What Is Your VA Scanner Really Doing?

It’s clear from social media and first hand reports, that the awareness of what VA (Vulnerability Assessment) scanners are really doing in testing scenarios is quite low. So I setup up a test box with Ubuntu 18 and exposed some services which are well known to the hacker community and also still popular in production business use cases: Secure Shell (SSH) and an Apache web service.

This post isn’t an attack on VA products at all. It’s aimed at setting a more healthy expectation, and I will cover a test scenario with a packet sniffer (Wireshark), Nessus Professional, and OpenVAS, that illustrates the point.

I became aware 20 years ago, from validating VA scanner output, that a lot of what VA scanners barf out is alarmist (red flags, CRITICAL [fix NOW!]) and also based purely on guesswork – when the scanner “sees” a service, it grabs a service banner (e.g. “OpenSSH 7.6p1 Ubuntu 4ubuntu0.3”), looks in its database for public disclosed vulnerability with that version, and flags vulnerability if there are any associated CVEs. Contrary to popular belief, there is no actual interaction in the way of further investigating or validating vulnerability. All vulnerability reporting is based on the service banner. So if i change my banner to “hi OpenVAS”, nothing will be reported. And in security, we like to advise hiding product names and versions – this helps with drive-by style automated attacks, in a much more effective way than for example, changing default service ports.

This article then demonstrates the VA scanner behaviour described above and covers developments over the past 20 years (did things improve?) with the two most commonly found scanners: Nessus and OpenVAS, which even if are not used directly, are used indirectly (vendors in this space do not recreate the wheel, they take existing IP – all legal I’m sure – and create their own UI for it). It was fairly well-known that Nessus was the basis of most commercial VAs in the 00s, and it seems unlikely that scenario has changed a great deal.

Test Setup

So if I look at my test box setup I see from port scan results (nmap):

PORT STATE SERVICE VERSION
22/tcp open ssh OpenSSH 7.6p1 Ubuntu 4ubuntu0.3 (Ubuntu Linux; protocol 2.0)
25/tcp open smtp Postfix smtpd
80/tcp open http Apache httpd 2.4.29 ((Ubuntu))
139/tcp open netbios-ssn Samba smbd 3.X - 4.X (workgroup: WORKGROUP)
445/tcp open netbios-ssn Samba smbd 3.X - 4.X (workgroup: WORKGROUP)
3000/tcp open http Apache httpd 2.4.29 ((Ubuntu))
5000/tcp open http Docker Registry (API: 2.0)
8000/tcp open http Apache httpd 2.4.29

So…naughty, naughty. Apache is not so old but still I’d expect to see some CVEs flagged, and I can say the same for the SSH service. Samba is there too in a default format. Samba is Linux’s implementation of MS Windows SMB (Server Message Block) and is full of holes. The Postfix mail service is also quite old, and there’s a Docker API exposed! All this would get an attacker quite excited, and indeed there’s plenty of automated attack scenarios which would work here.

There was also an EOL Phpmyadmin and EOL jQuery wrapped up in the web service.

Developments in Two Decades

So there has been some changes. For want of a better word, there’s now more honesty. In the case of OpenVAS, for vulnerability that involves grabbing a banner and assuming vulnerability based on this, there is a Quality of Detection (QoD) rating, which is set as default at around 70%. This is a kind of probability rating for a finding not being a false positive. Interestingly those findings that involve a banner grab are way down there under 50, and most are no longer flagged as “critical”.

Nessus, for its banner-grabbed vulnerabilities, is more explicit and it is report will state “Note that Nessus has not tested for this issue but has instead relied only on the application’s self-reported version number.”

Even 7 years ago, there would be lots of issues reported for an outdated Apache or SSH service, many of which would be flagged wrongly as CRITICAL, but not necessarily exploitable, and the existance of the vulnerability was based only on a text banner. So these more recent VA versions are an improvement, but its clear the awareness out there of these issues is still quite low. The problem is now – we do want to see if services are downlevel, so please $VENDOR, don’t hide them (more on this later).

First Scan – Banners On Display

So using Wireshark, sniffing HTTP on port 80 (plain text) we have the following…

Wireshark window showing the OpenVAS interaction with the text box target

The packets highlighted in black are the only two of any interest, wherein OpenVAS has used the HTTP GET method to request for “/”, and receives a response where the header shows the product (Apache) and version (2.4.29).

