Module: Msf::Exploit::Remote::DCERPC::KerberosAuthentication

Defined in:
lib/msf/core/exploit/remote/dcerpc/kerberos_authentication.rb

Overview

This class implements an override for RubySMB's default authentication method to instead use a kerberos authenticator

Instance Attribute Summary collapse

Instance Method Summary collapse

Instance Attribute Details

#krb_encryptor ⇒ Object

Returns the value of attribute krb_encryptor.



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# File 'lib/msf/core/exploit/remote/dcerpc/kerberos_authentication.rb', line 132

def krb_encryptor
  @krb_encryptor
end

Instance Method Details

#auth_provider_complete_handshake(response, options) ⇒ Object 



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# File 'lib/msf/core/exploit/remote/dcerpc/kerberos_authentication.rb', line 84

def auth_provider_complete_handshake(response, options)
  begin
    @kerberos_authenticator.validate_response!(response.auth_value, accept_incomplete: true)
    gss_api = OpenSSL::ASN1.decode(response.auth_value)
    security_blob = ::RubySMB::Gss.asn1dig(gss_api, 0, 2, 0)&.value
    ap_rep = Rex::Proto::Kerberos::Model::ApRep.decode(security_blob)
    ap_rep_enc_part = ap_rep.decrypt_enc_part(@session_key.value)
  rescue ::Rex::Proto::Kerberos::Model::Error::KerberosDecodingError,
           ::Rex::Proto::Kerberos::Model::Error::KerberosError,
           OpenSSL::ASN1::ASN1Error => e
    raise RubySMB::Dcerpc::Error::BindError, e.message # raise the more context-specific BindError
  end
  server_sequence_number = ap_rep_enc_part.sequence_number
  # Now complete the handshake - see [MS-KILE] 3.4.5.1 - https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-kile/190ab8de-dc42-49cf-bf1b-ea5705b7a087
  response_ap_rep = build_ap_rep(@session_key, server_sequence_number)

  wrapped_ap_rep = OpenSSL::ASN1::ASN1Data.new([
    OpenSSL::ASN1::Sequence.new([
      OpenSSL::ASN1::ASN1Data.new([
        OpenSSL::ASN1::OctetString(response_ap_rep.encode)
      ], 2, :CONTEXT_SPECIFIC)
    ])
  ], 1, :CONTEXT_SPECIFIC).to_der

  alter_ctx = RubySMB::Dcerpc::AlterContext.new(options)
  alter_ctx.pdu_header.call_id = @call_id

  add_auth_verifier(alter_ctx, wrapped_ap_rep)

  send_packet(alter_ctx)

  begin
    dcerpc_response = recv_struct(RubySMB::Dcerpc::AlterContextResp)
  rescue RubySMB::Dcerpc::Error::InvalidPacket
    raise RubySMB::Dcerpc::Error::BindError, e.message # raise the more context-specific BindError
  end

  self.krb_encryptor = @kerberos_authenticator.get_message_encryptor(ap_rep_enc_part.subkey,
                                                                              @client_sequence_number,
                                                                              server_sequence_number)
  # Set the session key value on the parent class - needed for decrypting attribute values in e.g. DRSR
  @session_key = ap_rep_enc_part.subkey.value
end

#auth_provider_decrypt_and_verify(dcerpc_response) ⇒ Boolean

Decrypt the value in dcerpc_response, and validate its signature. This function modifies the request object in-place, and returns whether the signature was valid.

Parameters:

  • dcerpc_response (Response) —

    The Response packet to decrypt and verify in-place

Returns:

  • (Boolean) —

    Is the packet's signature valid?

