Executive Summary

CVE-2026-9758 is a vulnerability in the certificate validation process of the S2OPC server or client. The issue occurs during the final validation step called the Trust List Check, where the comparison between the peer certificate chain and the trusted certificates list is done incorrectly.

Instead of comparing the entire certificate length, only the first bytes corresponding to the subject length are compared. This partial comparison allows a well-formed untrusted certificate to be considered trusted if it shares the same subject name and certificate length as a trusted certificate, even if the keys differ.

An attacker can exploit this by creating a self-signed certificate with the same subject and length as a trusted certificate but with a different key. Because the comparison uses the subject length rather than the full certificate length, the attacker's certificate can match the trusted one by having identical initial bytes, allowing unauthorized access.

This vulnerability affects both MbedTLS and CycloneCrypto wrapper implementations and has been present since release 1.5.0. The fix involves correcting the comparison to use the full certificate length.

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Compliance Impact

This vulnerability allows an attacker to forge a valid but untrusted certificate that is accepted as trusted by the S2OPC system. Such unauthorized access due to improper certificate validation can lead to compromised confidentiality, integrity, and availability of sensitive data.

As a result, organizations using affected implementations may face challenges in maintaining compliance with common security standards and regulations such as GDPR and HIPAA, which require strong authentication and protection of sensitive data against unauthorized access.

Failure to properly validate certificates and prevent unauthorized access could lead to data breaches or exposure of personal or protected health information, thereby violating these regulations.

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Impact Analysis

This vulnerability can allow an attacker to forge a valid but untrusted certificate that is accepted as trusted by the S2OPC system.

As a result, an attacker could gain unauthorized access to systems or data protected by certificate-based authentication, potentially leading to data breaches, unauthorized actions, or system compromise.

Because the attacker can impersonate trusted entities by exploiting the flawed certificate comparison, the integrity and confidentiality of communications and operations relying on these certificates are at risk.

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Detection Guidance

This vulnerability involves improper certificate validation where an untrusted certificate with the same subject and length as a trusted certificate is accepted. Detection involves inspecting the certificate validation process, especially the comparison logic between peer certificates and the trusted list.

To detect exploitation attempts on your system or network, you should monitor for certificates that have identical subject names and lengths as trusted certificates but differ in their keys.

While no specific commands are provided, you can use standard certificate inspection tools such as OpenSSL to compare certificates. For example, you can extract and compare certificate subjects and lengths using commands like:

• openssl x509 -in <certificate.pem> -noout -subject -serial -fingerprint
• openssl x509 -in <certificate.pem> -noout -text | grep -A1 'Subject:'

Additionally, reviewing logs of the S2OPC server or client for certificate validation errors or unexpected trusted certificate acceptance may help detect this vulnerability.

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Mitigation Strategies

The immediate mitigation step is to update the S2OPC software to a version where the certificate comparison logic has been fixed to use the full certificate length instead of only the subject length.

Until the fix is applied, you should carefully audit and restrict the trusted certificates list to prevent certificates with similar subjects and lengths but different keys from being accepted.

Additionally, monitor for suspicious certificates and consider implementing additional certificate validation checks outside of the vulnerable component.

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