File Browser has incorrect access control for public directory shares via rule path rebasing

Summary

File Browser's public share handlers rebase the share owner's filesystem root to the shared directory and then evaluate descendant paths against the owner's global and per-user rules using the rebased relative path instead of the original path relative to the owner's scope.

As a result, an attacker who knows a public directory share URL can access files and subdirectories that the owner explicitly blocked with rules, as long as those blocked paths are located underneath the shared directory. In the simplest case this is an unauthenticated information disclosure through GET /api/public/share/* and GET /api/public/dl/*.

Details

The public share flow first resolves the original shared path under the owner's filesystem, but then switches d.user.Fs to a new BasePathFs rooted at the shared directory. The follow-up authorization check is still performed by d.Check, which compares the request path to rule strings using prefix matching.

When the share target is a directory, the path passed to d.Check becomes relative to the shared directory, while the rules remain relative to the owner's original scope. A deny rule such as /projects/private therefore no longer matches a public share request for /private/secret.txt, even though the rebased filesystem resolves that request to the real path /projects/private/secret.txt.

Core vulnerable code path:

// http/public.go
if file.IsDir {
    basePath = filepath.Clean(link.Path)
    filePath = ifPath
}

d.user.Fs = afero.NewBasePathFs(d.user.Fs, basePath)

file, err = files.NewFileInfo(&files.FileOptions{
    Fs:      d.user.Fs,
    Path:    filePath,
    Expand:  true,
    Checker: d,
})

// http/data.go and rules/rules.go
func (d *data) Check(path string) bool {
    allow := true
    for _, rule := range d.settings.Rules {
        if rule.Matches(path) {
            allow = rule.Allow
        }
    }
    for _, rule := range d.user.Rules {
        if rule.Matches(path) {
            allow = rule.Allow
        }
    }
    return allow
}

func (r *Rule) Matches(path string) bool {
    if path == r.Path {
        return true
    }
    prefix := r.Path
    if prefix != "/" && !strings.HasSuffix(prefix, "/") {
        prefix += "/"
    }
    return strings.HasPrefix(path, prefix)
}

The issue is reachable from the public endpoints registered in http/http.go for /api/public/share/* and /api/public/dl/*.

PoC

The attacker only needs a directory share URL. No authenticated session is required if the share is not password protected.