In the world of cybersecurity, rarely a day goes by without news of a new vulnerability or exploit within a wide range of technological systems. Recently, researchers have discovered a remote code execution vulnerability in the Layer 2 Tunneling Protocol (L2TP), a popular VPN protocol. Identified as CVE-2023-41767, this critical issue can lead to potential attacks by malicious actors, granting them unauthorized access to sensitive information and enable them to wreak havoc on vulnerable networks. In this post, we will dive into the details of this vulnerability, analyze the source code snippet, discuss the exploit, and provide useful links to the original references.


L2TP is a tunneling protocol used to support virtual private networks (VPNs) or as part of the delivery of services by ISPs. It does not provide any encryption or confidentiality by itself. Instead, it relies on an encryption protocol that it passes within the tunnel to provide privacy. When the layer 2 tunneling protocol fails to properly validate incoming packets, attackers can exploit this weakness to execute arbitrary code on the affected system.

CVE-2023-41767 Analysis

The CVE-2023-41767 vulnerability lies in the L2TP subsystem, wherein an improper validation of user-supplied data allows an attacker to execute code remotely on the targeted device. This security issue stems from a buffer overflow in the packet handling mechanism that can be triggered by sending specially crafted L2TP packets.

To help you better understand the vulnerability, let's take a look at a simplified code snippet

/* In the following example, buffer_size represents the fixed size of the buffer
   that can store L2TP packets. */
#define buffer_size 256

void process_l2tp_packet(char *incoming_packet, size_t incoming_packet_size) {
  char l2tp_packet_buffer[buffer_size];

  /* Copies the incoming_packet into the local l2tp_packet_buffer memory area */
  memcpy(l2tp_packet_buffer, incoming_packet, incoming_packet_size);
  /* ... */

In this code snippet, the issue emerges due to the memcpy function which copies the incoming_packet directly into the fixed-size buffer l2tp_packet_buffer without checking if the incoming_packet_size is larger than buffer_size. Consequently, an attacker can send a larger-than-allowed packet, leading to a buffer overflow vulnerability.

Exploit Details

The exploitation of CVE-2023-41767 involves an attacker leveraging the buffer overflow vulnerability by crafting a specially tailored L2TP packet and sending it to the vulnerable system. The malicious packet, when processed by the vulnerable implementation, can cause a buffer overflow and ultimately result in remote code execution. The attacker can then gain control over the affected system, potentially leading to unauthorized access to sensitive data or resources.

Embed malicious payload within the specially crafted L2TP packet.

3. Send the packet to the targeted device, triggering the buffer overflow and executing the malicious payload.

Original References

1. CVE: CVE-2023-41767 - CVE identifier for this vulnerability.
2. NIST NVD: CVE-2023-41767 Detail - Provides technical details, CVSS score, and impact analysis of the vulnerability.
3. L2TP RFC: RFC 2661 - Layer Two Tunneling Protocol (L2TP) - Detailed documentation about the L2TP protocol.


CVE-2023-41767 is a critical remote code execution vulnerability discovered in the Layer 2 Tunneling Protocol (L2TP). Exploiting this vulnerability could allow a malicious actor to gain unauthorized access to sensitive data or resources on vulnerable systems. In this article, we examined the vulnerability's origin, analyzed the code snippet, and discussed the exploit details. To protect against this security threat, it is crucial to follow best practices and ensure that proper security updates are applied to the affected systems.


Published on: 10/10/2023 18:15:18 UTC
Last modified on: 10/12/2023 22:17:57 UTC