CVE-2025-54983

Executive Summary

CVE-2025-54983 is a vulnerability in the Zscaler Client Connector (ZCC) for Windows, affecting versions 4.6 prior to 4.6.0.216 and 4.7 prior to 4.7.0.47. The issue involves a health check port that, under specific circumstances, is not properly released after use. This failure can allow network traffic to bypass ZCC forwarding controls, potentially exposing the endpoint to unauthorized network flows or enabling data exfiltration. The vulnerability has a CVSS score of 5.2 (Medium), with a local attack vector and low attack complexity. No public exploit code or proof-of-concept is currently available, and the patch status remains unknown.

Supply Chain & Package Security

This vulnerability is specific to the Zscaler Client Connector, a proprietary Windows application. It is not related to open-source package ecosystems (NPM, PyPI, Go, Ruby). The risk is limited to the ZCC software supply chain, where users must rely on vendor updates.

CVSS Breakdown & Threat Level

• CVSS Score: 5.2 (Medium)
• Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H
• Attack Vector (AV): Local (L) – The attacker must have local access to the system.
• Attack Complexity (AC): Low (L) – No special conditions are required.
• Privileges Required (PR): Low (L) – The attacker needs basic user privileges.
• User Interaction (UI): None (N) – No user action is needed.
• Scope (S): Unchanged (U) – The vulnerability does not affect other components.
• Confidentiality (C): None (N) – No data disclosure.
• Integrity (I): Low (L) – Limited ability to modify traffic.
• Availability (A): High (H) – Potential for denial of service or traffic bypass.

Threat Level: Medium. The vulnerability requires local access, but it can lead to traffic bypass, which may be exploited in multi-user environments or via malware.

Deep Dive & Mechanics

The Zscaler Client Connector uses a health check port to monitor its operational status. This port is intended to be opened temporarily for diagnostics and then released. In vulnerable versions, under specific conditions (e.g., rapid connection/disconnection cycles or race conditions), the port is not properly closed after use. This leaves the port in a lingering state, allowing network traffic to be routed through it instead of the ZCC forwarding controls.

Root Cause: Improper resource management (CWE-404: Improper Resource Shutdown or Release). The health check port is not released after the diagnostic session ends, causing a resource leak.

Impact: Traffic that should be inspected or blocked by ZCC can bypass these controls. This could allow an attacker to:

• Exfiltrate data without detection.
• Communicate with command-and-control servers.
• Bypass network policies.

Affected Versions:

• ZCC for Windows 4.6 < 4.6.0.216
• ZCC for Windows 4.7 < 4.7.0.47

Vulnerable vs. Patched Code

No specific code snippets are available. However, the vulnerability likely involves a socket or port handle that is not closed in all code paths. A patched version would ensure that the health check port is released after use, possibly by adding proper cleanup in error handling or timeout routines.

Vulnerable Code (Conceptual):

// Vulnerable: Port not released after use
void health_check() {
    int sock = socket(AF_INET, SOCK_STREAM, 0);
    bind(sock, ...);
    listen(sock, 1);
    // ... handle connection ...
    // Missing: close(sock);
}

Patched Code (Conceptual):

// Patched: Port released after use
void health_check() {
    int sock = socket(AF_INET, SOCK_STREAM, 0);
    bind(sock, ...);
    listen(sock, 1);
    // ... handle connection ...
    close(sock); // Proper cleanup
}

Exploitation & Lab Setup

Red Team

• Prerequisites: Local access to a Windows system with vulnerable ZCC (versions 4.6.x < 4.6.0.216 or 4.7.x < 4.7.0.47).
• Exploitation Steps:
  1. Identify the health check port (likely a high port, e.g., 8080 or 8443).
  2. Trigger the condition that prevents port release (e.g., rapid connection attempts).
  3. Once the port is leaked, use it to tunnel traffic that bypasses ZCC controls.
  4. Example: Use netsh or a custom proxy to route traffic through the leaked port.

Proof-of-Concept (No public exploit available):

# Hypothetical: Scan for leaked health check port
netstat -ano | findstr :<port>
# Use the port to bypass ZCC
curl --proxy http://127.0.0.1:<port> http://malicious.com

Blue Team

• Detection: Monitor for unexpected open ports on ZCC processes.
• Mitigation: Restrict local user privileges to prevent exploitation.
• Hunting: Look for traffic patterns that bypass ZCC inspection.

Indicators of Compromise (IOCs)

• File Hashes: None available.
• Network Indicators:
  • Unexpected open ports on ZCC processes (e.g., port 8080, 8443, or other high ports).
  • Traffic from ZCC process to external IPs without inspection.
• Process Indicators:
  • ZCC process (zscaler-client-connector.exe) with multiple open sockets.
• Registry Keys: None known.

Detection Engineering

YARA Rule

rule CVE_2025_54983_Zscaler_HealthCheckPort
{
    meta:
        description = "Detects Zscaler Client Connector with leaked health check port"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-54983"
    strings:
        $s1 = "healthcheck" nocase
        $s2 = "zscaler" nocase
    condition:
        $s1 and $s2
}

Sigma Rule

title: Zscaler Client Connector Health Check Port Leak
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects potential exploitation of CVE-2025-54983 where ZCC health check port is not released
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-54983
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\zscaler-client-connector.exe'
        CommandLine|contains: 'healthcheck'
    condition: selection
falsepositives:
    - Legitimate health check operations
level: medium

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-54983 Zscaler Health Check Port Bypass"; flow:to_server; content:"|00|"; depth:1; reference:cve,2025-54983; sid:1000001; rev:1;)

Remediation & Mitigation

• Immediate: Upgrade to ZCC versions 4.6.0.216 or 4.7.0.47 or later.
• Workaround: If patching is not possible, restrict local user permissions and monitor for unexpected open ports.
• Long-term: Implement proper resource cleanup in all code paths.

Valters IT
DevSecOps & Cyber Threat Intelligence Expert