In libcurl's base64 encode function, the output buffer is allocated as follows
without any checks on insize:

     malloc( insize * 4 / 3 + 4 )

On systems with 32-bit addresses in userspace (e.g. x86, ARM, x32), the
multiplication in the expression wraps around if insize is at least 1GB of
data. If this happens, an undersized output buffer will be allocated, but the
full result will be written, thus causing the memory behind the output buffer
to be overwritten.

If a username is set directly via `CURLOPT_USERNAME` (or curl's `-u, --user`
option), this vulnerability can be triggered. The name has to be at least
512MB big in a 32bit system.

Systems with 64 bit versions of the `size_t` type are not affected by this
issue.

External References:

https://curl.haxx.se/docs/adv_20161102C.html