[BUG] OpenSSL Security

In general, X.509 RFCs state that all critical extensions must be validated. There is one Microsoft-specific critical extension (OID 1.3.6.1.4.1.311.21.10 -- Microsoft Application Policies) that is used in Microsoft-issued authenticode certificates and is not validated in the OpenSSL implementation.

Moreover, the implementation uses the flag X509_V_FLAG_IGNORE_CRITICAL when verifying signatures. From the OpenSSL docs:

X509_V_FLAG_IGNORE_CRITICAL disabled critical extension checking. By default any unhandled critical extensions in certificates or (if checked) CRLs results in a fatal error. If this flag is set unhandled critical extensions are ignored. WARNING setting this option for anything other than debugging purposes can be a security risk. Finer control over which extensions are supported can be performed in the verification callback.

Start with src/msix/PAL/Signature/OpenSSL/SignatureValidator.cpp, especially the verification path around the X509_V_FLAG_IGNORE_CRITICAL use cited in the issue, and review the relevant OpenSSL certificate-verification behavior. Done means critical extensions are not silently ignored and the Microsoft Application Policies extension is handled during authenticode certificate validation.