Note the Wireshark filter used (tcp.port == 80 and http). Other than the initial exchange where a banner was grabbed, there was no further interaction. This was the same for Nessus.

What was reported? Well, for OpenVAS, a handful of potential CVEs were reported but I had to lower the QoD to see them! Which is interesting. If anything this is moving the bar too far in the opposite direction. I mean as an owner of this system, I do want to know if i am running old warez!

For Nessus, 6 Apache CVEs were reported with either critical or “high” severity. Overall, I had a similar experience with that of OpenVAS except to even see the Apache issues reported I had to beg the scanner with the following scan configuration setup:

  • Settings –> Assessment –> Override normal accuracy and show potential false alarms
  • Settings –> Assessment –> perform thorough tests
  • Settings –> Advanced –> enable safe checks on (and i also tried the “off” option)
  • Settings –> Advanced –> plugins –> web servers –> enabled. This is the Apache vulnerability section

For the SSH service, OpenVAS reported 3 medium issues which is roughly what i was expecting. Nessus did not report any at all! Answers on a postcard for that one.

Banners Concealed

What was interesting was that the Secure Shell service doesn’t present an option to hide the banner any more, and on investigation, the majority-held community-version of this story is that the banner is needed in some cases.

Apache however did present a banner obfuscation option. For Ubuntu 18 and Apache 2.4.29, this involved:

  • apt install libapache2-mod-security2
  • a2enmod security2
  • edit /etc/apache2/conf-available/security.conf
  • ServerTokens set to “Prod”
  • systemctl restart apache2

This setup results in the following banner for Apache: Apache httpd – so no version number.

The outcome? As expected, all mention of Apache has now ended. Neither OpenVAS or Nessus reported anything to do with Apache of any note.

What DID The Scanners Find?

Just to summarise the findings when the banners were fully on display…it wasn’t a blank slate. There were some findings. Here are the highlights – for OpenVAS:

  • All Critical issues detected were related to PHPMyAdmin, plus one related to jQuery being EOL, but not stating any particular vulnerability. These version numbers are remotely queriable and this is the basis on which these issues were reported.
  • The SSH and Apache issues.
  • Other lower criticality issues were around certificate ciphers.
  • Some CVSS 6, medium issues with Samba – again these are banner-grabbed guesswork findings.

Nessus didn’t report anything outside of what OpenVAS flagged. OpenVAS reported significantly more issues.

It should be said that both scanners did a lot of querying for HTTP application layer issues that could be seen in the packet sniffer output. For example, queries were made for Python/Django settings.py (database password), and other HTTP gotchas.

Unauthenticated Versus Credentialed Testing

With VA Scanners, the picture hasn’t really changed in 20 years. If anything the picture is worse now because the balance with banner-grabbing guesswork has swung too far the other way, and we have to plead with the scanners to tell us about downlevel software versions. This is presumably an effort to reduce the number of false positives, but its not an advisable strategy. It’s perfectly ok to let us know we are running old wares and if we want, we should be able to see the CVEs associated with our listening services, even if many of them are false positives (and I can say from 20 years of network penetration testing, there will be plenty).

With this type of unauthenticated VA scanning though, the real problem has always been false negatives (to the extent that an open Docker API wasn’t flagged as a problem by either scanner), but none of the other commercial tools out there (I have tried a few in recent years) will be in a better position, because there is hard-limit that can be achieved non-locally with no adminstrative authentication credentials.

Both Nessus and OpenVAS allow use of credentialled based testing but its clear this aspect was never a part of the core design. Nessus has expanded its portfolio of credentialed tests but in the time allocated I could not get it to work with SSH public key authentication. In any case, a CIS benchmark approach will always be not-so-great, for reasons outside the scope of this article. We also have to be careful about where authentication credentials are stored. In the case of SSH keys, this means storing a private key, and with some vendors the key will be stored in their cloud somewhere out there.

Conclusion

This post focusses on one major aspect of VA scanning that is grabbing banners and reporting on vulnerability based on the findings from the banner. This is better than nothing but its futility is hopefully illustrated here, and this approach is core to most of what VA scanners do for us.

The market priority has always been towards unauthenticated scanning. Little focus was ever given to credentialed scanning. This has to change because the unauthenticated approach is like trying to diagnose a problem with your car without ever lifting the bonnet/hood, and moreover we could be moving into an era where accreditation bodies mandate credentialed scanning.

