In Mongoose before version 7.23, clients that use a multi‑certificate CA bundle trust any certificate chain that matches a hostname without verifying its signature. The library stores the bundle but never sets the length of the DER data, and the receiving function accepts a CN match without calling the built‑in signature verifier. A network attacker can therefore forge a self‑signed certificate, satisfy both the hostname and the certificate verification checks, and impersonate the server. This flaw can lead to credential disclosure, traffic modification, and delivery of malicious responses. The weakness is a classic example of insecure certificate validation (CWE‑295).

All installations of the Mongoose embedded web server and network library running a version earlier than 7.23 are affected. The issue specifically impacts configurations that enable a multi‑certificate CA bundle for TLS connections. Any client built with cesanta::mongoose that relies on the built‑in TLS subsystem falls under this risk.

The CVSS score of 9.3 reflects a high‑severity vulnerability that an attacker could exploit over the network. Although an EPSS score is not currently available, the flaw remains exploitable because an adversary only needs to present a suitable certificate during the TLS handshake. The vulnerability is not yet listed in the CISA KEV catalog, but its critical nature means that an active attacker could achieve full interception and manipulation of TLS traffic with minimal effort.