libcurl contains a buffer overrun flaw in the NTLM authentication code.

The internal function `Curl_ntlm_core_mk_ntlmv2_hash` sums up the lengths of
the user name + password (= SUM) and multiplies the sum by two (= SIZE) to
figure out how large storage to allocate from the heap.

The SUM value is subsequently used to iterate over the input and generate
output into the storage buffer. On systems with a 32 bit `size_t`, the math to
calculate SIZE triggers an integer overflow when the combined lengths of the
user name and password is larger than 2GB (2^31 bytes). This integer overflow
usually causes a very small buffer to actually get allocated instead of the
intended very huge one, making the use of that buffer end up in a buffer
overrun.

This is only an issue on 32 bit systems. It also requires the user and
password fields to use more than 2GB of memory combined, which in itself
should be rare.

- Affected versions: libcurl 7.36.0 to and including 7.56.1
- Not affected versions: libcurl < 7.36.0 and >= 7.57.0