A flaw in the Cesanta Mongoose library, present up to version 7.20, allows an attacker to manipulate the GCM authentication tag verification performed by mg_aes_gcm_decrypt in src/tls_aes128.c. The failure to properly validate the cryptographic signature can lead to acceptance of tampered TLS traffic, potentially compromising confidentiality or integrity of encrypted data. The weakness is characterized by CWE‑345 and CWE‑347, indicating a lack of proper authentication checks.

The vulnerability affects all installations of Cesanta Mongoose up to and including 7.20. The vendor has delivered a fix in version 7.21; therefore, any deployment using 7.20 or earlier is vulnerable, regardless of operating system or build environment.

The CVSS v3.1 score of 6.3 rates this as a medium severity weakness, with the EPSS score indicating a very low likelihood of exploitation (<1%). It is not listed in CISA KEV and is considered difficult to exploit, though the attack can be performed remotely and requires high procedural complexity.