flatpak build-init with --sdk-extensions or --base-extensions copies extension files into the build directory. The target path comes from the directory key in the extension point metadata and is resolved using g_file_resolve_relative_path, which resolves .. components. A malicious extension with directory=../../ causes its files to be written outside the build tree.

Before copying, the target path is cleaned with flatpak_rm_rf, so existing files at the traversed path are deleted and replaced with the extension's content.