CVE-2024-38520 in SoftEtherVPN
Summary
by MITRE • 06/26/2024
SoftEtherVPN is a an open-source cross-platform multi-protocol VPN Program. When SoftEtherVPN is deployed with L2TP enabled on a device, it introduces the possibility of the host being used for amplification/reflection traffic generation because it will respond to every packet with two response packets that are larger than the request packet size. These sorts of techniques are used by external actors who generate spoofed source IPs to target a destination on the internet. This vulnerability has been patched in version 5.02.5185.
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Analysis
by VulDB Data Team • 06/26/2024
CVE-2024-38520 represents a critical denial-of-service vulnerability in SoftEtherVPN software that exploits the Layer 2 Tunneling Protocol (L2TP) implementation to facilitate distributed denial-of-service (DDoS) amplification attacks. This vulnerability specifically affects deployments where L2TP functionality is enabled on the SoftEtherVPN server, creating a condition where the system responds to every incoming packet with two response packets that exceed the original request size. The flaw stems from the protocol handling mechanism that does not properly validate incoming L2TP packets, allowing malicious actors to exploit this behavior for amplification purposes. The vulnerability aligns with CWE-400, which covers "Uncontrolled Resource Consumption" and specifically addresses the improper handling of network traffic that leads to excessive resource utilization. The technical implementation involves the L2TP server responding to spoofed source IP addresses with oversized packets, creating a scenario where a small request can generate multiple larger responses that can be used to overwhelm target systems. This behavior directly maps to ATT&CK technique T1498.001, which describes "Network Denial of Service" through the use of amplification attacks. The operational impact of this vulnerability extends beyond simple service disruption as it enables attackers to leverage compromised SoftEtherVPN servers as amplification points in larger DDoS campaigns. When an attacker spoofs the source IP address of a target system and sends L2TP packets to a vulnerable SoftEtherVPN server, the server responds to the spoofed address with oversized packets, effectively using the server as a weapon to flood the intended victim. The amplification factor occurs because the response packets are typically two to three times larger than the original request, allowing attackers to generate significant traffic volume from relatively small initial packets. This vulnerability particularly affects organizations that deploy SoftEtherVPN with L2TP enabled in public-facing environments, as these deployments become attractive targets for malicious actors seeking to leverage the amplification effect for launching large-scale attacks against other systems. The patch released in version 5.02.5185 addresses the root cause by implementing proper packet validation and response sizing controls that prevent the generation of oversized responses to malformed or spoofed L2TP packets. Organizations should immediately update their SoftEtherVPN installations to the patched version and consider implementing network-level mitigations such as rate limiting and source IP validation to prevent exploitation. Additionally, security teams should monitor for unusual traffic patterns that may indicate exploitation attempts and ensure that L2TP functionality is disabled on servers that do not require it for legitimate operations. The vulnerability demonstrates the importance of proper input validation in network protocol implementations and highlights how seemingly benign features can be weaponized for large-scale attacks when not properly secured against spoofing and amplification techniques.