## Solution
CyberChef can be used to base64 decode the string, then BSON deserialize the result:

The result is a string, and a list of integers. Use the list of integers as indexes into the string to get the flag:
```python
>>> indexes = [5,11,0,6,27,18,14,13,26,14,5,26,0,26,1,18,14,13,28]
>>> s = "abcdefghjiklmnopqrstuvwxyz_{}"
>>> flag = ""
>>> for i in indexes:
... flag += s[i]
...
>>> print(flag)
flag{son_of_a_bson}
```

## Flag
**flag{son_of_a_bson}**

Original writeup (https://malcrypt.gitlab.io/blog/ctfs/2021/tenable/misc/not_json/).