libcurl: SMTP end-of-response out-of-bounds read - CVE-2019-3823

How it was found

libcurl contains a heap out-of-bounds read in the code handling the
end-of-response for SMTP.

If the buffer passed to `smtp_endofresp()` isn't NUL terminated and contains
no character ending the parsed number, and `len` is set to 5, then the
`strtol()` call reads beyond the allocated buffer. The read contents will not
be returned to the caller.

The issue was reported to the project on 18 January 2019. A patch was sent to me on 19 January 2019. curl 7.64.0 was released on 6 January 2019.

https://curl.haxx.se/docs/CVE-2019-3823.html

Impact

If the buffer passed to smtp_endofresp() isn't NUL terminated and contains no character ending the parsed number, and len is set to 5, then the strtol() call reads beyond the allocated buffer.