Cause: libio vtable validation is a security hardening technique that aims to make it harder for attackers to injection libio vtables and functions pointers after exploiting a memory corruption vulnerability. However, some old i386 applications do supply their own vtables, so the validation needs to be disabled, and the original implementation did not do this for interposed stdio handles. Consequence: Some old applications would fail with an error message, `Fatal error: glibc detected an invalid stdio handle`. Fix: glibc now detects interposition of stdio handle data structures and disables vtable validation in that case. Result: Old applications which were impacted by this defect work again.