Re: [mod-security-users] Performance woes - larger JSON payloads with CRS

[Christian F. <chr...@ne...>]

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
> >
> >
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> >
> >
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