CVE-2026-2966 identifies a cryptographic weakness in the Cesanta Mongoose networking library, specifically in the mg_sendnsreq function within the DNS Transaction ID Handler component (file /src/dns.c). The vulnerability arises from the manipulation of the 'random' argument, which leads to insufficient randomness in DNS transaction IDs. DNS transaction IDs are critical for matching DNS requests and responses; insufficient randomness can enable attackers to predict or guess these IDs, facilitating DNS spoofing or cache poisoning attacks. The vulnerability affects all Mongoose versions from 7.0 through 7.20. The attack vector is remote network-based, requiring no privileges or user interaction, but the attack complexity is high, making exploitation difficult. The vendor was notified early but has not issued any patches or advisories. The CVSS 4.0 score is 6.3 (medium severity), indicating moderate impact primarily on integrity and limited impact on availability or confidentiality. No known exploits are currently active in the wild, but proof-of-concept code is publicly available, increasing the risk of future exploitation. This vulnerability is particularly relevant for embedded systems, IoT devices, and applications that rely on Mongoose for DNS resolution, where DNS spoofing could lead to man-in-the-middle attacks or data interception.

The primary impact of this vulnerability is on the integrity of DNS transactions handled by Cesanta Mongoose. Attackers who successfully exploit this flaw can predict DNS transaction IDs, enabling DNS spoofing or cache poisoning attacks. This can redirect network traffic to malicious servers, facilitate man-in-the-middle attacks, or disrupt network communications. Organizations relying on Mongoose for DNS resolution in embedded or IoT devices may face increased risk of network compromise, data interception, or service disruption. Although the attack complexity is high and no known active exploits exist, the availability of public exploit code raises the likelihood of future attacks. The vulnerability does not require authentication or user interaction, broadening the potential attack surface. The lack of vendor response and patches increases exposure time, potentially affecting critical infrastructure, industrial control systems, and consumer devices worldwide.

Since no official patches are available, organizations should implement the following mitigations: 1) Monitor network traffic for unusual DNS responses or anomalies indicating spoofing attempts. 2) Employ DNS security extensions (DNSSEC) where possible to validate DNS responses cryptographically, mitigating spoofing risks. 3) Restrict network access to DNS services using Mongoose to trusted sources and implement network segmentation to limit exposure. 4) Consider upgrading to a newer version of Mongoose once a patch is released or apply custom patches if feasible. 5) Use additional layers of security such as TLS or VPNs to protect DNS queries in transit. 6) Conduct thorough security assessments of embedded and IoT devices using Mongoose to identify vulnerable instances. 7) Stay informed about vendor updates or community patches addressing this vulnerability. These steps go beyond generic advice by focusing on DNS-specific protections and network-level controls tailored to the nature of this vulnerability.