Arcane Backend: OS Command Injection in Volume Browser ListDirectory via path query parameter

Summary

GET /environments/{id}/volumes/{volumeName}/browse accepts a path query parameter that is passed to a shell command (sh -c "find … | while …") inside an Arcane helper container. The path sanitiser blocks ../ traversal but does not strip Bourne-shell metacharacters such as $() or backticks, and strconv.Quote only escapes Go string metacharacters, not shell substitution sequences. Any authenticated user with access to a browseable volume can execute arbitrary commands inside the helper container; command output is reflected back in the 500 error body.

Details

The execution flow is:

1. BrowseDirectoryInput.Path (query: path) — backend/internal/huma/handlers/volumes.go:148
2. VolumeHandler.BrowseDirectory calls volumeService.ListDirectory(ctx, volumeName, input.Path) — backend/internal/huma/handlers/volumes.go:858-865. Note the route registration at line 412–419 only declares BearerAuth/ApiKeyAuth; there is no checkAdmin(ctx) call (compare with customize.go, system.go, swarm.go, etc., which do enforce admin).
3. VolumeService.ListDirectory runs the user-supplied path through sanitizeBrowsePathInternal, then joins it under /volume, quotes it with strconv.Quote, and embeds it into a sh -c command:

// backend/internal/services/volume_service.go:286-300
sanitizedPath, err := s.sanitizeBrowsePathInternal(dirPath)
...
targetPath := path.Join("/volume", sanitizedPath)
quotedPath := strconv.Quote(targetPath)
cmd := []string{"sh", "-c", fmt.Sprintf(
    "find %s -mindepth 1 -maxdepth 1 | while IFS= read -r f; do out=$(stat -c \"%%s %%Y %%f %%A\" -- \"$f\" 2>/dev/null) || continue; printf \"%%s\\0%%s\\0\" \"$f\" \"$out\"; done",
    quotedPath)}
stdout, _, err := s.execInContainerInternal(ctx, containerID, cmd)

The sanitiser is insufficient (backend/internal/services/volume_service.go:1448-1467):

func (s *VolumeService) sanitizeBrowsePathInternal(input string) (string, error) {
    trimmed := strings.TrimSpace(input)
    if trimmed == "" || trimmed == "/" { return "/", nil }
    cleaned := path.Clean(trimmed)
    if !path.IsAbs(cleaned) { cleaned = "/" + cleaned }
    if strings.Contains(cleaned, "/../") || strings.HasSuffix(cleaned, "/..") || cleaned == "/.." {
        return "", fmt.Errorf("invalid path: path traversal not allowed")
    }
    if !strings.HasPrefix(cleaned, "/") { return "", fmt.Errorf("invalid path: must be absolute") }
    return cleaned, nil
}

Only ../ patterns are filtered. $(...), backticks, ;, &, |, >, etc. all pass through unchanged. strconv.Quote then wraps the path in Go-style double quotes, which sh -c interprets as a regular double-quoted string — and bash performs $(...) command substitution inside double quotes.

For the input /$( id): - sanitizeBrowsePathInternal returns /$( id) (no ../ present). - path.Join("/volume", "/$( id)") → /volume/$( id). - strconv.Quote(...) → "/volume/$( id)". - The shell runs find "/volume/$( id)" …, which expands to find "/volume/uid=0(root) gid=0(root) groups=0(root)" …. find fails because that path does not exist; the stderr containing the substituted command output is propagated by execInContainerInternal (volume_service.go:910-918) into a command exited with code N: … error, then re-wrapped by ListDirectory and returned to the client as a 500 response body.

Errors from the handler at volumes.go:863-864 are returned via huma.Error500InternalServerError(err.Error()), so the substituted output is reflected in plaintext.

Blast radius / mitigations actually present: - The helper container is created by createTempContainerInternal with NetworkDisabled: true, no privileged mode, no Docker socket mount, only the target Docker volume bind-mounted (:ro for browse). It is auto-removed. - Therefore the injection executes inside an isolated, network-disabled container that already has read access to the same files the browse API exposes. - However: the injection grants arbitrary command execution within that container (well beyond the find/stat/readlink/head primitives the API exposes), enables data exfiltration via error-message side channel, and lets an attacker probe the helper image / volume in ways the legitimate API forbids (e.g. read symlink targets the API explicitly censors at volume_service.go:336-356, read past size limits, etc.). - A non-admin authenticated Arcane user is sufficient (no role check on the volumes browser routes), which makes this a privilege/capability extension for users who otherwise cannot run arbitrary docker exec.

Secondary issue (same sanitiser): DeleteFile (volume_service.go:924-963) defends against deleting volume root with if sanitizedPath == "/". Input path=. yields path.Clean(".") == "." → prefixed to /., which fails the == "/" check, then path.Join("/volume", "/.") == "/volume", so the executed command is rm -rf /volume, recursively deleting all volume contents. This is a separate logic flaw worth fixing alongside the sanitiser hardening but is reported here only for completeness.

Impact

• Authenticated user (any role, including non-admin) can execute arbitrary shell commands inside the per-volume helper container.
• Output of those commands is reflected in HTTP 500 error bodies — usable as an exfiltration channel.
• Attacker gains capabilities the legitimate API withholds: bypass the symlink-target censoring at volume_service.go:336-356, bypass per-file byte limits, enumerate the helper image, mount-time inspection, etc.
• No host compromise: the container has NetworkDisabled: true, no privileged flag, no Docker socket; the volume is bind-mounted read-only for browse. Confidentiality/integrity/availability impact is therefore limited (CVSS C:L / I:L / A:L) but real.
• The same insufficient sanitiser additionally permits a destructive rm -rf /volume by sending path=. to DELETE /environments/{id}/volumes/{volumeName}/browse, which any authenticated user can also reach.