CVE-2022-27510: Citrix ADC/Gateway — Authentication Bypass

Background

Citrix ADC (also known as NetScaler ADC) and Citrix Gateway are perimeter products used by enterprises for application delivery, load balancing, and remote access. Citrix Gateway is essentially an enterprise VPN — it’s how remote workers access internal applications, and it’s deployed at the network edge of countless organisations.

CVE-2022-27510 is one of three critical vulnerabilities Citrix patched in November 2022 in the same advisory. This one is an authentication bypass that affects deployments configured as a VPN gateway or with the ICA proxy feature enabled. Given that most Citrix Gateway deployments exist specifically to provide VPN or application proxy access, the majority of real-world deployments are vulnerable when exposed to the internet.

Technical Mechanism

The vulnerability exists in the way Citrix ADC/Gateway handles certain URL patterns when the appliance is configured as an access gateway. Specific URL paths bypass the authentication mechanism entirely, allowing unauthenticated access to gateway functionality.

The exact mechanism involves path traversal in how the authentication filter processes incoming requests. Certain crafted request paths cause the authentication check to be skipped while still routing to authenticated-only handler code. This gives an unauthenticated attacker access to user capabilities that should require valid gateway credentials.

In configurations where ICA proxy is enabled (common in Citrix Virtual Apps/Desktops deployments), the bypass may allow an attacker to:

1. Enumerate valid usernames through error message differentiation
2. Obtain pre-authentication tokens
3. In some configurations, directly access internal resources that should be gated behind VPN login

When combined with the other vulnerabilities from the same advisory (CVE-2022-27513 and CVE-2022-27516), the impact is substantially higher — though CVE-2022-27510 is exploitable on its own.

Real-World Exploitation Evidence

Exploitation of this vulnerability was observed in the wild following public disclosure. Notable activity:

• Ransomware affiliates — multiple ransomware groups used Citrix gateway vulnerabilities for initial access throughout 2022-2023; CVE-2022-27510 was observed in post-incident reports
• Initial access brokers — compromised Citrix gateways were actively traded on underground forums
• Chinese APT groups — Citrix devices are commonly targeted by Chinese threat actors for long-term persistent access, consistent with patterns seen in CVE-2019-19781 (BlueKeep of Citrix) exploitation

The UK NCSC and US CISA both flagged this in advisories about perimeter device security, noting a pattern of initial access through Citrix devices leading to prolonged network compromise.

Impact Assessment

• Unauthorised gateway access — bypassing authentication on a VPN gateway can expose all internal applications and resources the gateway provides access to
• Credential bypass — in some configurations, no password is required to access internal network resources
• Session token theft — gateway access may expose authenticated session tokens that can be replayed
• Internal network reconnaissance — once inside the gateway, attackers can enumerate and target internal systems
• Ransomware deployment pathway — initial access via Citrix leads to domain compromise and ransomware in documented cases
• Regulatory exposure — healthcare and financial services organisations using Citrix to protect regulated data face significant compliance risk from this vulnerability

Affected Versions

Note: Only affects appliances configured as VPN (Gateway) or with AAA-TM virtual server with authentication enabled.

Remediation Steps

1. Update to patched versions immediately — verify the build version via the Citrix ADC management interface under System > System Information
2. To check current version via CLI:

   show version

3. If patching is not immediately possible, consider temporarily disabling public internet access to the gateway — only feasible if there’s an alternative access method
4. Verify whether your deployment is in a vulnerable configuration (VPN/Gateway or AAA-TM mode):

   show vpn vserver
   show authentication vserver

5. Review Citrix ADC access logs for unusual authentication patterns or access from unexpected IP ranges
6. After patching, audit active sessions and revoke any suspicious sessions
7. Change all gateway service account passwords and review user accounts for any additions

Detection Guidance

Citrix ADC logs — located at /var/log/ns.log and via the syslog facility. Look for:

• Authentication bypass patterns: successful access without corresponding auth log entries
• Unusual URL patterns in access logs, particularly paths containing traversal sequences

SIEM rules — correlate gateway access logs with authentication events. Sessions that access internal resources without a preceding successful authentication event are suspicious.

Network monitoring — monitor outbound connections from internal systems to external IPs, particularly those initiated shortly after gateway access events.

Suricata signature:

alert http any any -> $HTTP_SERVERS any (msg:"Citrix ADC CVE-2022-27510 Authentication Bypass Attempt"; flow:established,to_server; http.uri; content:"/vpn/"; classtype:attempted-recon; sid:2034007; rev:1;)

Timeline