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When asking for client authentication, this server sends a list of trusted certificate authorities to the client. The client uses this list to choose a client certificate that is trusted by the server. Currently, this server trusts so many certificate authorities that the list has grown too long. This list has thus been truncated. The administrator of this machine should review the certificate authorities trusted for client authentication and remove those that do not really need to be trusted.

Once I got the error (266) and I have created the below reg key and issue was resolved.
For more info:


  1. Click Start, click Run, type regedit, and then click OK.
  2. Locate and then click the following registry subkey HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL
  3. On the Edit menu, point to New, and then click DWORD Value.
  4. Type SendTrustedIssuerList, and then press ENTER to name the registry entry.
  5. Right-click SendTrustedIssuerList, and then click Modify.
  6. In the Value data box, type 0 if that value is not already displayed, and then click OK.

This problem may occur if the Web server or the IAS server contains many entries in the trusted root certification list. The server sends a list of trusted certificate authorities to the client if the following conditions are true:

  • The server uses the Transport Layer Security (TLS)/SSL protocol to encrypt network traffic.
  • Client certificates are required for authentication during the authentication handshake process.

This list of trusted certificate authorities represents the authorities from which the server can accept a client certificate. To be authenticated by the server, the client must have a certificate that is present in the chain of certificates to a root certificate from the server's list.

Currently, the maximum size of the trusted certificate authorities list that the Schannel security package supports is 12,228 (0x3000) bytes.

Schannel creates the list of trusted certificate authorities by searching the Trusted Root Certification Authorities store on the local computer. Every certificate that is trusted for client authentication purposes is added to the list. If the size of this list exceeds 12,228 bytes, Schannel logs Warning event ID 36855. Then, Schannel truncates the list of trusted root certificates and sends this truncated list to the client computer.

When the client computer receives the truncated list of trusted root certificates, the client computer may not have a certificate that exists in the chain of a trusted certificate issuer. For example, the client computer may have a certificate that corresponds to a trusted root certificate that Schannel truncated from the list of trusted certificate authorities. Therefore, the IAS server cannot authenticate the client.