Looking at a sample build log
(https://buildd.debian.org/status/fetch.php?pkg=libreoffice&arch=m68k&ver=1%3A5.4.1-1&stamp=1504466495&raw=0)
one can see:

| ... analyzing package list ...
| ... creating log file /tmp/LibreOffice//logging/en-US/log_540_en-US.log
| ... creating installation set in /tmp/LibreOffice//install/LibreOffice_5.4.1.2.0_Linux ...
| ... removing old installation directories ...

What looks like a predictable /tmp path turns out to be one:

https://lists.freedesktop.org/archives/libreoffice/2017-August/078249.html

Another local user may use this vulnerability to gain privileges of a
user who is building libreoffice from source. I did not request a CVE
for this issue.

Helmut

JFTR, we don't treat these as security issues from jessie onwards since kernel
hardening renders these non-exploitable:
https://www.debian.org/releases/jessie/amd64/release-notes/ch-whats-new.en.html#security

Cheers,
        Moritz

Dear submitter,

as the package libreoffice has just been removed from the Debian archive
experimental we hereby close the associated bug reports.  We are sorry
that we couldn't deal with your issue properly.

For details on the removal, please see https://bugs.debian.org/1069123

The version of this package that was in Debian prior to this removal
can still be found using https://snapshot.debian.org/.

Please note that the changes have been done on the master archive and
will not propagate to any mirrors until the next dinstall run at the
earliest.

This message was generated automatically; if you believe that there is
a problem with it please contact the archive administrators by mailing
ftpmaster@ftp-master.debian.org.

Debian distribution maintenance software
pp.
Scott Kitterman (the ftpmaster behind the curtain)

Humanitarian Grant of 1.5M for you. Reply for claims