McAfee Labs has closely monitored the activity around the ransomware WannaCry. Many sources have reported on this attack and its behavior, including this post by McAfee’s Raj Samani and Christiaan Beek and this post by Steve Grobman. In the last 24 hours, we have learned more about this malware. These findings mainly concern the malware’s network propagation, Bitcoin activity, and differences in observed variants.
Malware network behavior
WannaCry uses the MS17-010 exploit to spread to other machines through NetBIOS. The malware contains exploits in its body that are used during the exploitation phase. These are related to CVE-2017-0143, CVE-2017-0144, CVE-2017-0145, CVE-2017-0146, CVE-2017-0147, and CVE-2017-0148, all based on the MS17-10 security bulletin.
In many reports we read that the malware generates a list of internal IPs. We found that the malware generates random IP addresses, not limited to the local network. The following is an example attempt at propagation:
With this, the malware can spread not only to other machines in same network, but also across the Internet if sites allow NetBIOS packets from outside networks. This could be one reason for the widespread infection seen in this outbreak and why many people are unsure about the initial infection vector of the malware.
Another interesting characteristic of the malware is that once a machine with an open NetBIOS port is found, the malware will send three NetBIOS session setup packets to it. One has the proper IP of the machine being exploited, and the other two contain two IP addresses hardcoded in the malware body:
The preceding packet contains the IP of the machine being exploited. It uses the test network 192.168.0.0/24. The other two packets, below, contain different IPs that the malware has in its code:
This activity and the presence of two hardcoded IP addresses (192.168.56.20, 172.16.99.5) could be used to detect the exploit using network intrusion prevention systems.
Server message block (SMB) packets also contain the encrypted payload, which consists of exploit shellcode and the file launcher.dll. During our analysis, we found the malware is encrypted using a 4-byte XOR key, 0x45BF6313.
Encrypted payload with the key 0x45BF6313.
Decrypted launcher.dll payload.
We also found following x64 shellcode being transferred during network communication over SMB.
Machine A at left, Machine B at right.
The infection flow to the vulnerable host (Machine B).
Kernel mode at left, user mode at right.
Infection using kernel exploit
In our analysis, we found that on infected machines the SMB driver srv2.sys is vulnerable in kernel module and is exploited by the malware to spread using SMB communication.
A compromised srv2.sys will inject launcher.dll into the user-mode process lsass.exe, which acts as the loader for mssecsvc.exe. This DLL contains only one export, PlayGame:
The code simply extracts the ransomware dropper from the resource shown previously, and starts it using the function CreateProcess:
Injected launcher.dll in the lsass.exe address space.
Malware variants in the wild
As reported by several sources, the malware dropper contains code to check to two specific domains before executing its ransomware or the network exploit codes.
While looking for more samples in our malware database, we came across several other droppers (MD5: 509C41EC97BB81B0567B059AA2F50FE8) that did not exhibit this same behavior. These other droppers did not have the code to exploit machines through NetBIOS or to check for the kill-switch domain. With these samples, the ransomware code would be executed in all cases.
These samples were found in the wild, which means they are capable of infecting and spreading, but in a much less aggressive way. Once the ransomware infects a machine, it also tries to infect any network shares mounted as local disks. Anyone accessing these shares could execute the malware sample by mistake and infect themselves. This infection vector is not as effective as the network exploit but could nonetheless wreak havoc in a corporate environment.
We also examined the droppers (for example, MD5: DB349B97C37D22F5EA1D1841E3C89EB4) that had the exploit code to compare with the other samples. We found that this exploit-aware dropper is a wrapper around the other droppers.
Looking at the exploit-aware sample, we found that one of the resources contains a 3.4MB .exe file that is the same as the other type of droppers:
The preceding resource is extracted after the remote host is exploited and sent to the victim and installed as a service. This event starts the infection on the remote machine.
WannaCry offers free decryption for some random number of files in the folder C:\McAfee\<random folder name>\f.wnry. We have seen 10 files decrypted for free.
In the first step, the malware checks the header of each encrypted file. Once successful, it calls the decryption routine, and decrypts all the files listed in C:\McAfee\<random folder name>\ f.wnry.
A code snippet of the header check:
The format of the encrypted file:
To decrypt all the files on an infected machine we need the file 00000000.dky, which contains the decryption keys. The decryption routine for the key and original file follows:
WannaCry uses three Bitcoin wallets to receive payments from its victims. Looking at the payment activity for these wallets gives us an idea of how much money the attackers have made.
The current statistics as of May 13 show that not many people have paid to recover their files:
- Wallet 12t9YDPgwueZ9NyMgw519p7AA8isjr6SMw
- Wallet 13AM4VW2dhxYgXeQepoHkHSQuy6NgaEb94
- Wallet 115p7UMMngoj1pMvkpHijcRdfJNXj6LrLn
The attackers appear to have earned a little over BTC 15.44 (US$27,724.22). That is not much considering the number of infected machines, but these numbers are increasing and might become much higher in the next few days. It’s possible that the sink holing of two sites may have helped slow things down:
Multiple organizations across more than 90 countries have been impacted, according to reports.
We will update this blog as we learn more.
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