This indicates an attack attempt to exploit a Use-After-Free vulnerability in LibreOffice.
The vulnerability is due to improper parsing of a crafted file by the vulnerable application. An attacker can exploit this by tricking an unsuspecting user into opening a malicious crafted file and execute arbitrary code within the context of the current user.
LibreOffice LibreOffice prior to 5.1.4
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the latest update from the vendor.