In testing snort 2.9 inline operation logs against OSSEC (3.6.0), I have
found something weird.
This “alert” event gets caught by the decoder:
10/17-21:23:32.374062 [**] [1:10000002:0] /etc/passwd test detected [**]
[Priority: 0] {TCP} 10.1.4.2:59240 -> 10.1.7.2:80
**Phase 1: Completed pre-decoding.
full event: '10/17-21:23:32.374062 [**] [1:10000002:0] /etc/passwd test
detected [**] [Priority: 0] {TCP} 10.1.4.2:59240 -> 10.1.7.2:80'
hostname: 'ossec'
program_name: '(null)'
log: '[**] [1:10000002:0] /etc/passwd test detected [**] [Priority: 0]
{TCP} 10.1.4.2:59240 -> 10.1.7.2:80'
**Phase 2: Completed decoding.
decoder: 'snort'
id: '1:10000002:0'
srcip: '10.1.4.2'
dstip: '10.1.7.2'
**Phase 3: Completed filtering (rules).
Rule id: '20101'
Level: '6'
Description: 'IDS event.'
**Alert to be generated.
This “drop” event, however, does not.
10/17-21:24:32.944406 [Drop] [**] [1:10000002:0] cmd.exe test detected [**]
[Priority: 0] {TCP} 10.1.4.2:59244 -> 10.1.7.2:80
**Phase 1: Completed pre-decoding.
full event: '10/17-21:24:32.944406 [Drop] [**] [1:10000002:0] cmd.exe test
detected [**] [Priority: 0] {TCP} 10.1.4.2:59244 -> 10.1.7.2:80'
hostname: 'ossec'
program_name: '(null)'
log: '[Drop] [**] [1:10000002:0] cmd.exe test detected [**] [Priority: 0]
{TCP} 10.1.4.2:59244 -> 10.1.7.2:80'
**Phase 2: Completed decoding.
No decoder matched.
For your reference here are the Snort decoder lines from
/var/ossec/etc/decoder.xml:
<decoder name="snort">
<program_name_pcre2>^snort</program_name_pcre2>
</decoder>
<decoder name="snort">
<type>ids</type>
<prematch_pcre2>^\[\*\*\] \[\d+:\d+:\d+\] </prematch_pcre2>
</decoder>
<decoder name="snort2">
<parent>snort</parent>
<type>ids</type>
<prematch_pcre2>^\[\*\*\] \[|^\[Drop\] \[\*\*\] \[|^\[</prematch_pcre2>
<pcre2 offset="after_prematch">(\d+:\d+:\d+)\] .+ (\S+?):?\d* ->
([^:]+)</pcre2>
<order>id,srcip,dstip</order>
<fts>name,id,srcip,dstip</fts>
</decoder>
So what’s happening is that the alert event that starts with the timestamp
then [**] [#:########:#] matches the second “snort” decoder and then gets
further tested against the “snort2” decoder.
However, the drop event that starts with the timestamp then [Drop] [**]
[#:#######:#] only matches the “snort2” decoder. This decoder catches
strings that start with [**] OR [Drop] [**] OR [ and then sorts out the ID
and IP info after the prematch. The problem is that since “snort2” declares
“snort” as it’s parent, and the “snort” decoder only catches events which
starts with [**] [#:########:#] only alert events make it to “snort2” for
further decoding.
I have 3 proposed solutions:
1) Remove the “snort” parent requirement from the “snort2” decoder. I don’t
see any value added by making it contingent on a prematch_pcre2 that would
ALSO be caught by the child decoder’s prematch_pcre2 but I may be missing
something. If this is OK, this could also allow for the complete removal of
the second “snort” decoder since both would catch log lines starting with
[**] [#:#######:#].
2) Change the second “snort” decoder’s prematch_pcre2 to: ^\[\*\*\]
\[\d+:\d+:\d+\]|^\[Drop\] \[\*\*\] \[\d+:\d+:\d+\] so it will match both
alert and drop events and allow access to the “snort2” decoder.
3) Add a third “snort” decoder with a prematch_pcre2 of ^\[Drop\] \[\*\*\]
\[\d+:\d+:\d+\]
Does anyone see any superiority between these approaches? Also, how does
one propose a change to the built-in decoder.xml?
Thanks,
Scott
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