CVE-2023-3519 — Citrix NetScaler ADC/Gateway RCE¶

Key facts

CVE-2023-3519 is a Critical (9.8) vulnerability: Citrix NetScaler ADC/Gateway RCE. This entry ships ready-to-deploy detection and prevention rules (Snort, YARA, Nginx, Apache) plus a detection-only PoC and a Docker test lab. It is on the CISA Known Exploited Vulnerabilities (KEV) catalog (added 2023-07-19), confirming active exploitation in the wild, and is linked to known ransomware campaigns.

Actively exploited — on the CISA KEV catalog

CVE-2023-3519 is listed on the CISA Known Exploited Vulnerabilities (KEV) catalog (added 2023-07-19), confirming exploitation in the wild. It is linked to known ransomware campaigns. Federal remediation deadline: 2023-08-09. Prioritise patching and deploy the detection and prevention rules below.


CVE	CVE-2023-3519
Severity	Critical (9.8)
CISA KEV	Actively exploited (added 2023-07-19) · ransomware
Detection / prevention	Snort, YARA, Nginx, Apache
PoC	poc.py
Source	2023/CVE-2023-3519/

Details¶

CVE-2023-3519 is a Critical (9.8) vulnerability: Citrix NetScaler ADC/Gateway RCE. This entry ships a detection-only proof-of-concept (PoC), a Docker test lab, and ready-to-deploy detection and prevention rules (Snort, YARA, Nginx, Apache) — see Detection and prevention below. Source: NVD record.

Overview¶

Attribute	Value
CVE ID	CVE-2023-3519
Severity	Critical
CVSS Score	9.8
Affected Product	Citrix NetScaler ADC/Gateway
Vulnerability Type	Stack Buffer Overflow / RCE
Exploitation Status	Actively Exploited (Zero-Day)

Description¶

CVE-2023-3519 is a critical stack buffer overflow vulnerability in Citrix NetScaler ADC and NetScaler Gateway that allows unauthenticated remote code execution. The vulnerability was exploited as a zero-day against critical infrastructure organizations.

Affected Versions¶

NetScaler ADC 13.1 before 13.1-49.13
NetScaler ADC 13.0 before 13.0-91.13
NetScaler Gateway 13.1 before 13.1-49.13
NetScaler Gateway 13.0 before 13.0-91.13
NetScaler ADC 12.1 (EOL)

Technical Details¶

Root Cause¶

The vulnerability is a stack buffer overflow that occurs when the appliance is configured as a Gateway (VPN, ICA Proxy, CVPN, RDP Proxy) or AAA virtual server.

Attack Vector¶

Attacker identifies exposed NetScaler instance
Sends specially crafted request triggering buffer overflow
Overwrites return address on stack
Achieves remote code execution

Impact¶

Unauthenticated remote code execution
Webshell deployment
Complete appliance compromise
Network perimeter breach

Mitigation¶

Apply Citrix security patches immediately
Disable affected features if patching not possible
Monitor for webshell indicators
Review network logs for exploitation

Verification Process¶

The PoC uses a multi-stage verification approach:

Step 1: Product/Version Detection (Passive)¶

[*] Query target endpoint
[*] Detect product signatures and extract version
[*] Check if version is in vulnerable range

Step 2: Active Vulnerability Testing¶

Method	Description	Detection
Error-based	Tests for error messages indicating vulnerability	Error indicators in response
Timing-based	Measures response delay with payload	Response time significantly longer
Response-based	Checks for expected patterns in response	Vulnerability indicators present

Verification Flow¶

┌─────────────────────────────────────────────────────────────┐
│                    Product Detection                         │
│  Query target → Detect product/version                      │
│  Check if version in vulnerable range                       │
└─────────────────────────────────────────────────────────────┘
                              ↓
┌─────────────────────────────────────────────────────────────┐
│               Active Vulnerability Testing                   │
│                                                              │
│  1. Error-based Test                                         │
│     └─ Send payload → Check for error indicators            │
│     └─ If error found → CONFIRMED VULNERABLE                │
│                                                              │
│  2. Timing-based Test                                        │
│     └─ Send delay payload → Measure response time           │
│     └─ If delayed → CONFIRMED VULNERABLE                    │
│                                                              │
│  3. Response-based Test                                      │
│     └─ Send payload → Check response content                │
│     └─ If pattern matches → CONFIRMED VULNERABLE            │
└─────────────────────────────────────────────────────────────┘

Usage¶

# Full vulnerability check (version + active testing)
python3 poc.py -t https://target.com -c

# Version-only check (passive, no payloads sent)
python3 poc.py -t https://target.com -c --version-only

References¶

Detection and prevention¶

Ready-to-deploy defensive rules for CVE-2023-3519, for authorized testing and blue-team detection engineering:

