diff --git a/libfreerdp/core/gateway/rpc_client.c b/libfreerdp/core/gateway/rpc_client.c
index 7edd2bfb0..d4432cbbd 100644
--- a/libfreerdp/core/gateway/rpc_client.c
+++ b/libfreerdp/core/gateway/rpc_client.c
@@ -982,7 +982,7 @@ BOOL rpc_client_write_call(rdpRpc* rpc, wStream* s, UINT16 opnum)
 	rpc_offset_pad(&offset, request_pdu.auth_verifier.auth_pad_length);
 	CopyMemory(&buffer[offset], &request_pdu.auth_verifier.auth_type, 8);
 	offset += 8;
-	Buffers[0].BufferType = SECBUFFER_DATA;  /* auth_data */
+	Buffers[0].BufferType = SECBUFFER_DATA | SECBUFFER_READONLY; /* auth_data */
 	Buffers[1].BufferType = SECBUFFER_TOKEN; /* signature */
 	Buffers[0].pvBuffer = buffer;
 	Buffers[0].cbBuffer = offset;