OpenBSD syslogd(8) escapes binary data with vis(3). Use the

OpenBSD syslogd(8) escapes binary data with vis(3).  Use the
VIS_NOSLASH option to avoid additional backslashes.  Although this
option prevents to decode binaries, it makes automatic post processing
easier.  Formats like JSON use backslash escaping themself, additional
escaping from syslogd would break SIEM systems.  vis protection was
introduced to avoid evil characters in log files and not to make
binary logging possible.
from Matthias Pitzl; OK deraadt@