Yesterday (May 14th 2019), Microsoft published a security advisory bulletin for a critical vulnerability in its remote login service…
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Critical RDP Flaw Leaves up to 2.3 Million Servers Exposed (CVE-2019–0708)

Daniel Andrew

Yesterday Microsoft published a security advisory bulletin for a critical vulnerability in its remote login service ‘Remote Desktop Services’, commonly known as RDP. The flaw allows a completely unauthenticated attacker to gain full access to the system, so in vulnerability terms, it’s about as bad as it gets.

RDP is popular among sysadmins as a way of remotely logging into Windows machines for admin purposes, and can often be used to gain access to corporate resources for remote workers.

Shodan shows the RDP login screens available to the whole internet

Because of this, RDP is very commonly exposed to the internet, and over 2.3 million servers expose this service worldwide. Windows servers running certain versions of Windows 7, and Windows 2008 are vulnerable, and these operating systems are still widely in use, and are supported by Microsoft. Out-of-support operating systems Windows XP and Windows 2003 are also affected.

At the time of writing, there is no publicly available exploit code, so we’re unlikely to be seeing mass exploitation just yet. However, Microsoft did patch this issue after seeing some exploitation in the wild, so we know at least targeted attacks are already taking place, and it is likely to be only a matter of time before the patch is reversed to uncover the details, and exploit code becomes more widely available.

The infamous ‘WannaCry’ attack back in 2017 used a similar bug in a Microsoft’s ‘SMB’ service, and led to the compromise of over 200,000 systems worldwide, impacting over 150 countries and costing approximately $4 billion in financial losses.

Exposed Remote Desktop services worldwide

Due to its similarities, this vulnerability is highly likely to be leveraged in the same way. It will probably only be a matter of time until news of the next worldwide attack breaks, again locking tens of thousands of corporate machines with ransomware.

Since 2015, Intruder has been recommending to its clients that both SMB and RDP services should not be exposed to the Internet, and should instead be placed behind a VPN for defence-in-depth purposes. While this may not be the time to say “I told you so”, it is an important reminder that vulnerabilities can be discovered in internet-facing services at any time, and rather than playing whack-a-mole by patching each vulnerability as it occurs, keeping your attack surface to a minimum is a much more prudent approach for the long-term.

For those who don’t already know, Intruder’s continuous vulnerability monitoring service lets you know which ports and services you have exposed to the Internet, detects vulnerabilities, and makes recommendations to reduce your organisation’s attack surface. So if you haven’t tried it already… then maybe today is a good day to start!

Release Date
Level of Ideal
Before CVE details are published
Limited public information is available about the vulnerability.

Red teamers, security researchers, detection engineers, threat actors have to actively research type of vulnerability, location in vulnerable software and build an associated exploit.

Tenable release checks for 47.43% of the CVEs they cover in this window, and Greenbone release 32.96%.
Day of CVE publish
Vulnerability information is publicly accessible.

Red teamers, security researchers, detection engineers and threat actors now have access to some of the information they were previously having to hunt themselves, speeding up potential exploit creation.

Tenable release checks for 17.12% of the CVEs they cover in this window, and Greenbone release 17.69%.
First week since CVE publish
Vulnerability information has been publicly available for up to 1 week.

The likelihood that exploitation in the wild is going to be happening is steadily increasing.

Tenable release checks for 10.9% of the CVEs they cover in this window, and Greenbone release 20.69%.
Between 1 week and 1 month since CVE publish
Vulnerability information has been publicly available for up to 1 month, and some very clever people have had time to craft an exploit.

We’re starting to lose some of the benefit of rapid, automated vulnerability detection.

Tenable release checks for 9.58% of the CVEs they cover in this window, and Greenbone release 12.43%.
After 1 month since CVE publish
Information has been publicly available for more than 31 days.

Any detection released a month after the details are publicly available is decreasing in value for me.

Tenable release checks for 14.97% of the CVEs they cover over a month after the CVE details have been published, and Greenbone release 16.23%.

With this information in mind, I wanted to check what is the delay for both Tenable and Greenbone to release a detection for their scanners. The following section will focus on vulnerabilities which:

These are the ones where an attacker can point their exploit code at your vulnerable system and gain unauthorised access.

We’ve seen previously that Tenable have remote checks for 643 critical vulnerabilities, and OpenVAS have remote checks for 450 critical vulnerabilities. Tenable release remote checks for critical vulnerabilities within 1 month of the details being made public 58.4% of the time, but Greenbone release their checks within 1 month 76.8% of the time. So, even though OpenVAS has fewer checks for those critical vulnerabilities, you are more likely to get them within 1 month of the details being made public. Let’s break that down further.

In Figure 10 we can see the absolute number of remote checks released on a given day after a CVE for a critical vulnerability has been published. What you can immediately see is that both Tenable and OpenVAS release the majority of their checks on or before the CVE details are made public; Tenable have released checks for 247 CVEs, and OpenVAS have released checks for 144 CVEs. Then since 2010 Tenable have remote released checks for 147 critical CVEs and OpenVAS 79 critical CVEs on the same day as the vulnerability details were published. The number of vulnerabilities then drops off across the first week and drops further after 1 week, as we would hope for in an efficient time-to-release scenario.

Figure 10: Absolute numbers of critical CVEs with a remote check release date from the date a CVE is published

While raw numbers are good, Tenable have a larger number of checks available so it could be unfair to go on raw numbers alone. It’s potentially more important to understand the likelihood that OpenVAS or Tenable will release a check of a vulnerability on any given day after a CVE for a critical vulnerability is released. In Figure 11 we can see that Tenable release 61% their checks on or before the date that a CVE is published, and OpenVAS release a shade under 50% of their checks on or before the day that a CVE is published.

Figure 11: Percentage chance of delay for critical vulnerabilities

So, since 2010 Tenable has more frequently released their checks before or on the same day as the CVE details have been published for critical vulnerabilities. While Tenable is leading at this point, Greenbone’s community feed still gets a considerable percentage of their checks out on or before day 0.

I thought I’d go another step further and try and see if I could identify any trend in each organisations release delay, are they getting better year-on-year or are their releases getting later? In Figure 12 I’ve taken the mean delay for critical vulnerabilities per year and plotted them. The mean as a metric is particularly influenced by outliers in a data set, so I expected some wackiness and limited the mean to only checks released 180 days prior to a CVE being published and 31 days after a CVE being published. These seem to me like reasonable limits, as anything greater than 6 months prior to CVE details being released is potentially a quirk of the check details and anything after a 1-month delay is less important for us.

What can we take away from Figure 12?

Figure 12: Release delay year-on-year (lower is better)

With the larger number of checks, and still being able to release a greater percentage of their remote checks for critical vulnerabilities Tenable could win this category. However, the delay time from 2019 and 2020 going to OpenVAS, and the trend lines being so close, I am going to declare this one a tie. It’s a tie.

The takeaway from this is that both vendors are getting their checks out the majority of the time either before the CVE details are published or on the day the details are published. This is overwhelmingly positive for both scanning solutions. Over time both also appear to be releasing remote checks for critical vulnerabilities more quickly.

Written by

Daniel Andrew

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