boxxy-rs

"If you implement boundaries and nobody is around to push them, do they even exist?". Have you ever wondered how your sandbox looks like from the inside? Tempted to test if you can escape it, if only you had a shell to give it a try? boxxy is a library that can be linked into a debug build of an existing program and drop you into an interactive shell. From there you can step through various stages of your sandbox and verify it actually contains™.

Development

cargo run --example boxxy

Linking with rust

Just put a dev-dependencies in your Cargo.toml and copy examples/boxxy.rs to your examples/ folder. Modify to include your sandbox.

[dev-dependencies]
boxxy = "*"

Linking with C

There is an example program, check the Makefile to see how it's built.

make cboxxy

Calling into machinecode

 [%]> # just RET to prompt
 [%]> jit ww==
 [%]> # print ohai and exit
 [%]> jit 6xpeuAEAAABIice6BQAAAA8FuDwAAABIMf8PBejh////b2hhaQo=

You can use the objdump utility to generate shellcode from assembly:

make sc/ohai && cargo run --example objdump sc/ohai

Debugging systemd security

There is a special ipc binary that automatically swaps its stdio interface with an unix domain socket so it can be used to debug security settings of a systemd unit.

Prepare ipc-boxxy:

cargo build --release --example ipc-boxxy
install -Dm755 target/release/examples/ipc-boxxy /usr/local/bin/ipc-boxxy

Prepare systemd unit:

sudo tee /etc/systemd/system/ipc-boxxy@.service <<EOF
[Unit]
Description=ipc boxxy debugger

[Service]
User=root
ExecStart=/usr/local/bin/ipc-boxxy /run/boxxy-%i.sock

NoNewPrivileges=yes
ProtectSystem=strict
ProtectHome=true
PrivateTmp=true
PrivateDevices=true
ProtectKernelTunables=true
ProtectKernelModules=true
ProtectControlGroups=true
RestrictAddressFamilies=AF_UNIX
MemoryDenyWriteExecute=true
CapabilityBoundingSet=
InaccessiblePaths=-/etc/ssh

EOF

Attach to shell:

sudo nc -Ul /run/boxxy-foo.sock &
sudo systemctl start ipc-boxxy@foo
fg

You can run arbitrary commands with exec, but stdio is still attached to the original process instead of your socket, to fix this setup a new listener in a different terminal:

sudo nc -Ul /run/boxxy-foo2.sock

And attach /bin/sh to this socket with ncat from your boxxy session:

exec ncat -c sh\ -i\ 2>&1 -U /run/boxxy-foo2.sock

Examples

There are vulnerable sandboxes (examples/vuln-*) as a challenge that can be exploited using the boxxy shell (no need to compile any exploits).

DO NOT POST SPOILERS

Start a challenge using eg. cargo run --example vuln-chroot

Warning

The shell is a basic interface for human input, do not write actual scripts, there be dragons.

Do not include boxxy in production builds.

License

This project is free software released under the LGPL3+ license.