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

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 organizations that run critical national infrastructure on-premises in favor 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.

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