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Fintechs and Security – Part Three

  • Prologue – covers the overall challenge at a high level
  • Part One – Recruiting and Interviews
  • Part Two – Threat and Vulnerability Management – Application Security
  • Part Three – Threat and Vulnerability Management – Other Layers
  • Part Four – Logging
  • Part Five – Cryptography and Key Management, and Identity Management
  • Part Six – Trust (network controls, such as firewalls and proxies), and Resilience
Threat and Vulnerability Management (TVM) – Other Layers

This article covers the key principles of vulnerability management for cloud, devops, and devsecops, and herein addresses the challenges faced by fintechs.

The previous post covered TVM from the application security point of view, but what about everything else? Being cloud and “dynamic”, even with Kubernetes and the mythical Immutable Architecture, doesn’t mean you don’t have to worry about the security of the operating systems and many devices in your cloud. The devil loves to hear claims to the effect that devops never SSHs to VM instances. And does SaaS help? Well that depends if SaaS is a good move – more on that later.

Fintechs are focussing on application security, which is good, but not so much in the security of other areas such as containers, IaaS/SaaS VMs, and little thought is ever given to the supply of patches and container images (they need to come from an integral source – preferably not involving pulling from the public Internet, and the patches and images need to be checked for integrity themselves).

And in general with vulnerability assessment (VA), we in infosec are still battling a popular misconception, which after a quarter of a decade is still a popular misconception – and that is the value, or lack of, of unauthenticated scanners such as OpenVAS and Nessus. More on this later.

The Overall Approach

The design process for a TVM capability was covered in Part One. Capabilities are people, process, and technology. They’re not just technology. So the design of TVM is not as follows: stick an OpenVAS VM in a VPC, fill it with target addresses, send the auto-generated report to ops. That is actually how many fintechs see the TVM challenge, or they just see it as being a purely application security show.

So there is a vulnerability reported. Is it a false positive? If not, then what is the risk? And how should the risk be treated? In order to get a view of risk, security professionals with an attack mindset need to know

  • the network layout and data flows – think from the point of view of an attacker – so for example if a front end web micro-service is compromised, what can the attacker can do from there? Can they install recon tools such as a port scanner or sniffer locally and figure out where the back end database is? This is really about “trust relationships”. That widget that routes connections may in itself seem like a device that isn’t worthy of attention, but it routes connections to a database hosting crown jewels…you can see its an important device and its configuration needs some intense scrutiny.
  • the location and sensitivity of critical information assets.
  • The ease and result of an exploit – how easy is it to gain a local shell presence and then what is the impact?

The points above should ideally be covered as part of threat modelling, that is carried out before any TVM capability design is drafted.

if the engineer or analyst or architect has the experience in CTF or simulated attack, they are in a good position to speak confidently about risk.

Types of Tool

I covered appsec tools in part two.

There are two types: unauthenticated and credentialed or authenticated scanners.

Many years ago i was an analyst running VA scans as part of an APAC regional accreditation service. I was using Nessus mostly but some other tools also. To help me filter false positives, I set up a local test box with services like Apache, Sendmail, etc, pointed Nessus at the box, then used Ethereal (now Wireshark) to figure out what the scanner was actually doing.

What became abundantly obvious with most services, is that the scanner wasn’t actually doing anything. It grabs a service banner and then …nothing. tumbleweed

I thought initially there was a problem with my setup but soon eliminated that doubt. There are a few cases where the scanner probes for more information but those automated efforts are somewhat ineffectual and in many cases the test that is run, and then the processing of the result, show a lack of understanding of the vulnerability. A false negative is likely to result, or at best a false positive. The scanner sees a text banner response such as “apache 2.2.14”, looks in its database for public disclosed vulnerability for that version, then barfs it all out as CRITICAL, red colour, etc.

Trying to assess vulnerability of an IaaS VM with unauthenticated VA scanners is like trying to diagnose a problem with your car without ever lifting the hood/bonnet.

So this leads us to credentialed scanners. Unfortunately the main players in the VA space pander to unauthenticated scans. I am not going to name vendors here, but its clear the market is poorly served in the area of credentialed scanning.

