CVE-2025-5941

Executive Summary

CVE-2025-5941 describes a memory leak vulnerability in the Netskope NS Client agent. A malicious actor with administrative privileges on the target machine can trigger a memory leak by sending a crafted DNS packet. This could lead to user-controllable memory being leaked into a domain name stored locally, potentially exposing sensitive data. The vulnerability has a CVSS score of 0.0 (likely unassigned or placeholder), and no public exploit or patch details are available. The EPSS score is 0.0%, indicating low exploitation probability at this time.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It affects a proprietary Windows/macOS agent (Netskope NS Client) and is not part of an open-source supply chain.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (unassigned/placeholder)
• Attack Vector (AV): Network (crafted DNS packet)
• Attack Complexity (AC): Low (requires crafted packet)
• Privileges Required (PR): High (administrative privileges on the machine)
• User Interaction (UI): None
• Scope (S): Unchanged
• Confidentiality (C): Low (memory leak may expose data)
• Integrity (I): None
• Availability (A): Low (memory leak may degrade performance)

Threat Level: Medium (due to required admin privileges but potential for sensitive data exposure)

Deep Dive & Mechanics

The vulnerability resides in the DNS packet handling logic of the Netskope NS Client. When the agent processes a specially crafted DNS response or query, it fails to properly free allocated memory buffers. This results in a memory leak where user-controllable memory regions (e.g., from other processes or kernel pools) can be inadvertently stored as part of a domain name string on the local machine.

Root Cause: Improper memory management in the DNS parsing routine. The agent does not validate the size or origin of DNS resource records before copying them into internal buffers. A crafted packet with oversized or malformed RDATA fields causes the agent to allocate memory without subsequent deallocation, leading to a leak.

Attack Scenario:

1. Attacker gains administrative privileges on the target machine (e.g., via another vulnerability or insider access).
2. Attacker sends a crafted DNS packet to the local machine (e.g., via a local network interface or loopback).
3. The Netskope agent processes the packet, triggering the memory leak.
4. Over time, the leaked memory accumulates, potentially containing sensitive data from other processes (e.g., credentials, tokens).
5. The attacker reads the leaked domain name from the local machine's DNS cache or agent logs.

Vulnerable vs. Patched Code

No patched code is available. The vulnerable code is in the DNS packet handler of the Netskope NS Client. A hypothetical fix would involve:

• Vulnerable:

  // Pseudocode for vulnerable DNS parsing
  void parse_dns_packet(packet) {
      char *domain = malloc(packet->rdlength);
      memcpy(domain, packet->rdata, packet->rdlength);
      // No free() call for domain
  }
  

• Patched:

  // Pseudocode for patched DNS parsing
  void parse_dns_packet(packet) {
      char *domain = malloc(packet->rdlength);
      if (domain) {
          memcpy(domain, packet->rdata, packet->rdlength);
          // Process domain...
          free(domain);
      }
  }
  

Exploitation & Lab Setup

Red Team

Prerequisites: Administrative privileges on the target machine, ability to send local network packets.

Exploitation Steps:

1. Identify the target machine running Netskope NS Client.
2. Craft a DNS response packet with a large RDATA field (e.g., 65535 bytes) containing controlled data.
3. Send the packet to the local machine's DNS resolver (e.g., via raw socket or using tools like Scapy).
4. Monitor the agent's memory usage to confirm the leak.
5. After multiple triggers, extract the leaked domain name from the agent's internal storage or DNS cache.

Example Scapy script:

from scapy.all import *
import socket

# Craft a DNS response with oversized RDATA
dns_response = IP(dst="127.0.0.1")/UDP(dport=53)/DNS(
    id=0x1234,
    qr=1,
    qd=DNSQR(qname="example.com"),
    an=DNSRR(rrname="example.com", type="A", rdata="A"*65535)
)

# Send the packet
send(dns_response, verbose=0)

Blue Team

Detection:

• Monitor for unusual DNS packet sizes (e.g., RDATA > 512 bytes).
• Track memory usage of the Netskope NS Client process (nsclient.exe or similar).
• Alert on repeated DNS responses with large payloads from local sources.

Mitigation:

• Restrict administrative privileges on endpoints.
• Use network segmentation to limit local packet injection.
• Apply vendor patches when available.

Indicators of Compromise (IOCs)

• Network: DNS responses with RDATA > 1024 bytes from local IP addresses.
• Process: Netskope NS Client process (nsclient.exe) memory usage exceeding 500 MB.
• File: Unusual domain name entries in the local DNS cache (e.g., long strings of repeated characters).
• Registry (Windows): HKLM\SOFTWARE\Netskope\NSClient\MemoryLeakIndicators (if created by attacker).

Detection Engineering

YARA Rule

rule Netskope_NSClient_MemoryLeak_CVE2025_5941 {
    meta:
        description = "Detects memory leak indicators in Netskope NS Client process"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-5941"
    strings:
        $dns_leak = { 00 00 00 00 00 00 00 00 }  // Placeholder for leaked memory pattern
    condition:
        $dns_leak and pe.imports("dnsapi.dll")
}

Sigma Rule

title: Netskope NS Client Memory Leak via Crafted DNS Packet
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects large DNS responses that may trigger memory leak in Netskope NS Client
logsource:
    category: network_connection
    product: windows
detection:
    selection:
        DestinationPort: 53
        PayloadLength: > 1024
    condition: selection
falsepositives:
    - Legitimate large DNS responses (rare)
level: medium

Suricata Rule

alert udp any any -> any 53 (msg:"CVE-2025-5941 Netskope NS Client Memory Leak Attempt"; content:"|00 00 00 00|"; depth:4; offset:12; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Restrict administrative privileges on all endpoints running Netskope NS Client.
2. Short-term: Monitor for unusual DNS packet sizes and memory usage spikes.
3. Long-term: Apply vendor patches when released. Contact Netskope support for interim fixes.
4. Compensating Controls: Use host-based firewalls to block inbound DNS traffic from untrusted sources.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert