This has been found out [1] coming from a bug in libpng older than 1.6.19, 1.5.24, 1.4.17, 1.2.54 and 1.0.64 (CVE-2015-8126).
DS [1] https://sourceforge.net/p/png-mng/mailman/png-mng-implement/thread/CA%2BPdXcuhLXJ89s6qjOEcm%2B99eWLmPBFcYSwcwJkajkLrNRLTeQ%40mail.gmail.com/#msg34581085 -- 4096R/DF5182C8 (sten...@debian.org) http://www.danielstender.com/