Hey folks! Today, we'll dive deep into a critical vulnerability found in the widely-used OpenSSL library, CVE-2011-4619. Before we get started, let's have a little background on what OpenSSL is. It's an open-source implementation of the SSL (Secure Sockets Layer) and TLS (Transport Layer Security) protocols, used by countless applications for secure communication over the internet.

The Vulnerability

This vulnerability exists in the handling of Server Gated Cryptography (SGC) handshake restarts in OpenSSL versions prior to .9.8s and 1.x prior to 1..f. SGC was designed to provide stronger encryption between web browsers and servers for legacy systems that can't handle newer SSL/TLS protocols.

The problem lies in a bug within the implementation that doesn't correctly handle handshake restarts, and as a result, remote attackers can cause a denial of service (DoS) attack by consuming the server's CPU. This obviously has severe implications, as it can lead to websites and services becoming unavailable to users.

Take a look at the relevant code in the 'statem/statem_clnt.c' function of OpenSSL

   if (s->s3->sgiRestart) {
      /* Send a HelloRequest if necessary */
      if (s->sgiWantsHelloRequest) {
          if (!ssl3_ambi_send_hello_request(s))
              goto f_err;
      }
      /* Process the ServerHello again */
      ...
  }

In the code snippet above, the issue arises when the server reuses the same SGC context without properly initializing it. Essentially, an infinite loop can be formed when the s->sgiWantsHelloRequest variable is always set to '1' (true). This causes the server's CPU to be unnecessarily consumed, leading to the denial of service.

Exploit Details

Although no publicly available exploit exists for this vulnerability, it's important to understand that skilled attackers could reverse-engineer the code and create an exploit on their own. Such an exploit would likely involve sending numerous handshake restart requests to the server with the intention of consuming the server's CPU and causing a DoS.

To further comprehend the scope and impact of this vulnerability, let's take a look at the original references:

1. OpenSSL Security Advisory: CVE-2011-4619
2. NIST National Vulnerability Database: CVE-2011-4619
3. Red Hat Bugzilla: CVE-2011-4619

Mitigation

To protect your server and applications from this vulnerability, all you have to do is update your OpenSSL library to version .9.8s or 1..f (or later). Don't forget to verify the update on your server after doing this. Not only will this fix keep you safe from CVE-2011-4619, but it'll also provide you with additional security updates and bug fixes.

Conclusion

CVE-2011-4619 serves as a reminder that even widely-used libraries such as OpenSSL can contain critical vulnerabilities. It's crucial to be aware of potential risks and regularly update your software, even if no known exploits are currently in the wild. Security should always be a top priority when developing and maintaining web applications and services.

Timeline

Published on: 01/06/2012 01:55:00 UTC
Last modified on: 08/23/2016 02:04:00 UTC