SOURCE

<div class="box">
    <div class="box2"></div>
    <div class="box3">
      <div class="box4"></div>
    </div>
  </div>

.box {
    display: flex;
    align-items: center;
    flex-direction: column;
    width: 500px;
    height: 500px;
    border: 1px solid red;
}

.box2 {
    width: 500px;
    height: 50px;
    flex-shrink: 0;
    border: 2px solid yellow;
}

.box3 {
    width: 500px;
    flex: 1;
    border: 2px solid green;
    overflow-y: auto;
}

.box4 {
    height: 500px;
}