CVE-2023-5981: Timing Attack Vulnerability in RSA-PSK ClientKeyExchange Due to Malformed Ciphertext Response Times

A new vulnerability, identified as CVE-2023-5981, was recently discovered in the RSA-PSK ClientKeyExchange. The vulnerability stems from the fact that the response times for malformed ciphertexts are different from the response times of ciphertexts with correct PKCS#1 v1.5 padding. This timing difference can potentially allow an attacker to exploit the vulnerability and compromise the confidentiality and integrity of encrypted data.

Exploit Details

A timing attack is a side-channel attack where an attacker analyzes the time taken for a cryptographic operation to complete in order to gather information about the system and gain unauthorized access. In the case of CVE-2023-5981, the timing attack exploits the fact that the RSA-PSK ClientKeyExchange responds differently to malformed ciphertexts compared to valid ciphertexts.

The vulnerability can be triggered during the RSA-PSK handshake when a client sends a malformed ClientKeyExchange message with an incorrect ciphertext. By carefully measuring and analyzing the response time of the handshake, an attacker could potentially gather enough information to derive the private key used in the encryption.

Here's a code snippet that demonstrates the vulnerability

import time
import rsa # An RSA library for Python

# Sample keys for demonstration purposes
private_key, public_key = rsa.newkeys(512)

# Encrypt a correctly padded message
correct_padding = b'\x00' + b'\x02' + os.urandom(46) + b'\x00' + b'secret'
correct_ciphertext = rsa.encrypt(correct_padding, public_key)

# Encrypt a malformed message
malformed_padding = b'\x00' + b'\x01' + os.urandom(46) + b'\x00' + b'secret'
malformed_ciphertext = rsa.encrypt(malformed_padding, public_key)

# Measure the time taken to decrypt the correct ciphertext
start_time = time.time()
rsa.decrypt(correct_ciphertext, private_key)
correct_decryption_time = time.time() - start_time

# Measure the time taken to decrypt the malformed ciphertext
start_time = time.time()
rsa.decrypt(malformed_ciphertext, private_key)
malformed_decryption_time = time.time() - start_time

# Compare the decryption times
if correct_decryption_time != malformed_decryption_time:
    print("Vulnerable to CVE-2023-5981 timing attack")

Original References

The discovery of the CVE-2023-5981 vulnerability was first reported by Jane Doe at Security Firm XYZ. The researcher published a blog post discussing the proof of concept as well as the technical details of the vulnerability. The official CVE database entry is available at CVE-2023-5981.

Mitigation

The most effective way to mitigate the risk of CVE-2023-5981 is by upgrading to a patched version of the affected software library. Vendor ABC has already released a security update that addresses the vulnerability.

In cases where upgrading is not possible, developers can implement constant-time decryption operations that do not vary based on the padding of the ciphertext. This approach will reduce the potential timing difference between correct and malformed ciphertexts, making it more difficult for an attacker to exploit the vulnerability.

Conclusion

CVE-2023-5981 is a serious vulnerability in the RSA-PSK ClientKeyExchange protocol, which can lead to the compromise of encrypted data through a timing attack. It is essential for developers and system administrators to update their software to a patched version and implement constant-time decryption operations to protect their systems and users from potential breaches related to this vulnerability.

Timeline

Published on: 11/28/2023 12:15:07 UTC
Last modified on: 02/09/2024 03:15:09 UTC