CVE-2026-23953

Incus is a system container and virtual machine manager. In versions 6.20.0 and below, 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 newline injection. This can allow adding arbitrary lifecycle hooks, ultimately resulting in arbitrary command execution 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). A fix is planned for versions 6.0.6 and 6.21.0, but they have not been released at the time of publication.

Incus is a system container and virtual machine manager. In versions 6.20.0 and below, 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 newline injection. This can allow adding arbitrary lifecycle hooks, ultimately resulting in arbitrary command execution 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). A fix is planned for versions 6.0.6 and 6.21.0, but they have not been released at the time of publication.

Incus container environment configuration newline injection in github.com/lxc/incus

### 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](https://github.com/user-attachments/files/24473682/environment_newline_injection.sh) [environment_newline_injection.patch](https://github.com/user-attachments/files/24473685/environment_newline_injection.patch)

Incus es un gestor de contenedores de sistema y máquinas virtuales. En las versiones 6.20.0 e inferiores, un usuario con la capacidad de lanzar un contenedor con una configuración YAML personalizada (por ejemplo, un miembro del grupo 'incus') puede crear una variable de entorno que contiene saltos de línea, la cual puede usarse para añadir elementos de configuración adicionales en el lxc.conf del contenedor debido a la inyección de saltos de línea. Esto puede permitir añadir ganchos de ciclo de vida arbitrarios, lo que en última instancia resulta en la ejecución arbitraria de comandos en el anfitrión. Explotar este problema en IncusOS requiere una ligera modificación de la carga útil para cambiar a un directorio diferente con permisos de escritura para el paso de validación (por ejemplo, /tmp). Esto puede confirmarse con un segundo contenedor con /tmp montado desde el anfitrión (Una acción privilegiada solo para validación). Se planea una solución para las versiones 6.0.6 y 6.21.0, pero no han sido lanzadas en el momento de la publicación.