[Snort-devel] Snort 3 - Custom file magic definitions

Y M snort at outlook.com
Tue Jun 12 13:30:33 EDT 2018


Thank you, Michael, and pardon my lack of Lua workings.

I tested the line provided and I got an odd behavior.  Running against a pcap containing a PDF transfer, for each PDF alert, the LNK alert was also generated. Running against a pcap with LNK file transfer , no alerts at all. Errors no longer show up.

I then tried each of the below commented lines separately, and the behavior was the same.

custom_file_magic.lua:

custom_file_magic =
{
    { type = "LNK", id = 1000, category = "Windows Shell Link Shortcut", rev = 1,
      magic = { { content = "| 4C 00 00 00 01 14 02 00 |",offset = 0 } } }
}

snort.lua:
..
dofile(conf_dir .. '/file_magic.lua')
dofile(conf_dir .. '/custom_file_magic.lua')

--for k,v in ipairs(custom_file_magic) do file_magic[k] = v end
--for k,v in ipairs(custom_file_magic) do table.insert(file_magic, v) end
--for i=1,#custom_file_magic do file_magic[#file_magic+1] = custom_file_magic[i] end
..

I then wrote a Lua script (thanks to the internet) to verify the merge. The script and output are attached, which shows that the tables are, well, merged. If this is an expected Lua behavior, then you can ignore this message and I will continue to dig into this. I can provide the pcaps I am testing with if they provide any help.

Thanks.
YM


________________________________
From: Snort-devel <snort-devel-bounces at lists.snort.org> on behalf of Michael Altizer via Snort-devel <snort-devel at lists.snort.org>
Sent: Monday, June 11, 2018 7:38 AM
To: snort-devel at lists.snort.org
Subject: Re: [Snort-devel] Snort 3 - Custom file magic definitions

On 06/10/2018 01:02 PM, Y M via Snort-devel wrote:
Hi,

What would be the best way to adding custom file magic definitions without altering the original file file_magic.lua?

Creating a custom file and including it in snort.lua overrides the original file_magic.lua, resulting in an error parsing rules that use file types from the original file_magic.lua file. This maybe a lua artifact as I understand it.

custom_file_magic.lua:
file_magic =
{
    { type = "LNK", id = 1000, category = "Windows Shell Link Shortcut", rev = 1,
      magic = { { content = "| 4C 00 00 00 01 14 02 00 |",offset = 0 } } }
}

snort.lua:
...
dofile(conf_dir .. '/file_magic.lua')
dofile(conf_dir .. '/custom_file_magic.lua')
...

local.rules:
alert file (msg:"PDF file in transit"; file_type:PDF; sid:9000000)
alert file (msg:"LNK file in transit"; file_type:LNK; sid:9000001)

Output:
...
ERROR: /usr/local/snort/etc/snort/../../rules/test.rules:1 Invalid file_type type 'PDF'. Not found in file_rules.
ERROR: /usr/local/snort/etc/snort/../../rules/test.rules:1 invalid argument file_type: = PDF
...

Otherwise, detection works as expected.

10/13-13:55:36.104000 [**] [1:9000001:0] "LNK file in transit" [**] [Priority: 0] [AppID: HTTP] {TCP} 10.10.10.2:80 -> 192.168.0.1:32641

Thanks.
YM

Simplest way to add a single element to the file_magic Lua table would probably be something like this sometime after file_magic.lua has been included:

file_magic[#file_magic+1] = { type = "LNK", id = 1000, category = "Windows Shell Link Shortcut", rev = 1,
      magic = { { content = "| 4C 00 00 00 01 14 02 00 |", offset = 0 } } }

If you want to add a lot of them, I'd probably make a separate table of them and then write a tiny bit of Lua to merge the tables.
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