In December 2011, the security community discovered a serious vulnerability in OpenSSL’s handling of block cipher padding in SSL 3.. Assigned as CVE-2011-4576, this bug potentially let remote attackers leak sensitive information by exploiting uninitialized memory used as padding. Let’s break down what happened, why it’s important, and see how attackers could have taken advantage of this weakness, with real code snippets and direct references.

1.. before 1..f

The problem? OpenSSL didn’t always initialize memory for the padding data in SSL 3. when using a block cipher (like AES or DES). This means that some bytes of padding, sent between computers as part of the SSL protocol, weren’t set to zero or any safe value.

If an attacker was able to decrypt or guess the padding, they might be able to read sensitive data left in the computer’s memory—things like private keys, passwords, or session details.

Why Is This Dangerous?

When computers set up a secure SSL connection, data is split into blocks (like turning a novel into pages). If the message doesn’t fill the last block fully, the space is padded out.

Normally, this padding doesn’t contain anything sensitive. But if the software doesn’t erase or initialize the block, the leftovers (“undeleted scenes” from memory) might be sent out in the open! In network protocols, this means random chunks of memory—whatever happened to be used last—could end up visible to attackers who know where to look.

The Code Flaw

The flaw occurred because of code that did not clean out the padding area. Here’s a simplified look at what the vulnerable C code might’ve looked like:

// Vulnerable code example
unsigned char pad[MAX_BLOCK_SIZE];
int padlen = block_size - (data_len % block_size);
memcpy(pad, data + data_len, padlen); // pad is used without being initialized

// ... pad is sent over SSL connection as the padding

> pad[] might contain whatever was left in memory—it wasn't wiped or filled!

Fixed Version

The fix was simple but crucial: always set the padding area to zero (or the proper padding byte) before using or sending it.

memset(pad, padlen, padlen); // each pad byte is set to padlen as per SSL3 standard

## Proof-of-Concept / Exploit Scenario

Imagine a malicious client or server purposely messing with padding or forcing lots of SSL handshakes, and picking up the undesired memory on the wire.

Here’s a high-level Python pseudocode snippet showing what an attack might look like using the vulnerable OpenSSL:

import socket
from tlslite import TLSConnection

# Connect to vulnerable SSL server
sock = socket.socket()
sock.connect(('victim.example.com', 443))
conn = TLSConnection(sock)

# Send regular handshake data
conn.handshakeClientCert()

# Request a large number of short, block-sized messages to maximize the odds of grabbing 'leaked' padding
for i in range(100):
    conn.send(b'A' * 15)    # 15 bytes, so the 16th is padding
    
    # Read the response; attacker sifts through extra bytes at the end for secrets
    data = conn.recv(1024)
    if b'secret' in data:
        print("Sensitive info found:", data)

Note: This is conceptual—the actual exploit would require low-level packet interception and careful analysis of the padding bytes.

How Was It Fixed?

OpenSSL patched the bug in .9.8s and 1..f by ensuring all padding bytes are initialized before encryption or decryption. Major distributions and vendors quickly shipped updates.

References

- OpenSSL Security Advisory (December 2011)
- CVE Details for CVE-2011-4576
- Initial commit fixing the issue (OpenSSL GitHub)
- Red Hat Security Bulletin

Update OpenSSL: Make sure you’re using updated versions: .9.8s or later, 1..f or later.

2. Scan for Old Libraries: Use tools like openssl version or ldd to look for old library files on your servers.
3. Monitor and Patch Regularly: Keep an eye on security advisories for cryptography libraries and patch them ASAP.

Summary

CVE-2011-4576 was a sneaky bug that could have exposed random memory contents to attackers during SSL 3. connections. The fix was straightforward: always clear out any memory used for padding before sending it across the wire.

This CVE is a classic example of why memory handling and initialization matter so much in cryptography. If you’re building or running secure systems, always keep your SSL/TLS libraries current!

Stay secure! Update your OpenSSL and double-check your servers.

_If you found this post helpful, share it with your sysadmin friends—let’s keep these kinds of bugs behind us._

Timeline

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