As of today, a critical security vulnerability with the identifier CVE-2023-0464 has been discovered in all supported versions of OpenSSL, the popular open-source software for SSL and TLS protocols. This vulnerability is related to the verification of X.509 certificate chains that include policy constraints, potentially exposing affected systems to a denial-of-service (DoS) attack. In this post, we'll discuss the details of the exploit, provide code snippets, and offer links to original references.

Code Snippets

The vulnerability occurs when policy processing is enabled in OpenSSL. By default, policy processing is disabled, but can be enabled either through command-line utilities or by invoking a specific function. To enable policy processing via command-line utilities, you can pass the -policy argument when running OpenSSL:

openssl verify -policy -CAfile ca.pem user_cert.pem

Alternatively, policy processing can be enabled programmatically using the X509_VERIFY_PARAM_set1_policies() function:

ASN1_OBJECT *pobj;
STACK_OF(ASN1_OBJECT) *policy_oids = NULL;

vpm = X509_VERIFY_PARAM_new();
pobj = OBJ_txt2obj("", );
policy_oids = sk_ASN1_OBJECT_new_null();
sk_ASN1_OBJECT_push(policy_oids, pobj);
X509_VERIFY_PARAM_set1_policies(vpm, policy_oids);
sk_ASN1_OBJECT_pop_free(policy_oids, ASN1_OBJECT_free);

X509_STORE_CTX_set_param(ctx, vpm);

Exploit Details

An attacker can exploit this vulnerability by creating a maliciously crafted X.509 certificate chain with policy constraints designed to cause a significant increase in computational resources. When the OpenSSL software attempts to verify such a certificate chain, the resource usage could spiral exponentially, resulting in a denial-of-service (DoS) against the affected system.

In practice, such an attack might manifest itself as an unresponsive website or application, causing inconvenience or disruption to both users and administrators. The effectiveness of this exploit depends on the victim system's computational resources and the complexity of the attacker's certificate chain, but under the right circumstances, it could potentially bring a server to its knees.

Original References

For a more technical and in-depth exploration of this vulnerability, please refer to the original references provided below:

1. OpenSSL Security Advisory:
2. CVE-2023-0464:

Mitigation Steps

To safeguard your systems against this vulnerability, it's highly recommended that administrators first assess whether policy processing is enabled in the OpenSSL installations. If policy processing is disabled (the default setting), the systems are not affected by this vulnerability.

If policy processing is enabled, however, administrators should carefully weigh the risks associated with continued usage and assess whether alternative certificate verification methods might be feasible. Additionally, it is advisable to monitor developments in this area closely, as OpenSSL may release patches or updates addressing this issue in the near future.

In conclusion, CVE-2023-0464 is a critical vulnerability potentially impacting a wide variety of systems using OpenSSL for X.509 certificate chain verification. By understanding the nature of the exploit and taking appropriate precautions, administrators can minimize their risk and help ensure their systems remain secure.


Published on: 03/22/2023 17:15:00 UTC
Last modified on: 03/29/2023 19:37:00 UTC