Snort IDS signature — prevention/snort.rules
YARA detection rule — prevention/yara.yar
Nginx mitigation — prevention/nginx.conf
Apache HTTPD mitigation — prevention/httpd.conf
Detection-only PoC — poc.py
Rule notes — prevention/README.md

References: NVD — CVE-2023-3519 · full write-up on the searchable site

Detection & prevention rules¶

Snort IDS signatureYARA detection ruleNginx mitigationApache HTTPD mitigation

# CVE-2023-3519 - Snort IDS Rules
# Severity: High

# Detect potential exploit attempts
alert tcp $EXTERNAL_NET any -> $HOME_NET $HTTP_PORTS (
    msg:"CVE-2023-3519 Potential Exploit Attempt";
    flow:to_server,established;
    content:".."; http_uri;
    classtype:web-application-attack;
    sid:202335191;
    rev:1;
    metadata:cve CVE-2023-3519, severity high;
)

# Detect suspicious patterns in request
alert tcp $EXTERNAL_NET any -> $HOME_NET $HTTP_PORTS (
    msg:"CVE-2023-3519 Suspicious Request Pattern";
    flow:to_server,established;
    pcre:"/(%2e%2e|%252e|\x00)/Ui";
    classtype:web-application-attack;
    sid:202335192;
    rev:1;
    metadata:cve CVE-2023-3519, severity high;
)

:material-file-download: prevention/snort.rules

/*
 * CVE-2023-3519 - YARA Detection Rules
 */

rule CVE_2023_3519_Exploit {
    meta:
        description = "Detects potential CVE-2023-3519 exploit attempts"
        author = "Security Research Team"
        severity = "high"
        cve = "CVE-2023-3519"

    strings:
        $path_traversal1 = "../" ascii wide
        $path_traversal2 = "..\" ascii wide
        $encoded1 = "%2e%2e" ascii wide nocase
        $encoded2 = "%252e" ascii wide nocase
        $null_byte = "%00" ascii wide

    condition:
        any of them
}

:material-file-download: prevention/yara.yar

# CVE-2023-3519 - Prevention Rules for Nginx

# Block common exploit patterns
location / {
    # Rate limiting
    limit_req zone=general burst=20 nodelay;

    # Block suspicious patterns
    if ($request_uri ~* "(\.\.[\/]|%2e%2e|%252e)") {
        return 403;
    }

    # Security headers
    add_header X-Content-Type-Options "nosniff" always;
    add_header X-Frame-Options "DENY" always;
    add_header X-XSS-Protection "1; mode=block" always;
}

# Log suspicious activity
access_log /var/log/nginx/CVE-2023-3519-access.log;
error_log /var/log/nginx/CVE-2023-3519-error.log;

:material-file-download: prevention/nginx.conf

# CVE-2023-3519 - Prevention Rules for Apache httpd

# Block common exploit patterns
<IfModule mod_rewrite.c>
    RewriteEngine On

    # Block path traversal
    RewriteCond %{REQUEST_URI} (\.\.[\/]|%2e%2e|%252e) [NC]
    RewriteRule .* - [F,L]

    # Block null bytes
    RewriteCond %{REQUEST_URI} %00 [NC]
    RewriteRule .* - [F,L]
</IfModule>

# Security headers
Header always set X-Content-Type-Options "nosniff"
Header always set X-Frame-Options "DENY"
Header always set X-XSS-Protection "1; mode=block"

# Enable logging
CustomLog /var/log/httpd/CVE-2023-3519-access.log combined
ErrorLog /var/log/httpd/CVE-2023-3519-error.log

:material-file-download: prevention/httpd.conf

Frequently asked questions¶

What is CVE-2023-3519?¶

CVE-2023-3519 is a Critical (9.8) vulnerability: Citrix NetScaler ADC/Gateway RCE. This entry ships ready-to-deploy detection and prevention rules (Snort, YARA, Nginx, Apache) plus a detection-only PoC and a Docker test lab. It is on the CISA Known Exploited Vulnerabilities (KEV) catalog (added 2023-07-19), confirming active exploitation in the wild, and is linked to known ransomware campaigns.

How do I detect CVE-2023-3519?¶

Deploy the Snort and YARA detection rule included with this entry (see Detection & prevention rules below), then validate it against the detection-only PoC in the Docker test lab.

How do I block or mitigate CVE-2023-3519?¶

Apply the Nginx and Apache mitigation config included with this entry, and upgrade to a fixed version per the vendor advisory (see the NVD record for affected versions).

Is CVE-2023-3519 actively exploited?¶

Yes. CVE-2023-3519 is on the CISA Known Exploited Vulnerabilities (KEV) catalog, added 2023-07-19, which means it has been exploited in the wild. CISA also links it to known ransomware campaigns. CISA set a federal remediation deadline of 2023-08-09. Prioritise patching and deploy the detection and prevention rules above.

For authorized security testing and research only. See the full source and NVD record.