CVE-2025-23419: How an Attacker Can Bypass Client Certificate Authentication in Nginx Servers using Session Resumption

As more and more people rely on the internet for their daily activities, ensuring the security of online services is crucial. One vulnerability that can pose a significant threat to the security of internet services is CVE-2025-23419. In this post, we will discuss this vulnerability in detail, as well as the steps you can take to protect your servers and applications.

What is CVE-2025-23419?

CVE-2025-23419 is a potential security vulnerability in servers that have multiple server blocks configured to share the same IP address and port. In this scenario, an attacker can use session resumption to bypass client certificate authentication requirements on these servers. This vulnerability arises when TLS Session Tickets (https://nginx.org/en/docs/http/ngx_http_ssl_module.html#ssl_session_ticket_key) are used, and/or the SSL session cache (https://nginx.org/en/docs/http/ngx_http_ssl_module.html#ssl_session_cache) are used in the default server, which is performing client certificate authentication.

Essentially, this vulnerability allows an attacker to resume an earlier session without needing to provide appropriate client certificates, which can potentially grant them access to sensitive information or unauthorized actions.

How does this vulnerability work?

In a typical setup, client certificate authentication is used to provide an additional layer of security – ensuring that only authorized clients can access certain resources on a server. This is done by requiring clients to present a valid, signed certificate as part of the SSL/TLS handshake process.

However, when multiple server blocks are configured to share the same IP address and port, and TLS Session Tickets or the SSL session cache are enabled, it is possible for an attacker to bypass this certificate verification process. They can do this by "resuming" an earlier session, using their own session ticket or cached session keys – without having to present a valid client certificate.

The following code snippet demonstrates a typical configuration that might be vulnerable to this issue:

server {
    listen 443 ssl default_server;
    server_name example1.com;
    ssl_certificate /path/to/example1/cert.pem;
    ssl_certificate_key /path/to/example1/key.pem;
    ssl_verify_client on;
    ssl_client_certificate /path/to/client/cert.pem;
    ssl_session_cache shared:SSL:5m;
    # ... (other configs)...
}

server {
    listen 443 ssl;
    server_name example2.com;
    ssl_certificate /path/to/example2/cert.pem;
    ssl_certificate_key /path/to/example2/key.pem;
    ssl_verify_client on;
    ssl_client_certificate /path/to/client/cert.pem;
    # ... (other configs)...
}

In this example, both the example1.com and example2.com server blocks are configured to use the same IP address and port (443, the default for HTTPS), as well as client certificate authentication. However, the default server (example1.com) also has the SSL session cache enabled, which could allow an attacker to bypass the client certificate authentication requirement.

To address this vulnerability, there are several steps you can take to protect your servers

1. Disable or limit the use of TLS Session Tickets and SSL session cache in your server configuration. This can be done by either not using ssl_session_ticket_key and ssl_session_cache directives or by appropriately restricting their usage.

2. Configure multiple server blocks that share the same IP address and port to use different ports or addresses. This will prevent the server configuration from being vulnerable to this issue.

3. Ensure that all server blocks which perform client certificate authentication do not use SSL/TLS session resumption features.

By implementing these measures, you can significantly reduce the risk of this vulnerability being exploited against your servers.

In conclusion, CVE-2025-23419 is a potentially serious vulnerability in specific configurations of servers and applications. By understanding the issue and implementing the correct measures, you can protect your applications and ensure that your users' data remains secure.

Timeline

Published on: 02/05/2025 18:15:33 UTC
Last modified on: 02/05/2025 20:15:45 UTC