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How bad is the Citrix Bleed vulnerability [CVE-2023-4966]?

Daniel Andrew

Citrix has confirmed reports of a critical vulnerability in some of its products being abused in the wild. CISA and Mandiant identified zero-day exploitation of this vulnerability in the wild back in late August 2023, so how bad is it and why is it making waves now?

What is the Citrix Bleed vulnerability?

The Citrix Bleed [CVE-2023-4966] vulnerability affects Citrix NetScaler ADC and NetScaler Gateway. NetScaler is an application delivery controller that’s widely used to manage remote access, often used by large enterprises and organisations that run critical national infrastructure on-premises in favour of cloud services.

Citrix claims to have more than 400,000 customers worldwide, including 99% of Fortune 100 companies and 98% of Fortune 500 companies, including Microsoft, CNET and eBay. In a blog post, Mandiant suggests victims so far include tech companies, government organizations and professional services companies.

What are the possible implications?

The NetScaler suite of products includes load balancing, firewall and VPN services, so one possible impact is compromised remote access to your private networks. NetScaler responds to certain requests by dumping memory back to the sender, which can contain access tokens for logged in users. The exploit is as bad as whatever you’ve given access to remotely through your NetScaler system. And because they're logged in sessions, MFA won't protect you.  

It’s not yet clear which user sessions are typically included in the memory dump from the information available on the exploit, although the public exploit doesn’t provide a way for an attacker to control which sessions are attacked. This may limit the impact, but since Citrix have rated the weakness as a CVSS 9.4, it’s better to err on the side of caution and assume it could include highly privileged accounts.

How severe is the vulnerability?

NIST rate CVE-2023-4966 as a High 7.5, but Citrix themselves rate it as Critical – and we agree. It does depend on which user sessions get exposed, and what permissions the affected user has.  

While CISA and Mandiant identified zero-day exploitation of this vulnerability in August, researchers at AssetNote looked into the vulnerability and have now developed a reliable exploit. You can see the exploit or test for exposure with their PoC here. The exploit is rather old-school and reminiscent of hacking in 1999 – send 24,812 "a" characters to the target system and receive a user session in reply.

How to fix the vulnerability

Citrix has released a patch for the flaw which you should apply immediately if you’re a NetScaler user. Make sure you fix properly by following the advice to kill all active and persistent sessions, which can be found under 'Recommended next steps', because compromised sessions can persist through a patch.

Further reading

Citrix Knowledge Center

Mandiant’s remediation advice

NIST database and advisories

How Intruder is helping

We’re monitoring the situation and will provide updates if more information becomes available. Our Security team are conducting Rapid Response  to detect vulnerable Citrix servers 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

Daniel Andrew

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