CVE-2026-23953 — Vulnerability in github.com/lxc/incus/v6 — CVE...

### 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)