete various files or processes and a memory dump reveals that it is connected to a remote access tool hosted on a Chinese website, ForShare 8.28.

The RAT can receive and execute commands from server, monitor the screen, capture audio and video, monitor the keyboard, transfer data, delete files, terminate processes, execute files, enumerate files and processes, download files, and control the machine.

Because the threat closes port 445, Cyphort believes the actor was aware of the EternalBlue vulnerability and was attempting to keep other malware out of the vulnerable machines.

“We believe that the group behind this attack is the same group that spreads Mirai via Windows Kaspersky discovered in February. We found similarities in terms of their IOCs,” the security researchers say.

In a report this week, Secdo also claims to have found evidence of malware abusing EternalBlue weeks before WannaCry emerged. One of the malicious programs appears to be a ransomware family that also steals user credentials.

A “new evasive attack that leaves no trace and has been infecting organizations using NSA exploits since the mid-April,” the researchers say. “The ransomware is the most apparent payload, yet under the surface a more sophisticated attack occurred that would have gone unnoticed.”

As part of this attack, the researchers say, actors were using an EternalBlue-based worm to infect all machines in a compromised network, and were also deploying a backdoor for persistence, or exfiltrated login credentials.

One of the attacks originated from a Russian IP (77.72.84.11). Using the NSA-linked exploit for compromise, attackers spawned a thread inside a legitimate application, and used it to download multiple modules, including SQLite DLL from SourceForge to steal login credentials from Firefox.

Stolen data is exfiltrated through the TOR network, after which “a ransomware variant of CRY128 that runs purely in-memory encrypts all the documents on the system,” the researchers say.

The recently discovered UIWIX ransomware that spreads via the EternalBlue exploit is also being executed only in memory, resulting in a fileless infection. UIWIX also contains code meant to steal a broad range of login credentials.

Another attack was linked to a Chinese actor and involved the distribution of a backdoor. The attack starts with process injection, similar to the above, but ends with the download of a known root-kit backdoor (based on Agony). The downloaded file, 666.exe, is blocked by antivirus programs.

“Based on these findings, we suspect that the scope of the damage is much greater than previously thought, and that there are at least 3 different groups that have been leveraging the NSA exploit to infect enterprise networks since late April,” Secdo notes.

In January, United States Computer Emergency Readiness Team (US-CERT) issued an alert after Shadow Brokers revealed they had a zero-day exploit targeting SMB up for sale. In February, a Windows’ SMBv3 0-day vulnerability (CVE-2017-0016) was assessed with a High severity rating, after initially believed to be Critical.

Related: Fileless Ransomware Spreads via EternalBlue Exploit

Related: Botnet Spread via NSA Hacking Tools for Weeks

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