Multiple security vulnerabilities have been discovered in XStream, a Java library to serialize objects to XML and back again. These vulnerabilities may allow a remote attacker to load and execute arbitrary code from a remote host only by manipulating the processed input stream. XStream itself sets up a whitelist by default now, i.e. it blocks all classes except those types it has explicit converters for. It used to have a blacklist by default, i.e. it tried to block all currently known critical classes of the Java runtime. Main reason for the blacklist were compatibility, it allowed to use newer versions of XStream as drop-in replacement. However, this approach has failed. A growing list of security reports has proven, that a blacklist is inherently unsafe, apart from the fact that types of 3rd libraries were not even considered. A blacklist scenario should be avoided in general, because it provides a false sense of security.