Citrix ADC and Gateway Directory Traversal Remote Code Execution (RCE) Vulnerability (CVE-2019-19781) Security Advisory
Executive & SecOps Summary
SECOPS ASSESSMENTCitrix Application Delivery Controller (ADC) and Gateway versions 10.5, 11.1, 12.0, 12.1, and 13.0
Unauthenticated remote attackers can exploit directory traversal to execute arbitrary code and gain full control of the target system
Immediately apply official Citrix security patches, or implement temporary HTTP/HTTPS request blocking policies (Responder Policies) prior to patching
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Upgrade to the latest secure firmware version provided by Citrix. Below is the standard procedure to upgrade the firmware via the CLI.
# 1. Access the Citrix appliance via SSH and enter the shell
shell
# 2. Extract the downloaded firmware package (example path)
tar -xvzf build_tgz_file.tgz -C /var/nsinstall/
# 3. Run the installation script to perform the upgrade
cd /var/nsinstall/
./installns
# 4. Reboot the system after installation completes
reboot2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible, apply the following Responder Policy via the Citrix CLI to block malicious directory traversal requests containing '/vpns/'.
# 1. Create a Responder Policy to detect and drop directory traversal attempts
add responder policy app_cve_2019_19781_bypass 'HTTP.REQ.URL.DECODE_USING_TEXT_MODE.CONTAINS("/vpns/") && (HTTP.REQ.URL.DECODE_USING_TEXT_MODE.CONTAINS("/../") || HTTP.REQ.URL.DECODE_USING_TEXT_MODE.CONTAINS("/\\../"))' DROP
# 2. Bind the policy globally to intercept incoming requests
bind responder global app_cve_2019_19781_bypass 1 -type REQ_OVERRIDE
# 3. Save the configuration permanently
save ns config3. Log Audit & IoC Verification Commands
Use the following shell script to inspect web server logs (/var/log/httpd.access_log) for directory traversal attempts and Perl script execution indicators.
#!/bin/sh
echo '[+] Checking Citrix HTTP Access Logs for CVE-2019-19781 Exploitation...'
# 1. Search for directory traversal patterns targeting '/vpns/'
grep -E '/vpns/.*\.\./' /var/log/httpd.access_log*
# 2. Search for unauthorized execution of 'newbm.pl'
grep 'newbm.pl' /var/log/httpd.access_log*
# 3. Check for recently modified or suspicious XML bookmark files
find /var/vpn/bookmark/ -name '*.xml' -mtime -30 -exec ls -la {} \;Stop Exposing VPN Ports to the Internet — Switch to Zero-Trust Access
Eliminate public-facing VPN concentrators and firewall management ports with identity-first Zero-Trust Network Access.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2019-19781) is a critical directory traversal and remote code execution (RCE) flaw in Citrix Application Delivery Controller (ADC) and Gateway products. The root cause lies in the web server component's failure to properly sanitize and validate user-supplied input within specific URL paths (e.g., '/vpn/../vpns/'). Attackers can exploit directory traversal sequences ('../') to bypass authentication mechanisms and directly access internal script engines and template files.
2. Exploitation Mechanics & Attack Scenario
An attacker sends a specially crafted HTTP GET or POST request to the Citrix VPN portal. Specifically, by performing a directory traversal attack targeting paths like '/vpn/../vpns/portal/scripts/newbm.pl', the attacker executes internal Perl scripts without authentication. Through these scripts, the attacker can write malicious XML macros or templates to arbitrary paths on the system and subsequently invoke them to execute arbitrary OS commands with system privileges (nsroot).
3. Infrastructure & Business Impact
This vulnerability has been assigned a CVSS 3.1 score of 9.8 (Critical) and is listed on the CISA KEV (Known Exploited Vulnerabilities) catalog due to active, widespread exploitation in the wild. Successful exploitation allows an attacker to gain complete administrative control over the Citrix appliance, which acts as the gateway to the internal network. This can lead to lateral movement, sensitive data exfiltration, ransomware deployment, and severe business disruption.
Official Vendor Advisories & References
An issue was discovered in Citrix Application Delivery Controller (ADC) and Gateway 10.5, 11.1, 12.0, 12.1, and 13.0. They allow Directory Traversal.
Frequently Asked SecOps Questions (CVE-2019-19781)
FAQQ1. How critical is CVE-2019-19781 and is it actively exploited in the wild?
CVE-2019-19781 is rated CVSS 9.8 (CRITICAL) with a 30-day EPSS exploitation probability of 100.0%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2019-19781?
Primary impacted vendors include citrix across application delivery controller, gateway. Scope summary: Citrix Application Delivery Controller (ADC) and Gateway versions 10.5, 11.1, 12.0, 12.1, and 13.0
Q3. What temporary mitigations or workarounds exist for CVE-2019-19781 before patching?
[Mitigation] Immediately apply official Citrix security patches, or implement temporary HTTP/HTTPS request blocking policies (Responder Policies) prior to patching Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
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