There is a well-documented exploit technique which involves writing pointers to custom vtables for the stdio streams, thus hijacking code execution.

By default, we should reject invalid vtables.  For backwards compatibility with really old binaries (from the GCC 2.95 era), we need to provide some way to disable this hardening on demand.

Further background information about the libio ABI is available here:

  https://sourceware.org/glibc/wiki/LibioVtables

This is just additional security hardening (despite existing exploits), so I'm flagging this issue as security-.