The vulnerability is located in the Poly1305 Authentication Tag Handler function mg_chacha20_poly1305_decrypt within Cesanta Mongoose up to version 7.20. Manipulating the authentication tag causes the function to accept an invalid or forged cryptographic signature as valid, effectively bypassing the authentication mechanism. The flaw is categorized as CWE-345 and CWE-347, indicating improper handling of cryptographic validation that may be exploitable by an attacker to deliver forged data or commands.

Affected systems include all deployments that incorporate Cesanta Mongoose, specifically version 7.20 and earlier. The flaw impacts any TLS session that relies on the ChaCha20/Poly1305 cipher suite. The CNA list identifies Cesanta:Mongoose as the affected vendor‑product pair and no other operating systems or subcomponents are mentioned.

Risk and exploitability assessment shows a CVSS score of 6.3, reflecting moderate severity. The EPSS score is below 1%, signaling a low likelihood of widespread exploitation at present. The vulnerability is not listed in the CISA KEV catalog. The attack can be launched remotely by an attacker who can craft an authentication tag with a forged Poly1305 tag, but the exploit is described as difficult, meaning that it requires the ability to generate a valid ChaCha20/Poly1305 payload. Successful exploitation bypasses integrity checks and can lead to unauthorized data processing or command execution within the affected application.