Previously, seq was computed as the result of |pending_commands|+1. This
opened the possibility to override queued commands, as well as logging
the same pending ftp reply multiple times.
For example, when commands 1, 2, 3 are pending, command 1 may be dequeued,
but the incoming command then receives seq 3 and overrides the already
pending command 3. The second scenario happens when ftp_reply() selected
command 3 as pending for logging, but is then followed by many ftp_request()
events. This resulted in command 3's response being logged for every
following ftp_request() over and over again.
Avoid both scenarios by tracking the command sequence as an absolute counter.
OSS-Fuzz generated traffic containing a CWD command with a single very large
path argument (427kb) starting with ".___/` \x00\x00...", This is followed
by a large number of ftp replies with code 250. The directory logic in
ftp_reply() would match every incoming reply with the one pending CWD command,
triggering path buildup ending with something 120MB in size.
Protect from re-using a directory command by setting a flag in the
CmdArg record when it was consumed for the path traversal logic.
This doesn't prevent unbounded path build-up generally, but does prevent the
amplification of a single large command with very many small ftp_replies.
Re-using a pending path command seems like a bug as well.
