CVE-2023-0401: OpenSSL PKCS7 Signature DoS Vulnerability

CVE-2023-0401 Overview

CVE-2023-0401 is a NULL pointer dereference vulnerability in OpenSSL that occurs when signatures are being verified on PKCS7 signed or signedAndEnveloped data. When the hash algorithm used for the signature is known to the OpenSSL library but the implementation of the hash algorithm is not available, the digest initialization fails. A missing check for the return value from the initialization function leads to invalid usage of the digest API, most likely resulting in an application crash.

The unavailability of an algorithm can be caused by using FIPS-enabled configuration of providers or more commonly by not loading the legacy provider. PKCS7 data is processed by the SMIME library calls and also by the time stamp (TS) library calls. While the TLS implementation in OpenSSL does not call these functions, third-party applications are affected if they call these functions to verify signatures on untrusted data.

Critical Impact

Attackers can trigger a denial of service condition by providing specially crafted PKCS7 signed data that causes a NULL pointer dereference crash in applications using vulnerable OpenSSL versions.

Affected Products

• OpenSSL (versions prior to the February 2023 security patch)
• Stormshield Management Center
• Third-party applications using OpenSSL SMIME or TS library functions for signature verification

Discovery Timeline

• 2023-02-08 - CVE CVE-2023-0401 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2023-0401

Vulnerability Analysis

This vulnerability is classified as CWE-476 (NULL Pointer Dereference). The flaw exists in OpenSSL's PKCS7 signature verification code path where the return value from the digest initialization function is not properly checked before subsequent API calls are made.

When an application attempts to verify a PKCS7 signature that uses a hash algorithm that is recognized by OpenSSL but has no available implementation (such as when FIPS mode is enabled or the legacy provider is not loaded), the EVP_DigestInit() or similar function returns an error. However, the calling code proceeds to use the uninitialized digest context, leading to a NULL pointer dereference.

The vulnerability can be exploited remotely over the network without requiring any privileges or user interaction. An attacker can craft malicious PKCS7 data that references an unavailable hash algorithm and send it to an application that uses OpenSSL's SMIME or timestamp verification functions, causing the application to crash.

Root Cause

The root cause is a missing error check on the return value from the digest initialization function in the PKCS7 signature verification code. When the hash algorithm implementation is unavailable (due to FIPS configuration or missing legacy provider), the initialization fails but the code continues execution as if it succeeded. This results in subsequent digest API calls operating on an invalid or NULL digest context.

Attack Vector

The attack is network-based and can be executed remotely. An attacker constructs PKCS7 signed or signedAndEnveloped data that specifies a hash algorithm known to OpenSSL but unavailable in the current configuration. When a vulnerable application processes this data for signature verification, the missing return value check causes the application to crash.

The vulnerability primarily affects applications that:

• Use OpenSSL's SMIME library functions for email signature verification
• Use OpenSSL's timestamp (TS) library functions
• Process untrusted PKCS7 data from external sources

The TLS implementation in OpenSSL is not directly affected as it does not use these specific code paths.

Detection Methods for CVE-2023-0401

Indicators of Compromise

• Unexpected application crashes with stack traces pointing to OpenSSL PKCS7 or SMIME functions
• Core dumps showing NULL pointer access in digest-related OpenSSL functions
• Increased volume of malformed PKCS7 or S/MIME messages targeting the application
• Log entries indicating signature verification failures followed by crashes

Detection Strategies

• Monitor application logs for crashes or segmentation faults in processes using OpenSSL for signature verification
• Implement input validation to detect and reject malformed PKCS7 data before passing it to OpenSSL
• Use application-level crash monitoring to detect patterns of DoS attempts
• Deploy network intrusion detection rules to identify suspicious PKCS7 or S/MIME traffic patterns

Monitoring Recommendations

• Enable crash reporting and analysis for applications using OpenSSL PKCS7/SMIME functions
• Monitor system stability metrics for applications processing untrusted signed data
• Track OpenSSL library versions across your infrastructure to identify vulnerable deployments
• Implement alerting for unusual rates of signature verification failures

How to Mitigate CVE-2023-0401

Immediate Actions Required

• Update OpenSSL to the patched version released in the February 2023 security update
• Review applications that use OpenSSL SMIME or timestamp (TS) library functions for signature verification
• Ensure the legacy provider is loaded if your application requires legacy hash algorithms
• Consider implementing input validation to reject PKCS7 data with unexpected or unsupported algorithms

Patch Information

OpenSSL has released a security patch addressing this vulnerability. The fix adds proper error checking for the return value from the digest initialization function, preventing the NULL pointer dereference.

Refer to the OpenSSL Security Advisory for official patch details. The specific code changes can be reviewed in the OpenSSL Commit Diff.

Additional vendor-specific advisories:

• Gentoo GLSA 202402-08
• SonicWall Vulnerability SNWLID-2023-0003

Workarounds

• Ensure the OpenSSL legacy provider is loaded if your application uses legacy hash algorithms for signature verification
• Implement pre-validation of PKCS7 data to reject messages using unsupported or unavailable hash algorithms before OpenSSL processes them
• Consider restricting PKCS7 signature verification to trusted data sources only until patching is complete
• If FIPS mode is required, ensure all necessary algorithm implementations are available in your FIPS provider configuration

# Example: Loading legacy provider in OpenSSL configuration
# Add to openssl.cnf or application configuration
[openssl_init]
providers = provider_sect

[provider_sect]
default = default_sect
legacy = legacy_sect

[default_sect]
activate = 1

[legacy_sect]
activate = 1