CVE-2026-2966: Cesanta Mongoose Weak Randomness Vulnerability

CVE-2026-2966 Overview

A cryptographic weakness has been identified in Cesanta Mongoose versions up to 7.20 affecting the DNS Transaction ID Handler component. The vulnerability exists in the mg_sendnsreq function within the /src/dns.c file, where manipulation of the random argument can lead to insufficiently random values being generated. This weakness could allow remote attackers to predict DNS transaction IDs, potentially enabling DNS cache poisoning or spoofing attacks.

Critical Impact

The use of weak random number generation for DNS transaction IDs may allow attackers to forge DNS responses, potentially redirecting network traffic to malicious servers or intercepting sensitive communications.

Affected Products

• Cesanta Mongoose up to version 7.20
• Applications using the Mongoose embedded web server library for DNS functionality
• IoT and embedded devices utilizing Mongoose DNS resolution capabilities

Discovery Timeline

• 2026-02-23 - CVE CVE-2026-2966 published to NVD
• 2026-02-23 - Last updated in NVD database

Technical Details for CVE-2026-2966

Vulnerability Analysis

This vulnerability falls under CWE-310 (Cryptographic Issues), specifically relating to insecure random number generation in the DNS transaction ID handling mechanism. The mg_sendnsreq function in Mongoose's DNS implementation fails to generate sufficiently random transaction IDs, which are critical for preventing DNS spoofing attacks.

DNS transaction IDs serve as a security mechanism to match DNS queries with their corresponding responses. When these IDs are predictable or exhibit insufficient randomness, an attacker positioned on the network can potentially guess the transaction ID and inject forged DNS responses before the legitimate server responds.

The attack requires network access and a high level of complexity to execute successfully. The exploitability is considered difficult due to the timing requirements and network positioning needed to successfully inject malicious DNS responses before legitimate ones arrive.

Root Cause

The root cause of this vulnerability lies in the improper implementation of random value generation within the mg_sendnsreq function. The random argument used to generate DNS transaction IDs does not provide cryptographically secure random values, resulting in predictable or insufficiently entropic identifiers. This implementation flaw violates the security requirements outlined in RFC 5452, which mandates that DNS implementations use strong random number generators for transaction IDs.

Attack Vector

The attack can be launched remotely over the network, though it requires specific conditions to be successful. An attacker must:

1. Monitor DNS query traffic from the vulnerable Mongoose instance to identify active DNS resolution requests
2. Predict or calculate the weak transaction ID being used for DNS queries
3. Race to inject a spoofed DNS response with the predicted transaction ID before the legitimate DNS server responds
4. Successfully poison the DNS cache or redirect traffic to attacker-controlled infrastructure

The vulnerability mechanism involves predictable transaction ID generation in the DNS query handling code. When mg_sendnsreq generates a DNS query, it uses a weak random number generation method that produces values with insufficient entropy. Technical details and proof-of-concept information are available in the GitHub PoC Repository. Additional analysis can be found at VulDB Analysis #347333.

Detection Methods for CVE-2026-2966

Indicators of Compromise

• Unusual patterns in DNS transaction IDs showing sequential or predictable values
• Unexpected DNS response mismatches or cache poisoning events
• Network traffic anomalies suggesting DNS spoofing attempts targeting Mongoose instances
• Log entries indicating DNS resolution failures followed by redirected traffic

Detection Strategies

• Monitor DNS traffic for anomalous response patterns, particularly responses arriving faster than typical DNS server latency
• Implement network-level DNS monitoring to detect transaction ID collisions or suspicious response timing
• Deploy intrusion detection signatures to identify potential DNS cache poisoning attempts
• Analyze Mongoose application logs for unexpected DNS resolution behavior or domain mismatches

Monitoring Recommendations

• Enable verbose logging for DNS-related functions in Mongoose deployments
• Implement DNSSEC validation where possible to detect forged responses
• Monitor network traffic for signs of man-in-the-middle positioning near DNS resolution paths
• Set up alerts for sudden changes in resolved IP addresses for critical domains

How to Mitigate CVE-2026-2966

Immediate Actions Required

• Audit all deployments using Cesanta Mongoose versions up to 7.20 for DNS functionality usage
• Consider implementing application-level DNS response validation as a temporary measure
• Evaluate upgrading to patched versions when available from Cesanta
• Where possible, use external DNS resolution mechanisms not dependent on Mongoose's built-in functionality

Patch Information

The vendor (Cesanta) was contacted early about this disclosure but did not respond. At the time of publication, no official patch is available. Organizations should monitor the Cesanta Mongoose GitHub repository for future security updates. The GitHub PoC Repository contains additional technical details about the vulnerability.

Workarounds

• Implement DNSSEC validation at the network level to reject unsigned or improperly signed DNS responses
• Use a local DNS resolver with strong randomization as a proxy for Mongoose applications
• Deploy network segmentation to limit attacker positioning opportunities near vulnerable instances
• Consider replacing Mongoose DNS functionality with an alternative DNS client library featuring cryptographically secure random number generation

# Example network-level mitigation using iptables to restrict DNS traffic
# Route DNS queries through a trusted local resolver

# Block direct outbound DNS queries from Mongoose instances
iptables -A OUTPUT -p udp --dport 53 -m owner --uid-owner mongoose_user -j DROP
iptables -A OUTPUT -p tcp --dport 53 -m owner --uid-owner mongoose_user -j DROP

# Allow DNS queries only to trusted local resolver
iptables -A OUTPUT -p udp --dport 53 -d 127.0.0.1 -j ACCEPT