The OCI delta stream parser read sizes as guint64 but passed them to GLib I/O and allocation functions expecting gsize (32 bits on 32-bit systems). The silent truncation causes undersized allocations while subsequent operations use the original 64-bit size, leading to heap buffer overflows. An attacker controlling an OCI registry can craft a delta stream that triggers this during flatpak install or flatpak update, potentially achieving code execution in the Flatpak client process.

On 64-bit systems, gsize and guint64 are the same width, so the truncation does not occur and the issue is not practically exploitable.