Executive brief
Cesanta Mongoose is a popular embedded networking library used to provide web, MQTT, and WebSocket capabilities to hardware devices. A security flaw in its built-in encryption handling allows a remote attacker to crash the device's secure services by sending a specially crafted connection request. This can lead to a denial-of-service, disrupting the device's ability to communicate securely or perform its primary functions.
Technical details
An out-of-bounds read vulnerability exists in the mg_tls_server_recv_hello() function within Cesanta Mongoose's built-in TLS implementation (MG_TLS_BUILTIN). The root cause is the use of an attacker-controlled session_id_len byte from a TLS ClientHello message as a buffer index without proper validation against the actual length of the received data. A remote, unauthenticated attacker can exploit this by sending a single crafted ClientHello packet with an oversized session ID length. This causes the server to read past the receive buffer, resulting in a crash and denial-of-service for any service utilizing the built-in TLS stack (such as HTTPS, MQTTS, or WSS). The issue is resolved in version 7.22.
Affected products
- Cesanta Mongoose before 7.22
Timeline
- 2026-06-24: patched: Version 7.22 released with fix
- 2026-07-09: disclosed: CVE published and advisory released