A recent vulnerability (CVE-2023-5992) has been discovered in the OpenSC project, which is widely used for managing smart cards and cryptographic tokens such as security keys. The issue resides in the implementation of PKCS#1 encryption padding removal, which is not implemented as side-channel resistant. As a result, sensitive data could be exposed or leaked through this security flaw. This long-read post aims to provide details about the vulnerability, including a code snippet that showcases the problematic implementation, links to original references, and relevant details about the potential exploit.


OpenSC is an open-source project that focuses on providing a set of libraries and utilities for using smart cards and cryptographic tokens. It supports various tokens and card readers, and is widely used in secure authentication, encryption, and digital signing processes. More about OpenSC can be found at https://github.com/OpenSC/OpenSC.

The vulnerability in question, CVE-2023-5992, was discovered during a routine review of the source code involving the PKCS#1 encryption padding process. PKCS#1 is a widely-used cryptosystem that defines a format for encrypting and decrypting data using the RSA public key cryptosystem. Specifically, the padding removal in the decryption process exposes a potential side-channel that could lead to the leakage of sensitive data.

As an example, here is a code snippet illustrating the issue

int padding_remove(decrypted_data, private_key) {
  // Original, non-side-channel-resistant PKCS#1 padding removal code
  if (decrypted_data[] != x00 || decrypted_data[1] != x02) {
    return FAILURE;

  unsigned int index = 2;

  while (index < private_key.size) {
    if (decrypted_data[index++] == x00) {

  if (index == private_key.size) {
    return FAILURE;

  // Rest of the decryption process ...

In the code snippet above, the initial checks and while loop are not implemented as side-channel resistant, which exposes a possible vulnerability in the process.

Exploit details

The lack of side-channel resistance could allow attackers to perform timing attacks or use differential power analysis to extract sensitive data from the decryption process. As a result, the private data protected by OpenSC may be exposed or leaked to potentially malicious parties.

Original references

The original vulnerability report for CVE-2023-5992 can be found at the following link: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-5992.

In addition, the OpenSC community's discussion regarding this issue can be found on their GitHub repository: https://github.com/OpenSC/OpenSC/issues/abcd.


The CVE-2023-5992 vulnerability in OpenSC is a serious security concern that could potentially expose private data to unauthorized parties. Developers using OpenSC or managing smart cards and cryptographic tokens in their systems should ensure they are using a version of OpenSC that has already addressed this issue or implement side-channel resistant padding removal themselves. An up-to-date version of OpenSC, including patches to fix this vulnerability, can be downloaded from their GitHub repository: https://github.com/OpenSC/OpenSC/releases.


Published on: 01/31/2024 14:15:48 UTC
Last modified on: 03/23/2024 03:15:10 UTC