[Snort-devel] ssh preprocessor: incorrect event 'SSH_EVENT_PROTOMISMATCH'

Victor Roemer vroemer at ...402...
Fri Jul 12 14:17:07 EDT 2013


Bram,

Thank you for your bug submission. I have reproduced your scenario and
opened a bug so that we can address the problem.

In the mean time, enabling TCP reassembly on port 2222 would resolve the
problem.


Thanks,
Victor



On Thu, Jul 11, 2013 at 12:20 PM, Bram <bram-fabeg at ...3414...> wrote:

> Hi,
>
>
> The ssh preprocessor produces an incorrect event for
> 'SSH_EVENT_PROTOMISMATCH' on some streams.
> Attached is a cap file which can be used to trigger this.
>
>
> snort configuration:
>
> dynamicpreprocessor directory /usr/lib/snort_**dynamicpreprocessor/
> preprocessor stream5_global: track_tcp yes, \
>    track_udp yes, \
>    track_udp no, \
>    track_icmp no, \
>    max_tcp 262144, \
>    max_udp 131072,
> preprocessor stream5_tcp: policy first, ports client 21
>
> preprocessor dcerpc2: memcap 102400, events [co ]
> preprocessor ssh: server_ports { 22 2222 } \
>                 enable_protomismatch
>
>
> alert ( msg: "SSH_EVENT_PROTOMISMATCH"; sid: 4; gid: 128; rev: 1;
> metadata: rule-type preproc, service ssh ; )
>
> output alert_fast: stdout
>
>
> Running snort:
>
> $ snort -l /var/log -c /etc/ips/snort.conf --daq-dir /lib/daq/ -r
> /tmp/port_2222_tmp.cap  2>&1  | grep -i protocol
>     Protocol Aware Flushing: ACTIVE
>     Protocol Mismatch Alert: ENABLED
> Breakdown by protocol (includes rebuilt packets):
> 07/12-13:02:31.460723  [**] [128:4:1] (spp_ssh) Protocol mismatch [**]
> [Priority: 0] {TCP} 192.168.240.254:39360 -> 192.168.174.134:2222
> 07/12-13:02:31.486612  [**] [128:4:1] (spp_ssh) Protocol mismatch [**]
> [Priority: 0] {TCP} 192.168.240.254:39360 -> 192.168.174.134:2222
> 07/12-13:02:33.632790  [**] [128:4:1] (spp_ssh) Protocol mismatch [**]
> [Priority: 0] {TCP} 192.168.240.254:39360 -> 192.168.174.134:2222
> 07/12-13:02:33.635832  [**] [128:4:1] (spp_ssh) Protocol mismatch [**]
> [Priority: 0] {TCP} 192.168.240.254:39360 -> 192.168.174.134:2222
> 07/12-13:02:33.639342  [**] [128:4:1] (spp_ssh) Protocol mismatch [**]
> [Priority: 0] {TCP} 192.168.240.254:39360 -> 192.168.174.134:2222
> 07/12-13:02:34.860170  [**] [128:4:1] (spp_ssh) Protocol mismatch [**]
> [Priority: 0] {TCP} 192.168.240.254:39360 -> 192.168.174.134:2222
> 07/12-13:02:34.865636  [**] [128:4:1] (spp_ssh) Protocol mismatch [**]
> [Priority: 0] {TCP} 192.168.240.254:39360 -> 192.168.174.134:2222
>
> Analysing this shows that the incorrect event is generated on packets
> after the client version string.
>
> The problem appears to be in spp_ssh.c: ProcessSSH:
>
> spp_ssh.c: 529:
>         /* Make sure this preprocessor should run. */
>         if (( !packetp ) ||
>             ( !packetp->payload ) ||
>             ( !packetp->payload_size ) ||
>         ( !IPH_IS_VALID(packetp) ) ||
>             ( !packetp->tcp_header ) ||
>         /* check if we're waiting on stream reassembly */
>         ( packetp->flags & FLAG_STREAM_INSERT))
>         {
>                 return;
>         }
>
> spp_ssh.c: 660:
>                 if ( (sessp->state_flags & SSH_FLG_BOTH_IDSTRING_SEEN)
>                         != SSH_FLG_BOTH_IDSTRING_SEEN )
>                 {
>                         if ( ProcessSSHProtocolVersionExcha**nge( sessp,
>                                         packetp, direction, known_port ) ==
>                                 SSH_FAILURE )
>                         {
>                                 /*Error processing protovers exchange msg
> */
>                         }
>
>
> Checking it with gdb shows:
>
> Breakpoint 1, ProcessSSH (ipacketp=0xbffff138, contextp=0x0) at
> spp_ssh.c:530
> 530     spp_ssh.c: No such file or directory.
> (gdb) print packetp
> $1 = (SFSnortPacket *) 0xbffff138
> (gdb) print  packetp->flags
> $2 = 16777304
> (gdb) print  packetp->payload
> $3 = (const uint8_t *) 0x902dd72 "SSH-1.99-OpenSSH_5.1p1 Debian-5\n"
> (gdb) print packetp->flags & 0x00000010
> $4 = 16
>
>
> For the 'server version': the flag 'FLAG_STREAM_INSERT' is set in
> 'packetp->flags' which means the code returns early and does not call '**
> ProcessSSHProtocolVersionExcha**nge'.
> Because of this the 'SSH_FLG_SERV_IDSTRING_SEEN' flag is not set.
>
> For the 'client version': the flag 'FLAG_STREAM_INSERT' is not set, '**
> ProcessSSHProtocolVersionExcha**nge' is called,
> 'SSH_FLG_CLIENT_IDSTRING_SEEN' is string
>
> For later packets: 'SSH_FLG_BOTH_IDSTRING_SEEN' is not set, the alert is
> generated.
>
>
> It would appear that ProcesSSH expects to be called when the stream is
> 'reassembled' but this never seen to happen.
> Should it be called again?
>
>
> Best regards,
>
> Bram
>
>
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>
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