Intruder's experts explain the severity of Cisco IOS XE zero-day vulnerability, whether you're affected, and how to mitigate any risk.
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Just how bad is the Cisco IOS XE zero-day vulnerability [CVE-2023-20198]?

Adam Oliver

A critical severity CVSS 10 zero-day vulnerability in Cisco’s IOS XE software has been exploited in the wild. Cisco hasn’t released a patch yet but it has provided some mitigation steps - but before you panic, let’s look at who’s actually affected.

What is the Cisco vulnerability (CVE-2023-20198)?

A security vulnerability is actively being exploited in the web user interface (UI) of Cisco’s IOS XE software which is installed on many Cisco controllers, switches, edge, branch and virtual routers.  

The web UI is an embedded GUI-based tool that can be used to provision, monitor, and troubleshoot the system, build configurations, simplify system deployment and manageability, and enhance the user experience. It’s not supposed to be exposed to the internet or untrusted networks, but the web interface is turned on by default, and you have to jump through hoops to turn it off.

Where the software is accessible from the internet, this gives remote, unauthenticated attackers the chance to create an account with privilege access level 15 on the impacted system. This can then be used to gain unauthorized control over the vulnerable system.

Should I be worried?

CVE-2023-20198 can be combined with a previous vulnerability, CVE-2021-1435, which Cisco patched back in 2021. This required authentication, but it can now be chained with the new vulnerability to bypass authentication so attackers can leverage the older vulnerability to execute system commands, which could then be used to pivot the internal network, if needed.

While this makes the vulnerability more severe if chained, the new vulnerability is still bad news if you have an attacker with level 15 privileges on the web UI, even without the RCE. It could possibly allow them to edit your Cisco configuration by poking holes in your firewall and providing external access to internal hosts. So CVE-2023-20198 is still critical even if it's not chained to the older, medium severity CVE-2021-1435.

What systems are at risk?

The vulnerability only affects Cisco IOS XE software if the web UI feature is enabled, but as the web interface is enabled by default it's definitely worth following Cisco's advisory and patching as soon as it's available, because the web UI may be exposed to internal networks and used for pivoting.

There are just shy of 50k IOS devices exposed to the internet at the moment which shouldn't be there but are – around 8,000 in the US, 1,000 in the UK and 1,000 in Canada – while Censys says there are about 49,000 XE web UIs that are exposed to the internet. This shows just how important attack surface management is – you need to know what’s exposed, so when something like this lands, you can find those assets, and mitigate or patch.

Is there a patch available/should you apply the patch?

As of now, no patch or workaround is currently available from Cisco, but they strongly recommend customers disable the HTTP Server feature on all their internet exposed IOS XE devices as a precaution.

How Intruder is helping

We’re actively monitoring the situation and will provide updates as more information becomes available. Intruder has various powerful scanners under the hood, but some Tenable detections rely on services which aren’t always exposed to the internet, so our Security team are conducting Rapid Response to detect any publicly exposed interfaces for Premium and Vanguard customers.

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

Adam Oliver

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