CVE-2023-0464: OpenSSL Certificate Chain DOS Vulnerability

CVE-2023-0464 Overview

A security vulnerability has been identified in all supported versions of OpenSSL related to the verification of X.509 certificate chains that include policy constraints. Attackers may be able to exploit this vulnerability by creating a malicious certificate chain that triggers exponential use of computational resources, leading to a denial-of-service (DoS) attack on affected systems.

Policy processing is disabled by default but can be enabled by passing the -policy argument to the command line utilities or by calling the X509_VERIFY_PARAM_set1_policies() function. This vulnerability affects systems that have explicitly enabled certificate policy verification, making them susceptible to resource exhaustion attacks through specially crafted certificate chains.

Critical Impact

Attackers can cause denial-of-service conditions by triggering exponential computational resource consumption during X.509 certificate chain verification with policy constraints enabled.

Affected Products

• OpenSSL (all supported versions with policy processing enabled)
• Systems using X509_VERIFY_PARAM_set1_policies() function
• Applications using OpenSSL command line utilities with -policy argument

Discovery Timeline

• 2023-03-22 - CVE-2023-0464 published to NVD
• 2025-05-05 - Last updated in NVD database

Technical Details for CVE-2023-0464

Vulnerability Analysis

This vulnerability is classified as CWE-295 (Improper Certificate Validation) and manifests within OpenSSL's X.509 certificate chain verification process when policy constraints are enabled. The flaw allows an attacker to construct a malicious certificate chain that, when processed, triggers exponential growth in computational resource usage.

The attack exploits the way OpenSSL handles policy constraint verification during certificate chain validation. When policy processing is enabled via the -policy command line argument or through the X509_VERIFY_PARAM_set1_policies() API function, the verification algorithm can be forced into an exponential time complexity scenario through carefully crafted certificate chains.

While policy processing is disabled by default, organizations that have enabled this feature for enhanced certificate validation are at risk. The network-based attack vector allows remote attackers to submit malicious certificates to vulnerable services without requiring authentication or user interaction.

Root Cause

The root cause lies in the algorithmic implementation of policy constraint processing within OpenSSL's X.509 certificate chain verification code. The verification algorithm exhibits exponential time complexity when processing certificate chains with certain policy constraint configurations. This algorithmic flaw allows attackers to create certificate chains that force the verification process into computationally intensive operations, consuming excessive CPU resources and potentially causing service unavailability.

Attack Vector

The vulnerability is exploitable over the network without requiring authentication or user interaction. An attacker can craft a malicious X.509 certificate chain containing policy constraints designed to maximize computational overhead during verification.

The attack scenario involves:

1. Identifying a target system that uses OpenSSL with policy processing enabled
2. Crafting a certificate chain with policy constraints that trigger exponential processing
3. Submitting the malicious certificate chain to the target service (e.g., during TLS handshake)
4. The verification process consumes excessive CPU resources, leading to denial of service

The vulnerability specifically targets the policy constraint verification mechanism. For detailed technical information about the vulnerability mechanism and patches, refer to the OpenSSL Security Advisory and the OpenSSL commit changes.

Detection Methods for CVE-2023-0464

Indicators of Compromise

• Unusual CPU spikes during TLS/SSL connection establishment
• Increased latency in certificate verification operations
• Service degradation or unresponsiveness in SSL-enabled applications
• Memory consumption increases during certificate chain processing

Detection Strategies

• Monitor for abnormal CPU utilization patterns in services using OpenSSL for certificate verification
• Implement logging for certificate verification operations and analyze for unusually long processing times
• Use network intrusion detection to identify anomalous certificate chain patterns in TLS handshakes
• Deploy application performance monitoring to detect degradation in SSL/TLS-dependent services

Monitoring Recommendations

• Establish baseline metrics for certificate verification performance and alert on deviations
• Monitor OpenSSL-dependent services for signs of resource exhaustion attacks
• Implement rate limiting on certificate verification operations where feasible
• Review logs for repeated certificate verification failures or timeouts

How to Mitigate CVE-2023-0464

Immediate Actions Required

• Update OpenSSL to the latest patched version addressing CVE-2023-0464
• Disable policy processing if not required by removing -policy arguments from command line utilities
• Audit applications for calls to X509_VERIFY_PARAM_set1_policies() and assess necessity
• Implement resource limits and timeouts for certificate verification operations

Patch Information

OpenSSL has released security patches to address this vulnerability. The fixes are available through the following commits:

• Commit 2017771e2db3e2b96f89bbe8766c3209f6a99545
• Commit 879f7080d7e141f415c79eaa3a8ac4a3dad0348b
• Commit 959c59c7a0164117e7f8366466a32bb1f8d77ff1

For Linux distributions, refer to vendor-specific security advisories including the Debian Security Advisory and Gentoo GLSA Report.

Workarounds

• Disable certificate policy verification if not required for your use case
• Implement connection rate limiting to reduce the impact of potential attacks
• Configure timeouts for certificate verification operations
• Consider using application-level firewalls to filter suspicious certificate submissions

# Configuration example - Verify OpenSSL version and check for policy usage
openssl version -a

# Check if applications are using policy verification (audit command line usage)
grep -r "\-policy" /etc/ssl/ /etc/pki/

# Update OpenSSL on Debian/Ubuntu systems
sudo apt-get update && sudo apt-get upgrade openssl

# Update OpenSSL on RHEL/CentOS systems
sudo yum update openssl