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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How To Break Into Information Security

I’ve been asked a few times recently, usually by operations folk, to give some advice about how to break into the security sector, so under much pain I decided to commit my thoughts on the subject to this web log post. I’ve commented on this subject before and more extensively in chapter 6 of Security De-engineering, but this version is more in line with the times (up to 2012 I was advising a wide pass-by trajectory of planet infosec) and it will be shorter – you have my word(s).

blog-image

First I’d just be wary about trying to get into security just because of financial reasons (David Froud has an excellent blog and one of his posts covered this point well). At the time of writing it is possible to get into the field just by having an IT background and a CISSP. But don’t do that unless you have what’s REALLY required (do not judge what is REALLY required for the field based on job descriptions – at the time of writing, there are still plenty of mistakes being made by organisations). Summarising this in a very brief way:

  • You feel like you have grown out of pure IT-based roles and sort of excelled in whatever IT field you were involved in. You’re the IT professional who doesn’t just clear their problem tickets and switch off. You are, for example, looking for ways to automate things, and self-teach around the subject.
  • Don’t think about getting into security straight from higher education. Whereas it is possible, don’t do it. Just…don’t. Operational Security (or opsec/devopssec) is an option but have some awareness of what this is (scroll down to the end for an explanation).
  • Flexibility: can jump freely from a Cisco switch to an Oracle Database on any Operating System. Taking an example: some IT folk are religious about Unix and experience a mental block when it comes to Windows – this doesn’t work for security. Others have some kind of aversion to Cloud, whereas a better mindset for the field is one that embraces the challenge. Security pros in the “engineer” box should be enthusiastic about the new opportunities for learning offered by extended use of YAML, choosing the ideal federated identity management solution, Puppet, Azure Powershell, and so on. [In theory] projects where on-premise applications are being migrated to Cloud are not [in theory] such a bad place to be in security [in theory].
  • You like coding. Maybe you did some Python or some other scripting. What i’ve noticed is that coding skills are more frequently being seen as requirements. In fact I heard that one organisation went as far as putting candidates through a programming test for a security role. Python, Ruby, Shell ([Li,U]nix) and Powershell are common requirements these days. But even if role descriptions don’t mention coding as a requirement – having these skills demonstrates the kind of flexibility and enthusiasm that go well with infosec. “Regex” comes up a lot but if you’ve done lots of Python/Ruby and/or Unix sed/awk you will be more than familiar with regular expressions.

There is a non-tech element to security (sometimes referred to as “GRC”) but this is something you can get into later. Being aware of international standards and checking to see what’s in a typical corporate security policy is a good idea, but don’t be under the impression that you need to be able to recite verses from these. Generally speaking “writing stuff” and communication is more of a requirement in security than other fields, but you don’t need to be polished at day zero. There are some who see the progression path as Security Analyst –> Security Consultant (Analyst who can communicate effectively).

Another common motivator is hacker conferences or Mr Robot. Infosec isn’t like that. Even the dark side – you see Elliott with a hoody writing code with electronic techno-beats in the background, but hackers don’t write code to compromise networks to any huge degree, if at all. All the code is written for them by others mostly. And as with the femtocell and Raspberry Pi incidents, they usually have to assume a physical presence on the inside, or they are an internal employee themselves, or they dupe someone on the inside of the organisation under attack. Even if you’re in a testing role on the light side, the tests are vastly restricted and there’s a very canned approach to the whole thing with performance KPIs based on reports or something else that doesn’t link to actual intellectual value. Its far from glamorous. There’s an awful lot of misunderstanding out there. What is spoken about at hacker confz is interesting but its not usually stuff that is required to prove the existence of vulnerability in a commercial penetration test – most networks are not particularly well defended, and very little attention is given to results, more so because in most cases the only concern is getting ticks in boxes for an audit – and the auditors are often 12 years old and have never seen a command shell. Quality is rarely a concern.

Its a good practice to build up a list of the more influential bloggers and build up a decent Twitter feed and check what’s happening daily, but also, here are the books that I found most useful in terms of starting out in the field:

  • TCP/IP Illustrated – there are 3 volumes. 1 and 2 are the most useful. Then…
  • Building Internet Firewalls – really a very good way to understand some of the bigger picture ideas behind network architecture design and data flows. I hear rolling of eyes from some sectors, but the same principles apply to Cloud and other “modern” ideas that are from the 90s. With Cloud you have less control over network aspects but network access control and trust relationships are still very much a concern.
  • Network Security Assessment – the earlier versions are also still pertinent unless you will never see a Secure Shell or SMB port (hint: you will).
  • Security Engineering – there’s a very good chapter on Cryptography and Key Management.
  • The Art of Software Security Assessment – whether or not you will be doing appsec for a living you should look at OWASP‘s site and check out Webgoat. They are reportedly looking to bolster their API security coverage, which is nice (a lot of APIs are full of the same holes that were plugged in public apps by the same orgs some years ago). But if you are planning on network penetration testing or application security as a day job, then read this book, its priceless and still very applicable today.
  • The Phoenix Project – a good background illustrative for gaining a better understanding of the landscape in devops.