Raises:

  • ArgumentError If the auth type is not SPNEGO (which ultimately wraps Kerberos)



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# File 'lib/msf/core/exploit/remote/dcerpc/kerberos_authentication.rb', line 41

def auth_provider_decrypt_and_verify(dcerpc_response)
  auth_type = dcerpc_response.sec_trailer.auth_type
  unless [RubySMB::Dcerpc::RPC_C_AUTHN_GSS_NEGOTIATE].include?(auth_type)
    raise ArgumentError, "Unsupported Auth Type: #{dcerpc_response.sec_trailer.auth_type}"
  end
  encrypted_stub = get_response_full_stub(dcerpc_response)
  signature = dcerpc_response.auth_value
  data = signature + encrypted_stub

  begin
    result = self.krb_encryptor.decrypt_and_verify(data)
  rescue Rex::Proto::Kerberos::Model::Error::KerberosError
    return false
  end
  set_decrypted_packet(dcerpc_response, result)

  true
end

#auth_provider_encrypt_and_sign(dcerpc_req) ⇒ Object

Encrypt the value in dcerpc_req, and add a valid signature to the request. This function modifies the request object in-place, and does not return anything.

Parameters:

  • dcerpc_req (Request) —

    The Request object to be encrypted and signed in-place



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# File 'lib/msf/core/exploit/remote/dcerpc/kerberos_authentication.rb', line 25

def auth_provider_encrypt_and_sign(dcerpc_req)
  auth_pad_length = get_auth_padding_length(dcerpc_req.stub.to_binary_s.length)
  plain_stub = dcerpc_req.stub.to_binary_s + "\x00" * auth_pad_length
  emessage, header_length, krb_pad_length = self.krb_encryptor.encrypt_and_increment(plain_stub)

  encrypted_stub = emessage[header_length..-1]
  signature = emessage[0,header_length]
  set_encrypted_packet(dcerpc_req, encrypted_stub, auth_pad_length)
  set_signature_on_packet(dcerpc_req, signature)
end

#auth_provider_init ⇒ Object

Initialize the auth provider using Kerberos

Returns:

  • Serialized message for initializing the auth provider



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# File 'lib/msf/core/exploit/remote/dcerpc/kerberos_authentication.rb', line 15

def auth_provider_init
  kerberos_result = @kerberos_authenticator.authenticate
  @application_key = @session_key = kerberos_result[:session_key]
  @client_sequence_number = kerberos_result[:client_sequence_number]
  kerberos_result[:security_blob]
end

#build_ap_rep(session_key, sequence_number) ⇒ Object 



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# File 'lib/msf/core/exploit/remote/dcerpc/kerberos_authentication.rb', line 60

def build_ap_rep(session_key, sequence_number)
  pvno = Rex::Proto::Kerberos::Model::VERSION
  msg_type = Rex::Proto::Kerberos::Model::AP_REP
  ctime = Time.now.utc
  cusec = ctime&.usec

  encrypted_part = Rex::Proto::Kerberos::Model::EncApRepPart.new(
    ctime: ctime,
    cusec: cusec,
    sequence_number: sequence_number,
    enc_key_usage: Rex::Proto::Kerberos::Crypto::KeyUsage::AP_REP_ENCPART
  )
  enc_aprep = Rex::Proto::Kerberos::Model::EncryptedData.new(
    etype: session_key.type,
    cipher: encrypted_part.encrypt(session_key.type, session_key.value)
  )

  Rex::Proto::Kerberos::Model::ApRep.new(
    pvno: pvno,
    msg_type: msg_type,
    enc_part: enc_aprep
  )
end

#get_auth_padding_length(plaintext_len) ⇒ Object 



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# File 'lib/msf/core/exploit/remote/dcerpc/kerberos_authentication.rb', line 128

def get_auth_padding_length(plaintext_len)
  (16 - (self.krb_encryptor.calculate_encrypted_length(plaintext_len) % 16)) % 16
end

#kerberos_authenticator=(kerberos_authenticator) ⇒ Object

Parameters:



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# File 'lib/msf/core/exploit/remote/dcerpc/kerberos_authentication.rb', line 9

def kerberos_authenticator=(kerberos_authenticator)
  @kerberos_authenticator = kerberos_authenticator
end