Re: [mod-security-users] Performance woes - larger JSON payloads with CRS
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Re: [mod-security-users] Performance woes - larger JSON payloads with CRS
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From: Henri C. <he...@pr...> - 2021-04-29 12:53:47
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I think what I need here is a way to exempt the request body from any
scanning when it's over 128kb (my chosen upper limit)...
I'll probably try implementing this as a phase 1 rule on Content-Length
unless anyone shouts there's a better way. `ProcessPartial` sounded like
the holy grail, but you can't partial process JSON (because it doesn't
parse!).
It'd be cool to be able to fallback to a fulltext scan of the partial JSON
if it's >128kb, I might have a try at looking into that also (again any
guidance appreciated)
Best Regards,
Henri
On Mon, 26 Apr 2021 at 15:24, Henri Cook <he...@pr...> wrote:
> The follow up problem to this is:
>
> Now i'm set to `SecRequestBodyLimit` 31457280 and
> `SecRequestBodyNoFilesLimit` 65536 and `SecRequestBodyLimitAction
> PartialProcess`
>
> In my mind this will process the first part of a request if it can, and
> ignore the rest. But:
>
> - Rule 200002 is triggered, which is saying the JSON can't be parsed.
> Presumably because in a large request it tries to process the beginning of
> the JSON and can't (because it won't parse, because the JSON is cut off so
> doesn't end)
>
> So I think I need to find a way to skip JSON parsing entirely when the
> payload is over 64kb (65536)? Does that sound right? Assuming 64kb is the
> limit I want to stick with. I hadn't really considered before this point
> that 'partial processing' of JSON was likely to be hairy but of course it
> makes sense.
>
>
>
> On Mon, 26 Apr 2021 at 07:17, Christian Folini <
> chr...@ne...> wrote:
>
>> Hey Henri,
>>
>> From a security practice, this is obviously lacking, but in wider
>> perspective,
>> I see it meet "industry standard", yes.
>>
>> When I teach, I tell my student, that the worst WAF is the one that is
>> switched off. So if you need to compromise and you can only apply 20% of
>> the rules because you run the risk of business demanding it's switched
>> off,
>> then that 20% WAF is still better than no WAF.
>>
>> Cheers,
>>
>> Christian
>>
>>
>> On Mon, Apr 26, 2021 at 06:57:54AM +0100, Henri Cook wrote:
>> > Thanks Christian, taking this in combination with Osama's point earlier
>> in
>> > the thread that most 'big four' (AWS/GCP/Cloudflare/Azure) WAFs seem to
>> > limit the payload they'll scan. From my reading to 128kb
>> (cloudflare+azure)
>> > or 8kb (aws+gcp) I think I'll be able to resolve our particular issue.
>> >
>> > I believe my modern application is already very robust in terms of
>> defence
>> > against sql injection as well as other OWASP top 10 attack vectors and
>> that
>> > a WAF primarily adds reassurance (for the business and clients who ask
>> if I
>> > have one) and minor frustration (for any potential attacker) layer. The
>> > spec is to add a WAF that meets (but notably does not necessarily have
>> to
>> > exceed) industry standards. I believe this means that I can switch
>> modsec
>> > to 128kb or 8kb partial parsing ('SecResponseBodyLimitAction
>> > ProcessPartial' - allowing through unscanned any payloads over those
>> sizes)
>> > and be able to say I've got scan-size-policy-parity with an AWS or a
>> > Cloudflare which means it is "industry standard".
>> >
>> > Please let me know if you think that's mad and thanks again
>> >
>> > Best Regards,
>> >
>> > Henri
>> >
>> > On Sun, 25 Apr 2021 at 21:39, Christian Folini <
>> chr...@ne...>
>> > wrote:
>> >
>> > > Hey Henri,
>> > >
>> > > You are in a bad situation and as far as I can see you are right, you
>> might
>> > > have to drop modsec/CRS in this situation.
>> > >
>> > > I've had a customer with a similar problem and we did a deep dive
>> > > investigation and I had to strike colors in the end.
>> > >
>> > > The point is not the JSON parser. That has shown to be really fast.
>> The
>> > > point
>> > > is several hundred variables that go into CRS afterwards. If you run
>> CRS
>> > > on a
>> > > standard web application you get forms with a few parameters and
>> that's
>> > > easy.
>> > > But several megabytes of JSON means hundreds of arguments and CRS
>> parses
>> > > them
>> > > all.
>> > >
>> > > So we tried to work with rule exclusions and skip the parameters we
>> did not
>> > > think dangerous, but here comes the bummer: ModSec 2.9 grew
>> substantially
>> > > slower the longer the ignore-lists of parameters became. This and a
>> few
>> > > very
>> > > odd behaviors.
>> > >
>> > > Given the customer wanted a generic WAF without tuning of individual
>> APIs
>> > > we
>> > > got to a dead end.
>> > >
>> > > However, if tuning was an option, then I would probably edit-CRS with
>> > > msc_pyparser and replace the target lists with arguments I was
>> interested
>> > > in.