It’s really very likely that sooner rather than later, accreditation schemes will mandate credentialed scanning. It is slowly but surely becoming a widespread realisation that unauthenticated scanners are limited to the above-mentioned testing methodology.

So overall, you will have a set of Technical Security Standards for different technologies such as Linux, Cisco IoS, Docker, and some others. There are a variety of tools out there that will get part of the job done with the more popular operating systems and databases. But in order to check compliance to your Technical Security Standards, expect to have to bridge the gap with your own scripting. With SSH this is infinitely feasible. With Windows, it is harder, but check Ansible and how it connects to Windows with Python.

Asset Management

Before you can assess for vulnerability, you need to know what your targets are. Thankfully Cloud comes with fewer technical barriers here. Of course the same political barriers exist as in the on-premise case, but the on-premise case presents many technical barriers in larger organisations.

Google Cloud has a built-in feature, and with AWS, each AWS Service (eg Amazon EC2, Amazon S3) have their own set of API calls and each Region is independent. AWS Config is highly useful here.

SaaS

I covered this issue in more detail in a previous post.

Remember the old times of on-premise? Admins were quite busy managing patches and other aspects of operating systems. There are not too many cases where a server is never accessed by an admin for more than a few weeks. There were incompatibilities and patch installs often came with some banana skins around dependencies.

The idea with SaaS is you hand over your operating systems to the CSP and hope for the best. So no access to SMB, RDP, or SSH. You have no visibility of patches that were installed, or not (!), and you have no idea which OS services are enabled or not. If you ask your friendly CSP for more information here, you will not get a reply, and if you do they will remind you that handed over your 50-million-lines-of-source-code OSes to them.

Here’s an example – one variant of the Conficker virus used the Windows ‘at’ scheduling service to keep itself prevalent. Now cloud providers don’t know if their customers need this or not. So – they verge on the side of danger and assume that they do. They will leave it enabled to start at VM boot up.

Note that also – SaaS instances will be invisible to credentialed VA scanners. The tool won’t be able to connect to SSH/RDP.

I am not suggesting for a moment that SaaS is bad. The cost benefits are clear. But when you moved to cloud, you saved on managing physical data centers. Perhaps consider that also saving on management of operating systems maybe taking it too far.

Patching

Don’t forget patching and look at how you are collecting and distributing patches. I’ve seen some architectures where the patching aspect is the attack vector that presents the highest danger, and there have been cases where malicious code was introduced as a result of poor patching.

The patches need to come from an integral source – this is where DNSSEC can play a part but be aware of its limitations – e.g. update.microsoft.com does not present a ‘dnskey’ Resource Record. Vendors sometimes provide a checksum or PGP cryptogram.

Some vendors do not present any patch integrity checksums at all and will force users to download a tarball. This is far from ideal and a workaround will be critical in most cases.

Redhat has their Satellite Network which will meet most organisations’ requirements.

For cloud, the best approach will usually be to ingress patches to a management VPC/Vnet, and all instances (usually even across differing code maturity level VPCs), can pull from there.

Delta Testing

Doing something like scanning critical networks for changes in advertised listening services is definitely a good idea, if not for detecting hacker shells, then for picking up on unauthorised changes. There is no feasible means to do this manually with nmap, or any other port scanner – the problem is time-outs will be flagged as a delta. Commercial offerings are cheap and allow tracking over long histories, there’s no false positives, and allow you to create your own groups of addresses.

Penetration Testing

There’s ideal state, which for most orgs is going to be something like mature vulnerability management processes (this is vulnerability assessment –> deduce risk with vulnerability –> treat risk –> repeat), and the red team pen test looks for anything you may have missed. Ideally, internal sec teams need to know pretty much everything about their network – every nook and cranny, every switch and firewall config, and then the pen test perhaps tells them things they didn’t already know.

Without these VM processes, you can still pen test but the test will be something like this: you find 40 holes of the 1000 in the sieve. But it’s worse than that, because those 40 holes will be back in 2 years.

There can be other circumstances where the pen test by independent 3rd party makes sense:

  • Compliance requirement.
  • Its better than nothing at all. i.e. you’re not even doing VA scans, let alone credentialed scans.

