Incus container environment configuration newline injection

Summary

A user with the ability to launch a container with a custom YAML configuration (e.g a member of the ‘incus’ group) can create an environment variable containing newlines, which can be used to add additional configuration items in the container’s lxc.conf due to the newline injection. This can allow adding arbitrary lifecycle hooks, ultimately resulting in arbitrary command execution on the host.

Details

When passing environment variables in the config block of a new container, values are not checked for the presence of newlines [1], which can result in newline injection inside the generated container lxc.conf. This can be used to set arbitrary additional configuration items, such as lxc.hook.pre-start. By exploiting this, a user with the ability to launch a container with an arbitrary config can achieve arbitrary command execution as root on the host.

Exploiting this issue on IncusOS requires a slight modification of the payload to change to a different writable directory for the validation step (e.g /tmp). This can be confirmed with a second container with /tmp mounted from the host (A privileged action for validation only).

[1] https://github.com/lxc/incus/blob/HEAD/internal/server/instance/drivers/driver_lxc.go#L1081

PoC

A proof-of-concept script exploiting this vulnerability can be found attached, named environment_newline_injection.sh, showing arbitrary command execution, which will write a file to the root filesystem (/newline_injection_command_exec_poc)

Manual Reproduction steps:

1. Launch a new container with a configuration file containing a multiline YAML string as an environment variable value, such as in the listing below.
2. Observe that the lxc.conf (/run/incus/user-1000_poc/lxc.conf in my case) contains an additional lxc.hook.pre-start item
3. Observe the creation of the file in the host root directory, with contents proving command execution as root.

incus launch images:alpine/edge --ephemeral poc << EOF
config:
  environment.FOO: |-
    abc
    lxc.hook.pre-start = /bin/sh -c "id > /newline_injection_command_exec_poc"
EOF

Impact

A user with the ability to launch a container with a custom YAML configuration (e.g a member of the ‘incus’ group) can achieve arbitrary command execution on the host.

Attachments

environment_newline_injection.sh environment_newline_injection.patch