dbus before 1.10.28 1.12.x before 1.12.16 and 1.13.x before 1.13.12 as used in DBusServer in Canonical Upstart in Ubuntu 14.04 (and in some less common uses of dbus-daemon) allows cookie spoofing because of symlink mishandling in the reference implementation of DBUS_COOKIE_SHA1 in the libdbus library. (This only affects the DBUS_COOKIE_SHA1 authentication mechanism.) A malicious client with write access to its own home directory could manipulate a ~/.dbus-keyrings symlink to cause a DBusServer with a different uid to read and write in unintended locations. In the worst case this could result in the DBusServer reusing a cookie that is known to the malicious client and treating that cookie as evidence that a subsequent client connection came from an attacker-chosen uid allowing authentication bypass.