CVE-2023-5992

Executive Summary

CVE-2023-5992 is a vulnerability in the OpenSC library, a widely used set of libraries and utilities for accessing smart cards. The flaw resides in the implementation of PKCS#1 encryption padding removal, which is not performed in a side-channel resistant manner. This means that an attacker with the ability to observe timing, power consumption, or electromagnetic emanations during the decryption process could potentially leak private data, such as the plaintext or the private key itself. The vulnerability has a CVSS score of 0.0, indicating that it has not been fully assessed, but the potential impact is high for environments where smart cards are used for authentication or encryption. No public proof-of-concept (PoC) or exploit code is available at this time.

Supply Chain & Package Security

OpenSC is a critical component in the smart card ecosystem, often used in government, enterprise, and personal security applications. The vulnerability is in the core library, affecting any application that relies on OpenSC for PKCS#1 decryption. This includes authentication systems, digital signature applications, and encrypted communication tools. The lack of a patch or official advisory from the vendor increases the risk, as users must rely on workarounds or alternative libraries.

CVSS Breakdown & Threat Level

The CVSS score is 0.0, meaning no official assessment has been published. However, based on the description, the vulnerability is likely to have a high confidentiality impact if exploited. The attack vector is local (requiring physical proximity or access to the same system), with low attack complexity. The threat level is considered Medium due to the lack of a known exploit and the requirement for side-channel access.

Deep Dive & Mechanics

PKCS#1 v1.5 padding is used in RSA encryption to add randomness and structure to the plaintext before encryption. The padding removal process involves checking the first two bytes (0x00 0x02) and then scanning for a 0x00 byte to separate the padding from the data. In a non-constant-time implementation, the time taken to find this separator byte can vary based on the data, allowing an attacker to perform a Bleichenbacher-style attack or a timing side-channel attack.

The vulnerable code in OpenSC likely uses a loop that breaks upon finding the 0x00 byte, without masking the timing differences. An attacker who can measure the decryption time (e.g., via network latency or power analysis) can infer information about the plaintext. Over multiple observations, this can lead to full plaintext recovery or even private key extraction if the key is used repeatedly.

Vulnerable vs. Patched Code

Since no official patch is available, the following is a conceptual comparison.

Vulnerable Code (Conceptual):

int pkcs1_remove_padding(unsigned char *data, int len) {
    if (data[0] != 0x00 || data[1] != 0x02) return -1;
    int i;
    for (i = 2; i < len; i++) {
        if (data[i] == 0x00) {
            // Found separator
            break;
        }
    }
    // Copy data after separator
    memmove(data, data + i + 1, len - i - 1);
    return len - i - 1;
}

Patched Code (Conceptual):

int pkcs1_remove_padding_constant_time(unsigned char *data, int len) {
    if (data[0] != 0x00 || data[1] != 0x02) return -1;
    int i;
    int found = 0;
    int sep_index = 0;
    for (i = 2; i < len; i++) {
        // Constant-time comparison
        int is_sep = (data[i] == 0x00) & 1;
        found |= is_sep;
        // Only update sep_index if not already found
        sep_index = (found == 0) ? i : sep_index;
    }
    // Copy data after separator
    memmove(data, data + sep_index + 1, len - sep_index - 1);
    return len - sep_index - 1;
}

Exploitation & Lab Setup

Red Team

1. Setup: Use a smart card reader and a card that uses OpenSC for PKCS#1 operations. Alternatively, use a software emulator.
2. Observation: Use a logic analyzer or oscilloscope to capture power traces during decryption. Alternatively, use a high-resolution timer for timing attacks.
3. Exploit: Send crafted ciphertexts and measure the time or power consumption. Use statistical analysis to infer the padding separator position.
4. Data Leak: Over multiple queries, recover the plaintext or key material.

Blue Team

1. Detection: Monitor for unusual patterns of decryption operations on smart cards. Use anomaly detection on timing data.
2. Mitigation: Apply constant-time code patches if available. Use hardware that masks side-channel emissions.
3. Workaround: Disable PKCS#1 v1.5 padding and use OAEP (Optimal Asymmetric Encryption Padding) instead, which is designed to be side-channel resistant.

Indicators of Compromise (IOCs)

• Network: Unusual timing patterns in TLS handshakes or decryption operations.
• Host: High CPU usage during smart card operations, or repeated decryption attempts.
• File: Modified OpenSC library files (e.g., libopensc.so or opensc.dll).
• Log: Error logs indicating padding failures or unexpected decryption times.

Detection Engineering

YARA Rule

rule CVE_2023_5992_OpenSC_SideChannel {
    meta:
        description = "Detects vulnerable OpenSC PKCS#1 padding removal code"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-5992"
    strings:
        $code1 = { 00 02 ?? ?? ?? ?? 00 }  // Pattern for PKCS#1 padding
        $code2 = { 80 3? 00 74 ?? }        // Conditional jump after comparison (non-constant time)
    condition:
        $code1 and $code2
}

Sigma Rule

title: CVE-2023-5992 OpenSC Side-Channel Vulnerability
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects potential exploitation attempts of CVE-2023-5992 via timing anomalies
author: Valters IT
logsource:
    category: application
    product: opensc
detection:
    selection:
        EventID: 1001  # Example: OpenSC error event
        Message: "*padding*"
    condition: selection
falsepositives:
    - Legitimate smart card operations
level: medium

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2023-5992 OpenSC Side-Channel Attempt"; flow:to_server; content:"|00 02|"; offset:0; depth:2; content:"|00|"; within:256; sid:1000001; rev:1;)

Remediation & Mitigation

1. Patch: Apply any official patch from OpenSC when available. Monitor the OpenSC GitHub repository for updates.
2. Workaround: Use OAEP padding instead of PKCS#1 v1.5. OAEP is designed to be side-channel resistant.
3. Configuration: Disable PKCS#1 v1.5 in applications that use OpenSC, if possible.
4. Monitoring: Implement timing anomaly detection for smart card operations.
5. Hardware: Use smart cards with hardware countermeasures against side-channel attacks.

Valters IT
DevSecOps & Cyber Threat Intelligence Expert