In libressl-2.1.3/ssl/t1_lib.c the function ssl_add_clienthello_tlsext adds the TLS extension EllipticCurves to the ClientHello message:

const uint16_t *curves;
[..]
tls1_get_curvelist(s, 0, &curves, &curveslen);

In the function tls1_get_curvelist the default is used:

*pcurves = eccurves_default;  // global
*pcurveslen = sizeof(eccurves_default);  // 56

Later the size of the global eccurves_default in bytes is used to index into the curves array. This leads to an out-of-bounds read access which leaks adjacent memory to the ClientHello message.

for (i = 0; i < curveslen; i++)
  s2n(curves[i], ret);

A pointer to function tls1_new is nearby eccurves_default in my test build. The vulnerability could be used to leak the pointer.

This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.