CVE-2026-5188: wolfSSL Integer Underflow Vulnerability

CVE-2026-5188 Overview

An integer underflow vulnerability exists in wolfSSL when parsing the Subject Alternative Name (SAN) extension of X.509 certificates. A malformed certificate can specify an entry length larger than the enclosing sequence, causing the internal length counter to wrap during parsing. This results in incorrect handling of certificate data. The issue is limited to configurations using the original ASN.1 parsing implementation, which is off by default.

Critical Impact

Integer underflow in certificate parsing could lead to improper certificate validation, potentially affecting confidentiality or availability in non-default wolfSSL configurations.

Affected Products

• wolfSSL (configurations using original ASN.1 parsing implementation)

Discovery Timeline

• April 10, 2026 - CVE CVE-2026-5188 published to NVD
• April 13, 2026 - Last updated in NVD database

Technical Details for CVE-2026-5188

Vulnerability Analysis

This vulnerability is classified as CWE-191 (Integer Underflow), occurring in wolfSSL's X.509 certificate parsing logic. The flaw manifests when processing the Subject Alternative Name (SAN) extension of certificates. When a malformed certificate specifies an entry length that exceeds the bounds of its enclosing ASN.1 sequence, the internal length counter underflows during parsing operations.

The vulnerability requires network access but involves high attack complexity and depends on user interaction, making exploitation challenging. The impact is limited to potential information disclosure and minor availability degradation. Importantly, this issue only affects wolfSSL deployments that have explicitly enabled the original ASN.1 parsing implementation, which is disabled by default.

Root Cause

The root cause stems from insufficient bounds checking when parsing the length field of SAN entries within X.509 certificates. When the parser encounters a SAN entry with a declared length larger than the remaining bytes in the enclosing sequence, the subtraction operation on the internal length counter results in an integer underflow. This causes the counter to wrap to a large positive value, leading to incorrect parsing behavior and potential mishandling of subsequent certificate data.

Attack Vector

The attack vector involves presenting a specially crafted X.509 certificate to a wolfSSL-enabled application. An attacker would need to:

1. Craft a malformed certificate with a SAN extension containing an oversized length field
2. Present this certificate to a vulnerable wolfSSL instance during TLS handshake or certificate verification
3. The attack requires the target to be using the non-default original ASN.1 parsing implementation

The attack is network-based but requires high complexity due to the non-default configuration requirement and the need for user interaction in the certificate verification process.

Due to the specific nature of this vulnerability, technical details should be referenced from the wolfSSL Pull Request #10024 which contains the authoritative fix and technical context for this integer underflow condition in the ASN.1 parsing logic.

Detection Methods for CVE-2026-5188

Indicators of Compromise

• Unusual certificate parsing errors or failures in wolfSSL logs
• Malformed X.509 certificates with abnormally large SAN entry length fields
• Certificate validation anomalies or unexpected behavior during TLS handshakes

Detection Strategies

• Monitor wolfSSL error logs for ASN.1 parsing failures or length-related errors
• Implement certificate validation monitoring to detect malformed SAN extensions
• Review wolfSSL configuration to determine if the original ASN.1 parser is enabled

Monitoring Recommendations

• Enable verbose logging for certificate parsing operations in wolfSSL
• Deploy network-level monitoring for malformed TLS certificates
• Audit wolfSSL configurations across the environment to identify non-default ASN.1 parser usage

How to Mitigate CVE-2026-5188

Immediate Actions Required

• Verify wolfSSL configuration to confirm whether the original ASN.1 parsing implementation is enabled
• If using the non-default original ASN.1 parser, prioritize applying the security patch
• Consider switching to the default ASN.1 parsing implementation as an immediate mitigation

Patch Information

A fix for this vulnerability is available in the wolfSSL GitHub Pull Request #10024. Organizations using wolfSSL with the original ASN.1 parsing implementation should apply this patch or update to a version containing the fix.

Workarounds

• Disable the original ASN.1 parsing implementation and use the default parser instead
• Implement additional certificate validation at the application layer before passing to wolfSSL
• Restrict certificate sources to trusted Certificate Authorities to reduce exposure to malformed certificates

# Verify wolfSSL configuration
# Check if original ASN.1 parser is enabled in your build configuration
grep -r "WOLFSSL_ASN_ORIGINAL" /path/to/wolfssl/config

# If the original parser is enabled, consider rebuilding without this flag
# or apply the security patch from pull request #10024