Snort mailing list archives
ssh preprocessor: incorrect event 'SSH_EVENT_PROTOMISMATCH'
From: Bram <bram-fabeg () mail wizbit be>
Date: Thu, 11 Jul 2013 18:20:12 +0200
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_protomismatchalert ( 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 ( ProcessSSHProtocolVersionExchange( 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 = 16For the 'server version': the flag 'FLAG_STREAM_INSERT' is set in 'packetp->flags' which means the code returns early and does not call 'ProcessSSHProtocolVersionExchange'.
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, 'ProcessSSHProtocolVersionExchange' 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 ---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program.
Attachment:
port_2222_tmp.cap
Description:
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Current thread:
- ssh preprocessor: incorrect event 'SSH_EVENT_PROTOMISMATCH' Bram (Jul 11)
- Re: ssh preprocessor: incorrect event 'SSH_EVENT_PROTOMISMATCH' Victor Roemer (Jul 12)
- Re: ssh preprocessor: incorrect event 'SSH_EVENT_PROTOMISMATCH' Bram (Jul 15)
- Re: ssh preprocessor: incorrect event 'SSH_EVENT_PROTOMISMATCH' Victor Roemer (Jul 16)
- Re: ssh preprocessor: incorrect event 'SSH_EVENT_PROTOMISMATCH' Bram (Jul 15)
- Re: ssh preprocessor: incorrect event 'SSH_EVENT_PROTOMISMATCH' Victor Roemer (Jul 12)