What is antijack?

antijack was inspired by ttyjack and is its counterpart in some sense, hence the name.

antijack's mission is threefold:

• demo execution of a program in a way where it cannot inject commands via ioctls TIOCSTI and/or TIOCLINUX into the surrounding controlling terminal, e.g. try antijack ttyjack echo nope.
• generate and dump a seccomp syscall filter (a BPF program) that blocks ioctls TIOCSTI and TIOCLINUX into a file for use with e.g. bubblewrap a la bwrap --seccomp 3 [..] 3< <(antijack --dump /dev/stdout).
• demo mitigation at syscall level for Linux leveraging libseccomp. May not be enough!, more on that below.

It should be noted that:

• Alternative options for mitigation include:
  • setsid(2) (or setsid(1)) with drawbacks or
  • use of a PTY.
• With security in mind, we need to ask "why should access to the controlling terminal be granted?" not "why should it be taken away?" Use of a PTY by default is a consequence of that. The fact that TIOCLINUX attacks came to awareness later than TIOCSTI indicates that when the next attack like these will be discovered, those who are blocking single ioctls will have to adjust while those using a PTY may already by protected.
• The defaults for su and sudo are known-vulnerable as of 2023-03-16.
  • For su it takes --pty.
  • For sudo it takes Defaults use_pty in /etc/sudoers.
• Both util-linux and GNU coreutils have reverted their use of libseccomp for mitigation:
  • https://github.com/util-linux/util-linux/commit/23f75093264aae5d58d61016cb1a29d8ebdfa157
  • https://github.com/coreutils/coreutils/commit/f5d7c0842ef7adc2be6e85f9ef66b35ebbbd6a61
• The syscall filter is easy to mis-implement, e.g. see CVE-2019-10063 and/or commit 5f6bd3aa6e6a15f644923afa66fb0068736e2b8d.

Requirements

• C99 compiler
• Linux build and target host
• glibc ≥ 2.32
• GNU make
• libseccomp

How to compile

$ make

Example output (on x86_64)

$ antijack --help
usage: antijack [-v|--verbose] [-o|--dump PATH.bpf] [--] [COMMAND [ARG ..]]
   or: antijack -h|--help

$ antijack -v -- ttyjack echo nope
[*] Initializing libseccomp...
[+]   Done.
[*] Adding rule block TIOCSTI ioctls...
[+]   Done.
[*] Adding rule block TIOCLINUX ioctls...
[+]   Done.
[*] Loading seccomp rules into the kernel...
#
# pseudo filter code start
#
# filter for arch x86_64 (3221225534)
if ($arch == 3221225534)
  # filter for syscall "ioctl" (16) [priority: 65532]
  if ($syscall == 16)
    if ($a1.hi32 & 0x00000000 == 0)
      if ($a1.lo32 & 0xffffffff == 21532)
        action KILL_PROCESS;
      if ($a1.lo32 & 0xffffffff == 21522)
        action KILL_PROCESS;
  # default action
  action ALLOW;
# invalid architecture action
action KILL;
#
# pseudo filter code end
#
[+]   Done.
[*] Releasing libseccomp...
[+]   Done.
[*] Running ttyjack...
Bad system call

$ antijack --dump filter.bpf

$ wc -c < filter.bpf
112

Related CVEs (not mine)

• CVE-2005-4890 for su of util-linux and sudo
• CVE-2006-7098 for Apache
• CVE-2007-1400 for plash
• CVE-2011-1408 for ikiwiki-mass-rebuild of ikiwiki
• CVE-2013-6409 for adequate
• CVE-2016-2568 for pkexec of Polkit
• CVE-2016-2779 for runuser of util-linux
• CVE-2016-2781 for chroot of GNU Coreutils
• CVE-2016-7545 for policycoreutils of SELinux
• CVE-2016-9016 for Firejail
• CVE-2016-10124 for lxc-attach of LXC
• CVE-2016-????? for runcon of GNU Coreutils
• CVE-2017-5226 for bubblewrap (or Flatpak)
• CVE-2019-7303 for snapd of Snap
• CVE-2019-10063 for Flatpak
• CVE-2019-11460 for gnome-desktop of GNOME
• CVE-2019-11461 for Nautilus of GNOME
• CVE-2020-13753 for WebKitGTK
• CVE-2021-????? for Homebrew
• CVE-2023-28100 for Flatpak
• CVE-2023-28339 for OpenDoas
• CVE-2023-????? for doas (except on OpenBSD >=6.2)
• CVE-2023-46277 for pleaser/please
• CVE-2023-????? for ssu
• CVE-2023-????? for sudo-rs

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Sebastian Pipping, Berlin, 2023