Hello,

The tempfile library does not check the prefix argument, which can be exploited to create files outside tmpdir by using directory traversal.

```
>>> import tempfile
>>> tempfile.gettempprefix()
'tmp'
>>> f = tempfile.NamedTemporaryFile(prefix="/home/mame/cracked")
>>> f.name
'/home/mame/crackedlt3y_ddm'
```

The same issue was found and treated as a vulnerability in PHP (CVE-2006-1494) and Ruby (CVE-2018-6914).

I first reported this issue to security@python.org at July 2018.  Some people kindly discussed it, and finally I was told to create a ticket here.