Introduction to the vulnerabilities of legacy encryption protocols for card data theft

[Student]

Encryption protocols such as SSL (Secure Sockets Layer) and its successor, TLS (Transport Layer Security), were designed to protect data in transit, including sensitive information like credit card numbers, CVV codes, and expiration dates. SSL emerged in the 1990s (versions 2.0 and 3.0), and TLS evolved from it, beginning with version 1.0 in 1999. However, early versions of these protocols contained fundamental weaknesses in their design, encryption algorithms, and authentication mechanisms, which were eventually identified by security researchers. These vulnerabilities allow attackers to intercept, decrypt, or manipulate traffic, which is particularly dangerous in online payment scenarios where card data is transmitted over HTTPS.

By 2025, most modern browsers and servers (e.g., in accordance with PCI DSS standards for payment systems) will require TLS 1.2 or 1.3, while older versions (SSL 2.0/3.0, TLS 1.0/1.1) are officially deprecated and disabled by default. However, on legacy systems, old devices, or poorly configured servers, they may still pose a risk. Vulnerabilities are often exploited through Man-in-the-Middle (MITM) attacks, where an attacker interferes with the connection between the client (browser) and the server (retailer's website), or through downgrade attacks, where the protocol is forcibly downgraded to a vulnerable version.

For educational purposes, below I will examine the key vulnerabilities in more detail: their mechanism, historical context, why they enable card data theft, and how to avoid them. The explanations will be conceptual, intended to understand the principles of cryptography and security, without going into the technical details of how attacks are implemented. These examples illustrate the evolution of protocols and the importance of updates.

1. POODLE (Padding Oracle On Downgraded Legacy Encryption)​

Mechanism and operating principle: POODLE exploits a weakness in SSL 3.0 related to the handling of "padding" (additional bytes added to align blocks of data during encryption). In CBC (Cipher Block Chaining) mode, the padding is checked on the server, and if it is incorrect, the connection is terminated. An attacker can manipulate packets to create an "oracle" (a mechanism that provides hints about correctness) and gradually decrypt the data. This requires many attempts (about 256 per byte), but allows one to extract fragments such as session cookies or form data.

Historical context: Discovered in 2014 by Google researchers, the attack was possible due to compatibility: browsers could "fallback" to SSL 3.0 if TLS was not working. In real-world scenarios, this led to leaks in systems such as legacy web servers.
Impact on card data theft: During payment transactions (e.g., when entering a card on a website), an attacker can decrypt parts of the traffic, including tokens or fragments of card numbers, if the connection is downgraded. This is especially risky on public Wi-Fi, where MITM attacks are easier.
Mitigation: Completely disable SSL 3.0 on servers. Use TLS 1.2+ with HSTS (HTTP Strict Transport Security) to prevent downgrades.

2. BEAST (Browser Exploit Against SSL/TLS)​

Mechanism and operating principle: BEAST targets TLS 1.0 and weaknesses in CBC mode. In CBC, each block is encrypted based on the previous one, using an initialization vector (IV). In TLS 1.0, the IV is predictable (based on the previous block), which allows an attacker in control of part of the plaintext (e.g., via JavaScript) to predict and extract the encrypted data byte by byte.

Historical context: Discovered in 2011 (CVE-2011-3389). This was the first major attack on TLS, showing that even "secure" protocols are vulnerable to adaptive attacks. Fixed in TLS 1.1 by randomizing the IV.
Impact on card data theft: An attacker can inject malicious code into the browser (e.g., via XSS) and decrypt form data, including card numbers, during submission. This is relevant for older e-commerce sites.
Mitigation: Switch to TLS 1.1+ or use modes like GCM instead of CBC. Browsers have introduced workarounds like 1/n-1 record splitting.

3. CRIME (Compression Ratio Info-leak Made Easy)​

Mechanism and operating principle: CRIME uses data compression in TLS (for example, DEFLATE), where repeated strings are compressed better. The attacker adds guesses to the request (via JavaScript) and analyzes the size of the compressed packet: if the guess matches a secret (for example, a cookie), the size is reduced, revealing information.

Historical context: Discovered in 2012. Similar to BREACH (for HTTP compression). Demonstrated the risks of combining encryption and compression.
Impact on card data theft: Can reveal session tokens or parts of card data in forms if the site uses compression. In payments, this leads to session hijacking.
Mitigation: Disable TLS compression (default in modern versions). Use CSRF tokens to protect forms.

4. Heartbleed​

Mechanism and operating principle: This is a bug in the OpenSSL library (not in the protocol, but in the SSL/TLS implementation). In the Heartbeat extension, the client requests an echo of data, but can specify a larger size than it sent, forcing the server to return extra bytes from memory (up to 64 KB).

Historical context: Discovered in 2014 (CVE-2014-0160). Affected millions of servers; led to mass leaks.
Impact on card data theft: The server could "emit" private keys, passwords, or buffers with card data from active sessions.
Mitigation: Update OpenSSL to a patched version. Regularly scan for vulnerabilities.

5. DROWN (Decrypting RSA with Obsolete and Weakened eNcryption)​

Mechanism and operating principle: DROWN exploits SSL 2.0 support on the server (even if it is not used). The attacker uses weak SSL 2.0 "export" ciphers to attack RSA keys, then decrypts modern TLS sessions with the same key.

Historical context: Discovered in 2016. Related to export restrictions of the 1990s (weak keys for international export).
Impact on card data theft: Allows decryption of all traffic, including payments, if the server supports SSL 2.0.
Mitigation: Disable SSL 2.0 completely. Use unique keys for sessions.

6. Other vulnerabilities (FREAK, Logjam, MITM on weak ciphers)​

• FREAK (Factoring RSA Export Keys): Exploits export ciphers, downgrades to weak keys (512-bit RSA), allowing brute-force attacks. Discovered in 2015; affects TLS 1.0/1.1.
• Logjam: A weakness in Diffie-Hellman key exchange; allows downgrade to weak groups, decrypting traffic.
• MITM on weak protocols: In SSL/TLS with RC4 or 3DES, an attacker interferes by exploiting predictability or weak cryptography.

Impact: All make it easier to intercept payment data on unsecured networks.
Mitigation: Use strong ciphers (AES-GCM, ChaCha20), forward secrecy (ECDHE), and monitor configuration using tools like SSL Labs.

Conclusion and Lessons​

These vulnerabilities highlight how cryptosystems evolve, from simple padding errors to complex compression attacks. In 2025, the risks are minimal in updated systems, but legacy devices (such as old POS terminals) are still vulnerable. For education: study cryptography through resources like OWASP or NIST. Always apply the "defense in depth" principle: encryption + card data tokenization + monitoring.