CURL-CVE-2018-1000300

curl might overflow a heap based memory buffer when closing down an FTP connection with very long server command replies.

When doing FTP transfers, curl keeps a spare "closure handle" around internally that is used when an FTP connection gets shut down since the original curl easy handle is then already removed.

FTP server response data that gets cached from the original transfer might then be larger than the default buffer size (16 KB) allocated in the "closure handle", which can lead to a buffer overwrite. The contents and size of that overwrite is controllable by the server.

This situation was detected by an assert() in the code, but that was of course only preventing bad stuff in debug builds. This bug is very unlikely to trigger with non-malicious servers.