Also – take a look at perhaps a Windows security standard from the range of CIS benchmarks.

Finally – as i alluded earlier – opsec is not security. Why do i say this? Because i did come across many who believe they made it as a security pro once they joined a SOC/NOC team and then switched off. Security is a holistic function that covers the entire organisation – not just its IT estate, but its people, management, availability and resilience concerns, and processes. As an example – you could be part of a SOC team analysing the alerts generated by a SIEM (BTW some of the best SIEM material online is that written by Dr Anton Chuvakin). This is a very product centric role. So what knowledge is required to architect a SIEM and design its correlation rules? This is security. The same applies to IDS. Responding to alerts and working with the product is opsec. Security is designing the rulebase on an internal node that feeds off a strategically placed network tap. You need to know how hackers work among other areas (see above). Security is a holistic function. A further example: opsec takes the alerts generated by vulnerability management enterprise suites and maybe does some base false positives testing. But how does the organisation respond effectively to a discovered vulnerability? This is security.

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Windows Vulnerability Management: A Cheat Sheet

So its been a long time since my last post. In fact it is almost an entire calendar year. But if we ask our delusional/non-cynical (delete as appropriate) colleagues, we discover that we should try to look to the positive. The last post was on 10th September 2013. So do not focus on the fact that its nearly 1 year since my last post, focus on the 1 week between this post and the 1 year mark. That is quite something! Remember people: positivity is key. 51 weeks instead of 52. All is good.

And to make up for the proportional short-fall of pearls of wisdom, I am hereby making freely available a spreadsheet I developed: its a compendium of information with regard to Windows 2008 Server vulnerabilities. In vulnerability management, the tool (McAfee VM 7.5 in this case, but it could be any tool) produces long lists of vulnerability. Some of these can be false positives. Others will be bona fide. Then, where there is vulnerability, there is risk, and the level of that risk to the business needs to be deduced.

The response of the organisation to the above-mentioned list of vulnerability is usually an IT response – a collaboration between security and operations, or at least it should be. Security typically is not in a position to deduce the risk and / or the remedial actions by themselves. This is where the spreadsheet comes in. All of the information is in one document and the information that is needed to deduce factors such as impact, ease of exploit, risk, false positive, etc…its all there.

Operating system security checklists are as thin on the ground (and in their content) as they are critical in the prevention (and also detection) world. Work in this area is seen as boring and unsexy. It doesn’t involve anything that could get you a place as a speaker at a Black Hat conference. There is nothing in this realm that involves some fanciful breach technique.

Overall, forget perimeter firewalls and anti-virus – operating system security is now the front line in the battle against unauthorised access.

The CIS Benchmarks are quoted by many as a source of operating system configuration security check items and fair play to the folks at CIS for producing these documents.

The columns are as such:

  • “CIS” : the CIS subtitle for the vulnerability
  • “Recommended VM Test”: yes or no, is this a test that is worth doing? (not all of them are worth it, some are outdated, some are just silly)
  • “McAfee Test Available”: yes or no
  • “McAfee Test ID”: numeric code of the test pattern with McAfee VM 7.5
  • “Comments”: summary of the CIS text that describes the vulnerability
  • “Test / Reg Key”: the registry key to check, or other test for the vulnerability
  • “Group Policy”: The GPO value related to the vulnerability if applicable
  • “Further comments”: Some rationale from experience. For example, likelihood of being a false positive, impact, risk, ease of exploit, how to exploit. Generally – more details that can help the Analyst when interfacing with Windows support staff.
  • “McAfee VM test notes”: This is a scratchpad for the Analyst to make notes, as a reference for any other Analysts who may be performing some testing. For example, if the test regularly yields a false positive, note the fact here.
  • “References”: URLs and other material that give some background information. Mostly these are relevant Microsoft Technet articles.

MVM-CIS-Spreadoe

So if anyone would like a copy of this spread sheet, please don’t hesitate to contact me. No – I will not spam you or share your contact details.

 

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