>> > >
>> > > https://coreruleset.org/20200901/introducing-msc_pyparser/
>> > >
>> > > As a complementary practice, one could think of performing allowlist
>> > > checks on
>> > > some / most of the JSON. Say you have a huge JSON payload with 500
>> > > parameters.
>> > > You examine it and discover that 300 of them actually contain simple
>> digits
>> > > and asciii characters and neither special chars nor escape sequences.
>> > > So you do a regex allowlist and apply it to these 300 parameters of
>> said
>> > > API. And the rest you can push into CRS. Or a subset of CRS.
>> > >
>> > > I have not done this and the problem is if ModSec is able to handle
>> the
>> > > large
>> > > target lists in a speedy manner.
>> > >
>> > >
>> > > Now you can turn to a CDN or alternative WAF. I would do an extensive
>> > > security
>> > > tests of such a system. As I said, the JSON parser can be really
>> fast. The
>> > > difficult thing is to check several hundred parameters without losing
>> > > performance.
>> > >
>> > > Good luck!
>> > >
>> > > Christian
>> > >
>> > >
>> > > On Sun, Apr 25, 2021 at 08:47:06PM +0100, Henri Cook wrote:
>> > > > Hi all,
>> > > >
>> > > > I'm in a situation where the only solution seems to be to drop
>> modsec/CRS
>> > > > and look at something like Cloudflare's WAF (and change our security
>> > > model
>> > > > out of necessity). I'm hoping the esteemed membership of this list
>> might
>> > > > have some thoughts.
>> > > >
>> > > > I've got about 1MB of JSON, payloads in our app might run to 20 or
>> even
>> > > > 30MB ultimately.
>> > > > This 1MB of somewhat nested JSON (7 or 8 levels deep) can take 40
>> seconds
>> > > > to process in mod sec 3.0.4 with CRS 3.2.0
>> > > >
>> > > > It takes 1 second to process in our API so the WAF element is a 39x
>> slow
>> > > > down. I appreciate there'll be some delays in WAF. Cloudflare's WAF
>> > > takes 5
>> > > > seconds to scan this payload - and that's my target.
>> > > >
>> > > > Has anyone got any idea how to improve performance? Reading blog
>> posts
>> > > > about the development of cloudflare's waf I see that memoization of
>> > > common
>> > > > function calls was one of their absolute best performance
>> improvements
>> > > over
>> > > > their modsec implementation (e.g. strlen(response_body) so it's only
>> > > > calculated once instead of once per rule OR contains('somestring',
>> > > > response_body)... you get the drift). Do we have anything like this
>> in
>> > > > modsec today? Is that already in place and my 39 seconds is after
>> that?
>> > > >
>> > > > I appreciate that mod sec is fast on its own and adding complex
>> rules can
>> > > > be said to slow it down. With CRS being by far the most common use
>> case
>> > > for
>> > > > mod sec (based on my googling) I'm surprised it's this slow, do you
>> think
>> > > > i've missed something?
>> > > >
>> > > > To note: I'm only scanning JSON payloads, typically much less than
>> 0.5MB
>> > > > but new, irregular ones that we need scanned in ideally <10 seconds
>> that
>> > > > can range from 1MB-30MB
>> > > >
>> > > > Best regards,
>> > > >
>> > > > Henri Cook
>> > >
>> > >
>> > > > _______________________________________________
>> > > > mod-security-users mailing list
>> > > > mod...@li...
>> > > > https://lists.sourceforge.net/lists/listinfo/mod-security-users
>> > > > Commercial ModSecurity Rules and Support from Trustwave's
>> SpiderLabs:
>> > > > http://www.modsecurity.org/projects/commercial/rules/
>> > > > http://www.modsecurity.org/projects/commercial/support/
>> > >
>> > >
>> > >
>> > > _______________________________________________
>> > > mod-security-users mailing list
>> > > mod...@li...
>> > > https://lists.sourceforge.net/lists/listinfo/mod-security-users
>> > > Commercial ModSecurity Rules and Support from Trustwave's SpiderLabs:
>> > > http://www.modsecurity.org/projects/commercial/rules/
>> > > http://www.modsecurity.org/projects/commercial/support/
>> > >
>>
>>
>> > _______________________________________________
>> > mod-security-users mailing list
>> > mod...@li...
>> > https://lists.sourceforge.net/lists/listinfo/mod-security-users
>> > Commercial ModSecurity Rules and Support from Trustwave's SpiderLabs:
>> > http://www.modsecurity.org/projects/commercial/rules/
>> > http://www.modsecurity.org/projects/commercial/support/
>>
>>
>>
>> _______________________________________________
>> mod-security-users mailing list
>> mod...@li...
>> https://lists.sourceforge.net/lists/listinfo/mod-security-users
>> Commercial ModSecurity Rules and Support from Trustwave's SpiderLabs:
>> http://www.modsecurity.org/projects/commercial/rules/
>> http://www.modsecurity.org/projects/commercial/support/
>>
>
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