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 (/src/dns.c). This function is responsible for generating random values used as DNS transaction IDs. Due to insufficient randomness in the argument 'random', the generated DNS transaction IDs can be predicted or manipulated by attackers. This predictability undermines the security of DNS transactions, potentially enabling DNS spoofing or cache poisoning attacks that redirect or intercept network traffic. The vulnerability affects all Cesanta Mongoose versions from 7.0 through 7.20. Exploitation is remote and requires a high level of attack complexity, with no privileges or user interaction needed. The CVSS 4.0 score is 6.3 (medium severity), reflecting network attack vector, high complexity, no privileges, no user interaction, and limited impact on integrity. The vendor was contacted early but did not respond, and no official patches or mitigations have been published. Public exploit code exists, increasing the risk of future exploitation despite no current known attacks in the wild.

The primary impact of this vulnerability is on the confidentiality and integrity of DNS transactions handled by Cesanta Mongoose. By predicting DNS transaction IDs, attackers can perform DNS spoofing or cache poisoning, redirecting users or devices to malicious servers, intercepting sensitive data, or disrupting network services. This can lead to man-in-the-middle attacks, data theft, or denial of service in critical IoT or embedded systems relying on Mongoose for DNS resolution. Given the widespread use of Mongoose in embedded devices and IoT applications, the vulnerability could affect a broad range of industries including telecommunications, industrial control systems, smart home devices, and critical infrastructure. The difficulty of exploitation reduces immediate risk, but the availability of exploit code and lack of vendor response increase long-term exposure. Organizations may face reputational damage, operational disruption, and potential regulatory consequences if exploited.

Until an official patch is released, organizations should implement the following mitigations: 1) Monitor network traffic for unusual DNS responses or anomalies indicative of DNS spoofing attempts. 2) Employ DNS security extensions (DNSSEC) to validate DNS responses cryptographically, mitigating spoofing risks. 3) Restrict network access to devices running vulnerable Mongoose versions, especially limiting exposure to untrusted networks. 4) Where possible, update or replace Cesanta Mongoose with versions beyond 7.20 once patches become available or consider alternative DNS handling libraries with stronger randomness guarantees. 5) Implement network-level protections such as DNS filtering, anomaly detection, and intrusion prevention systems to detect and block DNS manipulation attempts. 6) Conduct regular security assessments of embedded and IoT devices using Mongoose to identify and isolate vulnerable systems. 7) Engage with vendors or developers to demand timely patches and transparency regarding this vulnerability.