[Bug] x5c property is empty in JwtHeader after reading a JWT containing x5c in its header

[xatzipe]

Which version of Microsoft.IdentityModel are you using?

System.IdentityModel.Tokens.Jwt 7.2.0

Where is the issue?

• M.IM.JsonWebTokens
• M.IM.KeyVaultExtensions
• M.IM.Logging
• M.IM.ManagedKeyVaultSecurityKey
• M.IM.Protocols
• M.IM.Protocols.OpenIdConnect
• M.IM.Protocols.SignedHttpRequest
• M.IM.Protocols.WsFederation
• M.IM.TestExtensions
• M.IM.Tokens
• M.IM.Tokens.Saml
• M.IM.Validators
• M.IM.Xml
• S.IM.Tokens.Jwt
• Other (please describe)

Is this a new or an existing app?

Repro

        X509Certificate2 cert = CertificateFixture.GetP256Certificate("dummy-cert").Certificate;
        SecurityKey privateSecurityKey = new ECDsaSecurityKey(cert.GetECDsaPrivateKey());
        SigningCredentials signingCredentials = new SigningCredentials(privateSecurityKey, SecurityAlgorithms.EcdsaSha256);
        JwtHeader header = new JwtHeader(signingCredentials);

        X509Chain ch = new X509Chain();
        ch.Build(cert);
        List<string> x5CArray = ch.ChainElements
            .Select(element => Convert.ToBase64String(element.Certificate.Export(X509ContentType.Cert)))
            .ToList();
        header.Add(JwtHeaderParameterNames.X5c, x5CArray);

        var payload = new JwtPayload();

        SecurityToken securityToken = new JwtSecurityToken(header, payload);
        JwtSecurityTokenHandler tokenHandler = new JwtSecurityTokenHandler();
        string jwt = tokenHandler.WriteToken(securityToken);
        
        
        JwtSecurityToken token = tokenHandler.ReadJwtToken(jwt);
        string x5C = token.Header.X5c;
        Assert.False(x5C.IsNullOrEmpty());

Expected behavior
X5c property should return a string which contains a JSON array of the x5c header

Actual behavior
X5c property is null

Possible solution

Until version 6.35.0 x5c property was working property. After that, the internal method GetStandardClaim of the JwtHeader class was modified and stopped working.

In the new version, non string values are not handled and by default string.empty is returned. The value of x5c header at that point is List<string>

Additional context / logs / screenshots / links to code

JwtHeader->GetStandardClaim

Add any other context about the problem here, such as logs and screenshots or links to code.