The QNAP TS-239 Pro and TS-639 Pro with firmware 2.1.7 0613, 3.1.0 0627, and 3.1.1 0815 use the rand library function to generate a certain recovery key, which makes it easier for local users to determine this key via a brute-force attack.
Exploitability
AV:LAC:LPR:LUI:NScope
S:UImpact
C:HI:NA:N5.5/CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N