Prevent a bwrap sandbox from injecting commands into the controlling terminal (--new-session and the TIOCSTI problem)
domain: github.com/containers/bubblewrap · 8 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Requirements / auth: No credentials. Requires bubblewrap and unprivileged user namespaces.
Assume the risk is present whenever the sandbox shares your terminal: a process holding the controlling TTY can use the TIOCSTI ioctl to push characters into the terminal's input queue, which the shell outside the sandbox then executes (CVE-2017-5226).
Add `--new-session` to every interactive or TTY-attached bwrap invocation. It calls setsid(), creating a new terminal session and disconnecting the sandbox from the controlling terminal.
If you cannot use `--new-session` because the workload genuinely needs the controlling TTY (job control, an interactive REPL that requires it), block the ioctl instead: upstream states that if you are not filtering out TIOCSTI with seccomp filters, `--new-session` is needed.
Build that seccomp filter with libseccomp, denying `ioctl` with the TIOCSTI request value, export it as compiled cBPF, and pass it with `--seccomp FD`.
Note the tradeoff explicitly in whatever wraps bwrap: `--new-session` means the sandboxed program has no controlling terminal, so Ctrl-C from your shell will not reach it and programs that require a TTY may change behavior.
Pair `--new-session` with `--die-with-parent` so losing interactive control does not leave a sandbox running unattended.
Official documentation: https://github.com/containers/bubblewrap/blob/main/README.md | https://manpages.debian.org/unstable/bubblewrap/bwrap.1.en.html
Known gotchas
This is the single most commonly omitted bwrap flag. A sandbox that is otherwise perfectly locked down is fully escapable through the shared TTY if `--new-session` is missing and TIOCSTI is not filtered.
`--new-session` breaks job control and signal delivery from the launching shell. Scripts that relied on Ctrl-C reaching the sandboxed process need an explicit supervisor instead.
Some programs detect the absence of a controlling terminal and change behavior (disabling colour, refusing to prompt, or exiting). Test the workload with `--new-session` before assuming it is a free addition.
Filtering TIOCSTI is only equivalent protection if your filter covers every path to the ioctl for every descendant process. When in doubt, prefer `--new-session`.
Newer kernels may restrict TIOCSTI independently, but you cannot rely on the host kernel's configuration when writing a portable sandbox — set the flag yourself.
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