Wrap-up

  • It’s far from all about application security. This area was covered in part two.
  • Design a TVM capability (people, process, technology), don’t just acquire a technology (Qualys, Rapid 7, Tenable SC. etc), fill it with targets, and that’s it.
  • Use your VA data to formulate risk, then decide how to treat the risk. Repeat. Note that CVSS ratings are not particularly useful here. You need to ascertain risk for your environment, not some theoretical environment.
  • Credentialed scanning is the only solution worth considering, and indeed it’s highly likely that compliance schemes will soon start to mandate credentialed scanning.
  • Use a network delta tester to pick up on hacker shells and unauthorised changes in network services and firewalls.
  • Being dynamic with Kubernetes and microservices has not yet killed your platform risk or the OS in general.
  • SaaS may be a step too far for many, in terms of how much you can outsource.
  • When you SaaS’ify a service, you hand over the OS to a CSP, and also remove it from the scope of your TVM VA credentialed scanning.
  • Penetration testing has a well-defined place in security, which isn’t supposed to be one where it is used to inform security teams about their network! Think compliance, and what ideal state looks like here.

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Fintechs and Security – Part Two

  • Prologue – covers the overall challenge at a high level
  • Part One – Recruiting and Interviews
  • Part Two – Threat and Vulnerability Management – Application Security
  • Part Three – Threat and Vulnerability Management – Other Layers
  • Part Four – Logging
  • Part Five – Cryptography and Key Management, and Identity Management
  • Part Six – Trust (network controls, such as firewalls and proxies), and Resilience

Threat and Vulnerability Management (TVM) – Application Security

This part covers some high-level guider points related to the design of the application security side of TVM (Threat and Vulnerability Management), and the more common pitfalls that plague lots of organisations, not just fintechs. I won’t be covering different tools in the SAST or DAST space apart from one known-good. There are some decent SAST tools out there but none really stand out. The market is ever-changing. When i ask vendors to explain what they mean by [new acronym] what usually results is nothing, or a blast of obfuscation. So I’m not here to talk about specific vendor offerings, especially as the SAST challenge is so hard to get even close to right.

With vulnerability management in general, ${VENDOR} has succeeded in fouling the waters by claiming to be able to automate vulnerability management. This is nonsense. Vulnerability assessment can to some limited degree be automated with decent results, but vulnerability management cannot be automated.

The vulnerability management cycle has also been made more complicated by GRC folk who will present a diagram representing a cycle with 100 steps, when really its just assess –> deduce risk –> treat risk –> GOTO 1. The process is endless, and in the beginning it will be painful, but if handled without redundant theory, acronyms-for-the-sake-of-acronyms-for-the-same-concept-that-already-has-lots-of-acronyms, rebadging older concepts with a new name to make them seem revolutionary, or other common obfuscation techniques, it can be easily integrated as an operational process fairly quickly.

The Dawn Of Application Security

If you go back to the heady days of the late 90s, application security was a thing, it just wasn’t called “application security”. It was called penetration testing. Around the early 2000s, firewall configurations improved to the extent that in a pen test, you would only “see” port 80 and/or 443 exposing a web service on Apache, Internet Information Server, or iPlanet (those were the days – buffer overflow nirvana). So with other attack channels being closed from the perimeter perspective, more scrutiny was given to web-based services.

Attackers realised you can subvert user input by intercepting it with a proxy, modifying some fields, perhaps inject some SQL or HTML, and see output that perhaps you wouldn’t expect to see as part of the business goals of the online service.

At this point the “application security” world was formed and vulnerabilities were classified and given new names. The OWASP Top Ten was born, and the world has never been the same since.

SAST/DAST

More acronyms have been invented by ${VENDOR} since the early early pre-holocene days of appsec, supposedly representing “brand new” concepts such as SAST (Static Application Security Testing) and DAST (Dynamic Application Security Testing), which is the new equivalent of white box and black box testing respectively. The basic difference is about access to the source code. SAST is source code testing while DAST is an approach that will involve testing for OWASP type vulnerabilities while the software is running and accepting client connection requests.

The SAST scene is one that has been adopted by fintechs in more recent times. If you go back 15 years, you would struggle to find any real commercial interest in doing SAST – so if anyone ever tells you they have “20” or even “10” years of SAST experience, suggest they improve their creativity skills. The general feeling, not unjustified, was that for a large, complex application, assessing thousands of lines of source code at a vital organ/day couldn’t be justified.

