CVE-2023-6247 - OpenVPN 3 Core Library Vulnerability: PKCS#7 Parser Crash and Exploit Details

In this article, we will discuss a recently discovered vulnerability in the OpenVPN 3 Core Library - CVE-2023-6247. Found in versions up to and including 3.8.3, the flaw stems from the PKCS#7 parser that fails to properly validate parsed data. Consequently, this can lead to immediate application crashes and potential security threats. By dissecting the vulnerability, we will analyze its causes, potential impacts, and recommended mitigation steps.

As usual, we will provide code snippets, references to the original discoveries, and detailed explanations of the exploit to ensure readers have a comprehensive understanding of the issue at hand.

Background

The affected area of the OpenVPN code is the PKCS#7 parser found in the third-party library, mbedtls. PKCS#7 is a cryptographic standard that enables data to be securely bundled with digital signatures.

The OpenVPN 3 Core Library is widely used as the underlying technology in VPN applications and services across various platforms. Given its widespread use, this vulnerability could have severe consequences if left unpatched.

Technical Details of the Vulnerability (CVE-2023-6247)

The flaw exists due to improper validation of the parsed PKCS#7 data. Upon parsing a crafted PKCS#7 message, the application may lack the necessary error-checking mechanisms to catch incorrect values or malformed messages.

Here's an example of a potentially problematic code snippet in the mbedtls library

int mbedtls_pkcs7_parse_der( const unsigned char *buf, const int buflen,
                             mbedtls_pkcs7 *pkcs7 )
{
    int ret;
    unsigned char *p = (unsigned char *)buf;
    mbedtls_asn1_buf oid;

    /* Parse outer PKCS#7 layer */
    ret = mbedtls_asn1_get_tag( &p, buf + buflen, &pkcs7->version,
                                MBEDTLS_ASN1_CONSTRUCTED
                                | MBEDTLS_ASN1_CONTEXT_SPECIFIC );
    if( ret !=  )
        return( MBEDTLS_ERR_PKCS7_INVALID_FORMAT + ret );

    // ...
}

By not properly verifying the parsed data, it opens the door for attackers to insert malicious messages designed to crash the application. This can lead to scenarios like Denial of Service (DoS) attacks.

Exploit Details

While there appear to be no publicly available exploits for CVE-2023-6247 at the moment, a potential attacker with knowledge of the vulnerability could construct a malicious PKCS#7 message. When this message is fed to the OpenVPN service, the mbedtls library attempts to parse the message, and an application crash may occur.

It's important to note that even without publicly available exploits, the risk of attackers independently discovering this vulnerability is significant. Thus, it's crucial for developers and administrators to promptly patch their instances of OpenVPN to limit their exposure to potential attacks.

Mitigation Steps

To address this vulnerability, upgrades to the latest version of OpenVPN, which includes a patched mbedtls library, are recommended. OpenVPN has released a fix in version 3.8.4 that addresses the bug by adding proper validation to the PKCS#7 parser.

To update OpenVPN, follow these steps

1. Download the latest release - Visit the official OpenVPN website at https://openvpn.net/download-software/openvpn/ and download the appropriate version for your platform.

2. Replace existing OpenVPN installation - Uninstall the old version of OpenVPN and replace it with the newly downloaded 3.8.4 (or later) release.

3. Restart the OpenVPN service - In order to finalize the update, a restart of the OpenVPN service is required.

Finally, subscribe to security bulletins and monitor OpenVPN announcements for any future vulnerability discoveries or patches.

Conclusion

As demonstrated by CVE-2023-6247, even widely used, well-maintained software libraries like OpenVPN can contain critical vulnerabilities. Stay vigilant in proactively updating your software and staying up-to-date with the latest security news to minimize your risks.

Keep in mind that with the proper validation in place, the PKCS#7 parser can function safely, providing secure communication by utilizing digital signatures. By upgrading your OpenVPN installation, you ensure a stable working environment and aid in safeguarding your organization's data.

Timeline

Published on: 02/29/2024 01:42:34 UTC
Last modified on: 10/28/2024 00:35:00 UTC