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#24 in Debugging

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LGPL-3.0

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boxxy-rs Build Status crates.io docs.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 = "0.*"

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

Invoking from php

See autoboxxy for tooling to load boxxy from php, even if shell_exec and friends are disabled by php.ini.

Static binary

You may need to build a fully static binary, this is possible using the x86_64-unknown-linux-musl target.

cargo build --release --example boxxy --target x86_64-unknown-linux-musl
strip target/x86_64-unknown-linux-musl/release/examples/boxxy

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 target/debug/ipc-listener /run/boxxy-foo.sock 'systemctl start ipc-boxxy@foo'

You can run arbitrary commands with exec:

exec bash -i

AWS lambda

The example folder contains a reimplementation of lambdash, it automatically deploys boxxy as an aws lambda and allows you to execute commands on it. The client supports cross account access, but needs a preconfigured role that the lambda should use. You need to build a static binary first.

cargo run --features=aws --example lambdash -- \
    --assume-role arn:aws:iam::133713371337:role/AdminRole \
    --role arn:aws:iam::133337133337:role/lambda-test-role
    eu-west-1 boxxy

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.

Dependencies

~7.5MB
~161K SLoC