CVE-2026-30915: SFTPGo improperly sanitizes placeholders in group home directories/key prefixes

SFTPGo versions before v2.7.1 contain an input validation issue in the handling of dynamic group paths, for example, home directories or key prefixes.

When a group is configured with a dynamic home directory or key prefix using placeholders like %username%, the value replacing the placeholder is not strictly sanitized against relative path components. Consequently, if a user is created with a specially crafted username the resulting path may resolve to a parent directory instead of the intended sub-directory.