SAST is more of a common requirement these days. Why is that? The rise of fintechs, whose business is solely about generation of applications, is one side of it, and fintechs can (and do) go bust if they suffer a breach. Also – ${VENDOR}s have responded to the changing Appsec landscape by offering “solutions”. To be fair, the offerings ARE better than 10 years ago, but it wouldn’t take much to better those Hello World scripts. No but seriously, SAST assessment tools are raved about by Gartner and other independent sources, and they ARE better than offerings from the Victorian era, but only in certain refined scenarios and with certain programming languages.

If it was possible to be able to comprehensively assess lots of source code for vulnerability and get accurate results, then theoretically DAST would be harder to justify as a business undertaking. But as it is, SAST + DAST, despite the extensive resources required to do this effectively, can be justified in some cases. In other cases it can be perfectly fine to just go with DAST. It’s unlikely ever going to be ok to just go with SAST because of the scale of the task with complex apps.

Another point here – i see some fintechs using more than one SAST tool, from different vendors. There’s usually not much to gain from this. Some tools are better with some programming languages than others, but there is nothing cast in stone or any kind of majority-view here. The costs of going with multiple vendors is likely going to be harder and harder to justify as time goes on.

Does Automated Vulnerability Assessment Help?

The problem of appsec is still too complex for decent returns from automation. Anyone who has ever done any manual testing for issues such as XSS knows the vast myriad of ways in which such issues can be manifested. The blackbox/blind/DAST scene is still not more than Burp, Dirbuster, but even then its mostly still manual testing with proxies. Don’t expect to cover all OWASP top 10 issues for a complex application that presents an admin plus a user interface, even in a two-week engagement with four analysts.

My preferred approach is still Fred Flinstone’y, but since the automation just isn’t there yet, maybe its the best approach? This needs to happen when an application is still in the conceptual white board architecture design phase, not a fully grown [insert Hipster-given-name], and it goes something like this: white board, application architect – zero in on the areas where data flows involve transactions with untrusted networks or users. Crpyto/key management is another area to zoom in on.

Web Application Firewall

The best thing about WAFs, is they only allow propagation of the most dangerous attacks. But seriously, WAF can help, and in some respects, given the above-mentioned challenges of automating code testing, you need all the help you can get, but you need to spend time teaching the WAF about your expected URL patterns and tuning it – this can be costly. A “dumb” default-configured WAF can probably catch drive-by type issues for public disclosed vulnerabilities as long as you keep it updated. A lot depends on your risk profile, but note that you don’t need a security engineer to install a WAF and leave it in default config. Pretty much anyone can do this. You _do_ need an experienced security engineer or two to properly understand an application and configure a WAF accordingly.

Python and Ruby – Web Application Frameworks

Web application frameworks such as Ruby on Rails (RoR) and Django are in common usage in fintechs, and are at least in some cases, developed with security in mind in that they do offer developers features that are on by default. For example, with Django, if you design a HTML form for user input, the server side will have been automagically configured with the validation on the server side, depending on the model field type. So an email address will be validated client and server-side as an email address. Most OWASP issues are the result of failures to validate user input on the server side.

Note also though that with Django you can still disable HTML tag filtering of user input with a “| safe” in the template. So it’s dangerous to assume that all user input is sanitised.

In Django Templates you will also see a CSRF token as a hidden form field if you include a Form object in your template.

The point here is – the root of all evil in appsec is server-side validation, and much of your server-side validation effort in development will be covered by default if you go with RoR or Django. That is not the end of the story though with appsec and Django/RoR apps. Vulnerability of the host OS and applications can be problematic, and it’s far from the case that use of either Django or RoR as a dev framework eliminates the need for DAST/SAST. However the effort will be significantly reduced as compared to the C/Java/PHP cases.

Wrap-up

Overall i don’t want to too take much time bleating about this topic because the take away is clear – you CAN take steps to address application security assessment automation and include the testing as part of your CI/CD pipeline, but don’t expect to catch all vulnerabilities or even half of what is likely an endless list.

Expect that you will be compromised and plan for it – this is cheaper than spending zillions (e.g. by going with multiple SAST tools as i’ve seen plenty of times) on solving an unsolvable problem – just don’t let an incident result in a costly breach. This is the purpose of security architecture and engineering. It’s more to deal with the consequences of an initial exploit of an application security fail, than to eliminate vulnerability.

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