1. Phishing Attacks: Phishing attacks are one of the most common and effective methods used by cybercriminals to spread ransomware. These attacks involve tricking individuals into clicking on malicious links or downloading infected attachments. For example, an attacker may send an email impersonating a legitimate organization, urging the recipient to click on a link that leads to a ransomware infection.
2. Exploit Kits: Exploit kits are tools that take advantage of vulnerabilities in software to deliver malware, including ransomware. Cybercriminals can use exploit kits to target outdated or unpatched software on a victim's computer. For instance, the infamous exploit kit called "EternalBlue" was used to spread the WannaCry ransomware in 2017 by exploiting a vulnerability in the Windows operating system.
3. Remote Desktop Protocol (RDP) Attacks: RDP attacks involve gaining unauthorized access to a victim's computer or network by exploiting weak or compromised Remote Desktop Protocol connections. Once inside, attackers can deploy ransomware to encrypt files and demand a ransom. An example of this is the SamSam ransomware, which targeted organizations by brute-forcing RDP credentials.
4. Malvertising: Malvertising refers to the distribution of malicious advertisements that can lead to ransomware infections. Attackers inject malicious code into legitimate online advertisements, which, when clicked, redirect users to websites hosting ransomware. In 2016, the "Magnitude" exploit kit used malvertising to distribute the Cerber ransomware.
5. Remote Code Execution (RCE) Vulnerabilities: RCE vulnerabilities allow attackers to execute arbitrary code on a targeted system, making it an attractive entry point for ransomware. Notable examples include the Apache Struts vulnerability exploited by the infamous Equifax breach in 2017 and the "BlueKeep" vulnerability in Microsoft Windows Remote Desktop Services discovered in 2019.

References:

• https://www.cisco.com/c/en/us/products/security/what-is-ransomware.html
• https://www.symantec.com/security-center/threat-report
• https://www.kaspersky.com/resource-center/threats/ransomware