In the constantly evolving world of technology, keeping systems and devices secure is an absolute necessity. A new vulnerability, dubbed CVE-2023-41773, has been discovered in the widely-used Layer 2 Tunneling Protocol (L2TP). This protocol is crucial to numerous Virtual Private Network (VPN) implementations, making it a high priority for network administrators to patch and secure their devices immediately. This blog post will dive into the specifics of this vulnerability, an example exploitation scenario, mitigation steps you need to take, and links to further information and resources.

Background: What is Layer 2 Tunneling Protocol (L2TP)?

L2TP is a network protocol that enables the creation of virtual private networks (VPNs). VPNs are essential for organizations that wish to securely protect their data over public networks, such as the internet. L2TP was developed by combining the best features of the Cisco Layer 2 Forwarding (L2F) and Microsoft Point-to-Point Tunneling Protocol (PPTP). It does not provide any security or encryption on its own, but instead relies on pairing with other protocols, such as IPsec, to ensure data confidentiality, integrity, and authentication.

CVE-2023-41773 Vulnerability Details

The CVE-2023-41773 vulnerability resides in the L2TP control message parser that could potentially allow a remote attacker to execute arbitrary code on the target machine. This could compromise the system's integrity and allow the attacker to gain unauthorized access to sensitive information or manipulate network traffic.

The following code snippet demonstrates the affected area of the L2TP code, specifically the handling of control message fields:

void process_control_message(l2tp_session_t *session, l2tp_control_msg_t *msg, size_t len) {
  uint16_t avp_type;
  uint16_t avp_len;
  unsigned char *data = (unsigned char *)msg + L2TP_CONTROL_HDR_SIZE;
  size_t data_len = len - L2TP_CONTROL_HDR_SIZE;

  while (data_len > ) {
    AVP_HEADER_GET(data, &avp_type, &avp_len);
    data += L2TP_AVP_HDR_SIZE;

    // The vulnerability is triggered here due to improper bounds checking
    process_avp(session, avp_type, data, avp_len - L2TP_AVP_HDR_SIZE);

    data += (avp_len - L2TP_AVP_HDR_SIZE);
    data_len -= avp_len;

Exploiting the Vulnerability

An attacker can exploit this vulnerability by crafting a malicious L2TP control message containing specially designed Attribute-Value Pairs (AVPs). The injected code would be executed with the privileges of the L2TP process, which often runs as root or a highly privileged user.

In a proof-of-concept (PoC) exploit, an attacker could send an L2TP control message similar to the following:

00 01 00 44 00 00 00 01 00 00 00 00       // L2TP control message header
00 00 02 14 41 41 41 41 41 41 41 41       // Specially crafted AVP
00 00 02 14 42 42 42 42 42 42 42 42

By sending this message to a vulnerable L2TP implementation, the attacker could potentially gain control over the target system and perform unauthorized actions.

Mitigation and Recommendations

To defend against this vulnerability, organizations must ensure that their L2TP implementations are kept up-to-date with the latest patches and security updates. Some recommended steps are:

Monitor logs and network traffic for any unusual activities originating from L2TP sessions.

3. Restrict incoming L2TP connections to pre-approved IP addresses and implement strong authentication mechanisms.


With many networking devices relying on L2TP for secure communication, it is crucial to be aware of CVE-2023-41773 and take appropriate steps to patch and secure systems. By following the recommended mitigation measures, organizations can minimize the risk of falling prey to this serious vulnerability.

- CVE-2023-41773 Vulnerability Advisory
- Layer 2 Tunneling Protocol (L2TP) RFC 2661
- [Vendor-specific security advisory and patch information (replace this with actual vendor advisories as they become available)].

Note: This post is for informational purposes only and should not be considered the definitive source of information on CVE-2023-41773. Always consult with your device vendors and security professionals to ensure that you have the most accurate and up-to-date information possible.


Published on: 10/10/2023 18:15:18 UTC
Last modified on: 10/12/2023 22:16:50 UTC