Re: DNS Reverse Shell sig

[James Lay <jlay () slave-tothe-box net>]

On 2015-02-04 01:29 PM, rmkml wrote:
> Thx James for sharing,
>
> Length is always the same, add 0x3A (dns length) like this ?
> Add "-" for better performance ?
> modify pcre to use relative?
alert udp $HOME_NET any -> any 53 (msg:"Possible Python Reverse DNS 
Shell"; content:"|01 00 00 01 00 00 00 00 00 00 3A|"; depth:11; 
offset:2; fast_pattern; content:"-"; within:1; distance:1; 
pcre:"/^[a-z0-9]{56}/Ri"; 
reference:url,lockboxx.blogspot.com/2015/01/python-reverse-dns-shell.html; 
classtype:bad-unknown; sid:10000150; rev:2;)
> Best Regards
> @Rmkml
>
>
> On Wed, 4 Feb 2015, James Lay wrote:
>
> > In my testing of 
> > http://lockboxx.blogspot.com/2015/01/python-reverse-dns-shell.html I 
> > noticed that during the reverse shell session a semi-constant showed 
> > up...namely a character followed by a dash, followed by 56 other 
> > characters.  Pretty sure this could be changed in the python code, but 
> > this will catch this in it's current form.  It will not fire on each 
> > and every dns query, but will most likely fire at least during the 
> > session.
> >
> > alert udp $HOME_NET any -> any 53 (msg:"Possible Python Reverse DNS 
> > Shell"; content:"|01 00 00 01 00 00 00 00 00 00|"; depth:10; offset:2; 
> > fast_pattern; pcre:"/[a-z]-[a-z0-9]{56}/i"; 
> > reference:url,lockboxx.blogspot.com/2015/01/python-reverse-dns-shell.html; 
> > classtype:bad-unknown; sid:10000150; rev:1;)
> >
> > This WILL most likely FP if you're looking for a domain that matches 
> > the above style, but I couldn't find any such domain in my logs.  
> > Enjoy.
> >
> > James

That's a good idea..would that still catch the initial "[a-zA-Z]-" at 
the start though?  Or just the "-[a-z0-9]{56}"...thanks RM!

James

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