CVE-2023-0465: OpenSSL Auth Bypass Vulnerability

CVE-2023-0465 Overview

Applications that use a non-default option when verifying certificates may be vulnerable to an attack from a malicious CA to circumvent certain checks. Invalid certificate policies in leaf certificates are silently ignored by OpenSSL and other certificate policy checks are skipped for that certificate. A malicious CA could use this to deliberately assert invalid certificate policies in order to circumvent policy checking on the certificate altogether.

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 applications that have explicitly enabled certificate policy checking, which is not the default configuration.

Critical Impact
A malicious Certificate Authority can bypass certificate policy validation checks by using invalid certificate policies in leaf certificates, potentially allowing unauthorized certificates to be accepted as valid.

Affected Products

OpenSSL (multiple versions)
Applications using OpenSSL with certificate policy verification enabled
Systems that have explicitly enabled the -policy argument or X509_VERIFY_PARAM_set1_policies() function

Discovery Timeline

March 28, 2023 - CVE-2023-0465 published to NVD
February 18, 2025 - Last updated in NVD database

Technical Details for CVE-2023-0465

Vulnerability Analysis

This vulnerability represents an Improper Certificate Validation flaw (CWE-295) in OpenSSL's certificate policy verification mechanism. When applications enable the non-default policy checking feature, OpenSSL fails to properly handle invalid certificate policies present in leaf certificates. Rather than rejecting certificates with malformed or invalid policies, OpenSSL silently ignores these invalid entries and proceeds to skip subsequent policy checks entirely.

The security impact allows a malicious Certificate Authority to craft certificates containing deliberately invalid policy assertions. When such a certificate is processed by a vulnerable application with policy checking enabled, the entire policy verification process is bypassed, potentially allowing certificates that should be rejected to pass validation.

Root Cause

The root cause lies in OpenSSL's lenient error handling when encountering invalid certificate policies during the verification process. Instead of treating invalid policies as a validation failure, the implementation silently discards them and fails to perform the remaining policy checks. This behavior creates a security gap where a CA can effectively disable policy-based certificate restrictions by including malformed policy data.

The vulnerability specifically affects the certificate chain validation logic when processing X.509 certificate policies. The X509_VERIFY_PARAM_set1_policies() function and the -policy command line argument trigger the vulnerable code path.

Attack Vector

The attack vector is network-based, requiring the attacker to operate or compromise a Certificate Authority. The attacker crafts certificates with deliberately malformed or invalid certificate policy extensions. When a client application with policy verification enabled attempts to validate such a certificate, the invalid policies cause OpenSSL to skip policy verification entirely, allowing the certificate to be accepted despite potentially violating the expected certificate policies.

This attack can be leveraged in man-in-the-middle scenarios or when a malicious CA issues certificates that would otherwise be constrained by policy requirements. The attack requires no user interaction and can be executed remotely over the network.

Detection Methods for CVE-2023-0465

Indicators of Compromise

Certificates in use that contain malformed or unexpected certificate policy extensions
Certificate validation warnings or errors followed by successful connections to suspicious endpoints
Unusual certificate policy OIDs in leaf certificates that don't match expected organizational policies
Evidence of TLS connections established with certificates that should have been rejected by policy checks

Detection Strategies

Audit applications for usage of X509_VERIFY_PARAM_set1_policies() function calls to identify potentially vulnerable code paths
Monitor TLS handshakes for certificates with unusual or invalid policy extensions
Implement certificate transparency log monitoring to detect unexpected certificates issued for your domains
Review OpenSSL version inventories across your infrastructure to identify unpatched systems

Monitoring Recommendations

Deploy network monitoring to analyze X.509 certificate extensions in TLS handshakes
Implement centralized logging for certificate validation events across applications using OpenSSL
Configure alerts for certificate validation anomalies, particularly around policy extension processing
Regularly audit and inventory OpenSSL versions deployed across servers, containers, and application dependencies

How to Mitigate CVE-2023-0465

Immediate Actions Required

Update OpenSSL to the latest patched version as referenced in the OpenSSL Security Advisory March 2023
Identify all applications and services using OpenSSL with certificate policy verification enabled
Review certificate policy configurations and consider whether policy checking is required for your use case
Implement additional certificate validation controls such as certificate pinning where appropriate

Patch Information

OpenSSL has released patches to address this vulnerability. Multiple commits have been made to remediate the issue across different OpenSSL version branches. Administrators should update to the latest available OpenSSL version appropriate for their deployment.

Patch commits are available via the OpenSSL Git repository:

Distribution-specific patches are available via Debian Security Advisory DSA-5417, Debian LTS Security Announcement, and Gentoo GLSA 202402-08.

Workarounds

If policy checking is not required for your application, avoid enabling the -policy command line argument or calling X509_VERIFY_PARAM_set1_policies()
Implement additional certificate validation at the application layer beyond OpenSSL's built-in checks
Use certificate pinning to restrict accepted certificates to known, trusted certificates
Consider implementing certificate transparency monitoring as an additional layer of defense

bash

# Verify OpenSSL version to check if patched
openssl version -a

# Check if applications are using policy verification (example audit command)
grep -r "X509_VERIFY_PARAM_set1_policies" /path/to/source/code/

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

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