Much ado has been made (by this very author on this very blog!) about the incentives for attackers and defenders around ransomware. There is also a wealth of information on the internet about how to protect yourself from ransomware. One thing we want to avoid losing sight of, however, is just how we go from a machine that is working perfectly fine to one that is completely inoperable due to ransomware. We'll use MITRE's ATT&CK as a vague guide, but we don't follow it exactly.

Ransomware targeting and delivery

As LockFile is ravaging Exchange servers just a few short weeks after widespread exploitation was documented, we can draw two conclusions:

  1. If you're running an Exchange server, please stop reading this blog and go patch right away.
  2. When a widely exploitable vulnerability is available, ransomware actors will target it — no matter who you are.

Highly targeted attacks have certainly leveraged ransomware, but for nearly all of these actors, the goal is profit. That means widespread scanning for and exploitation of server-side vulnerabilities that allow for initial access, coupled with tools for privilege escalation and lateral movement. It also means the use of watering hole attacks, exploit kits, and spam campaigns.

That is to say, there are a few ways the initial access to a particular machine can come about, including through:

  1. Server-side vulnerability
  2. Lateral movement
  3. Watering hole or exploit kit
  4. Spam or phishing

The particular method tends to be subject to the availability of widely exploitable vulnerabilities and the preferences of the cybercrime group in question.

Droppers

The overwhelming majority of ransomware operators don't directly drop the ransomware payload on a victim machine. Instead, the first stage tends to be something like TrickBot, Qbot, Dridex, Ursnif, BazarLoader, or some other dropper. Upon execution, the dropper will often disable detection and response software, extract credentials for lateral movement, and — crucially — download the second-stage payload.

This all happens quietly, and the second-stage payload may not be dropped immediately. In some cases, the dropper will drop a second-stage payload which is not ransomware — something like Cobalt Strike. This tends to happen more frequently in larger organizations, where the potential payoff is much greater. When something that appears to be a home computer is infected, actors will typically quickly encrypt the machine, demand a payment that they expect to be paid, and move on with their day.

Once the attacker has compromised to their heart's content — one machine, one subnet, or every machine at an organization — they pull the trigger on the ransomware.

How ransomware works

By this point, nearly every security practitioner and many laypeople have a conceptual understanding of the mechanics of ransomware: A program encrypts some or all of your files, displays a message demanding payment, and (usually) sends a decryption key when the ransom is paid. But under the hood, how do these things happen? Once the dropper executes the ransomware, how does it work?

Launching the executable

The dropper will launch the executable. Typically, once it has achieved persistence and escalated privileges, the ransomware will either be injected into a running process or executed as a DLL. After successful injection or execution, the ransomware will build its imports and kill processes that could stop it. Many ransomware families will also delete all shadow copies and possible backups to maximize the damage.

The key

The first step of the encryption process is getting a key to actually encrypt the files with. Older and less advanced ransomware families will first reach out to a command and control server to request a key. Others come with built-in secret keys — these are typically easy to decrypt, with the right decrypter. Others still will use RSA or some other public key encryption so that no key needs to be imported. Many advanced families of ransomware today use both symmetric key encryption for speed and public key encryption to prevent defenders from intercepting the symmetric key.

For these modern families, an RSA public key is embedded in the executable, and an AES key is generated on the victim machine. Then, the files are encrypted using AES, and the key is encrypted using the RSA public key. Therefore, if a victim pays the ransom, they're paying for the private key to decrypt the key that was used to encrypt all of their files.

Encryption

Once the right key is in place, the ransomware begins encryption of the filesystem. Sometimes, this is file-by-file. In other cases, they encrypt blocks of bytes on the system directly. In both cases, the end result is the same — an encrypted filesystem and a ransom note requesting payment for the decryption key.

Payment and decryption

The typical “business" transaction involves sending a certain amount of bitcoin to a specified wallet and proof of payment. Then, the hackers provide their private key and a decryption utility to the victim. In most cases, the attackers actually do follow through on giving decryption keys, though we have recently seen a rise in double encryption, where two payments need to be made.

Once the ransom is paid and the files are decrypted, the real work begins.

Recovery

Recovering encrypted files is an important part of the ransomware recovery process, and whether you pay for decryption, use an available decrypter, or some other method, there's a catch to all of it. The attackers can still be in your environment.

As mentioned, in all but the least interesting cases (one machine on one home network), attackers are looking for lateral movement, and they're looking to establish persistence. That means that even after the files have been decrypted, you will need to scour your entire environment for residual backdoors, cobalt strike deployments, stolen credentials, and so on. A full-scale incident response is warranted.

Even beyond the cost of paying the ransom, this can be an extremely expensive endeavor. Reimaging systems, resetting passwords throughout an organization, and performing threat hunting is a lot of work, but it's necessary. Ultimately, failing to expunge the attacker from all of your systems means that if they want to double dip on your willingness to pay (especially if you paid them!), they just have to walk through the back door.

Proactive ransomware defense

Ransomware is a popular tactic for cybercrime organizations and shows no signs of slowing down. Unlike intellectual property theft or other forms of cybercrime, ransomware is typically an attack of opportunity and is therefore a threat to organizations of all sizes and industries. By understanding the different parts of the ransomware killchain, we can identify places to plug into the process and mitigate the issue.

Patching vulnerable systems and knowing your attack surface is a good first step in this process. Educating your staff on phishing emails, the threats of enabling macros, and other security knowledge can also help prevent the initial access.

The use of two-factor authentication, disabling of unnecessary or known-insecure services, and other standard hardening measures can help limit the spread of lateral movement. Additionally, having a security program that identifies and responds to threats in your environment is crucial for controlling the movement of attackers.

Having off-site backups is a really important step in this process as well. Between double extortion and the possibility that the group that attacked your organization and encrypted your files just isn't an honest broker, there are many ways that things could go wrong after you've decided to pay the ransom. This can also make the incident response process easier, since the backup images themselves can be checked for signs of intrusion.

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