The OpenBSD qsort() function is recursive, and not randomized, an attacker can construct a pathological input array of N elements that causes qsort() to deterministically recurse N/4 times. This allows attackers to consume arbitrary amounts of stack memory and manipulate stack memory to assist in arbitrary code execution attacks. This affects OpenBSD 6.1 and possibly earlier versions.
http://www.securityfocus.com/bid/99177
http://www.securitytracker.com/id/1039427
https://support.apple.com/HT208112
https://support.apple.com/HT208113
https://support.apple.com/HT208115
https://support.apple.com/HT208144
https://www.exploit-db.com/exploits/42271/
https://www.qualys.com/2017/06/19/stack-clash/stack-clash.txt
Source: MITRE
Published: 2017-06-19
Updated: 2017-10-24
Type: CWE-400
Base Score: 6.4
Vector: AV:N/AC:L/Au:N/C:N/I:P/A:P
Impact Score: 4.9
Exploitability Score: 10
Severity: MEDIUM
Base Score: 6.5
Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L
Impact Score: 2.5
Exploitability Score: 3.9
Severity: MEDIUM