In wallpaper.c in feh before v2.18.3, if a malicious client pretends to be the E17 window manager, it is possible to trigger an out-of-boundary heap write while receiving an IPC message. An integer overflow leads to a buffer overflow and/or a double free.
Gentoo Linux Security Advisory 201707-8 - A vulnerability in feh might allow remote attackers to execute arbitrary code. Versions less than 2.18.3 are affected.