piSignage 2.6.4 - Directory Traversal

# Exploit Title: piSignage 2.6.4 - Directory Traversal
# Date: 2019-11-13
# Exploit Author: JunYeong Ko
# Vendor Homepage: https://pisignage.com/
# Version:  piSignage before 2.6.4
# Tested on: piSignage before 2.6.4
# CVE : CVE-2019-20354

Summary:
The web application component of piSignage before 2.6.4 allows a remote attacker (authenticated as a low-privilege user) to download arbitrary files from the Raspberry Pi via api/settings/log?file=../ path traversal. In other words, this issue is in the player API for log download.

PoC:
1. Click the Log Download button at the bottom of the 'piSignage' administration page.
2. HTTP Packet is sent when the button is pressed.
3. Change the value of 'file' parameter to ../../../../../../../../../../etc/passwd.
4. You can see that the /etc/passwd file is read.

References:
https://github.com/colloqi/piSignage/issues/97