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SslStream server-side AIA certificate downloads disabled by default

Starting in .NET 11, SslStream doesn't download missing intermediate certificates using the Authority Information Access (AIA) extension by default when validating client certificates as a server.

Version introduced

.NET 11 Preview 3

Previous behavior

Previously, when SslStream validated client certificates as a server, it attempted to download missing intermediate certificates using the AIA extension if the client didn't provide them during the TLS handshake. This behavior occurred even when no custom CertificateChainPolicy was specified.

For example, the following code would attempt to download intermediate certificates via AIA if the client omitted them:

var sslStream = new SslStream(networkStream);

await sslStream.AuthenticateAsServerAsync(new SslServerAuthenticationOptions
{
    ServerCertificate = serverCertificate,
    ClientCertificateRequired = true,
    CertificateRevocationCheckMode = X509RevocationMode.Online
});

New behavior

Starting in .NET 11, SslStream disables AIA certificate downloads when operating as a server that validates client certificates. If the client doesn't provide all required intermediate certificates during the TLS handshake, the server no longer attempts to download them. The handshake fails with a certificate validation error unless the server is configured with the required intermediate certificates.

This change only applies when no custom CertificateChainPolicy is provided. If a custom X509ChainPolicy is specified, its DisableCertificateDownloads value is respected.

Type of breaking change

This change is a behavioral change.

Reason for change

Allowing AIA downloads during the TLS handshake can cause significant performance degradation if the AIA server is slow or unresponsive. Additionally, making outbound HTTP requests to client-provided endpoints introduces potential security risks, since an attacker could influence which external endpoints the server contacts. For more details, see the pull request that introduced this change.

Choose one of the following options:

  • Ensure the client sends all required intermediate certificates: Configure the client to include all intermediate certificates in the TLS handshake. On the client side, use ClientCertificateContext with an SslStreamCertificateContext that includes the full chain. For most scenarios, creating SslStreamCertificateContext handles intermediate certificate management automatically.

  • Provide intermediate certificates in the server's chain policy: Use ExtraStore to supply the necessary intermediate certificates to the server:

    var chainPolicy = new X509ChainPolicy
{
    // Disable AIA downloads (SslStream sets this to true on its internal policy by default in .NET 11;
    // when you supply a custom X509ChainPolicy, set it explicitly to match that behavior)
    DisableCertificateDownloads = true,

    // Add any necessary intermediate certificates
    ExtraStore = { intermediateCertificate },

    // If client certificates are issued by a private root CA, specify custom trust
    TrustMode = X509ChainTrustMode.CustomRootTrust,
    CustomTrustStore = { rootCertificate }
};

var sslStream = new SslStream(networkStream);

await sslStream.AuthenticateAsServerAsync(new SslServerAuthenticationOptions
{
    ServerCertificateContext = serverCertificateContext,
    ClientCertificateRequired = true,
    CertificateChainPolicy = chainPolicy
});

  • Explicitly allow AIA downloads (not recommended): Restore the previous behavior by setting DisableCertificateDownloads to false. This approach is not recommended due to the associated performance and security risks.

Affected APIs

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