open62541 CVE-2026-63362

open62541 is a widely embedded open-source C implementation of OPC UA (IEC 62541), the interoperability protocol for industrial automation maintained by o6 Automation GmbH and used in controllers, gateways, and edge devices across critical infrastructure. The root cause is CWE-191 (Integer Underflow / Wrap-around): the function verifyAndDecrypt() computes the length of the signed region by subtracting the signature size from the total buffer length using unsigned arithmetic, without first confirming the buffer is long enough to contain the signature. When a short message is received, the subtraction wraps to a value near SIZE_MAX. This near-maximum length is passed directly into the mbedTLS HMAC routine, which then reads far past the end of the actual allocation into unmapped memory, crashing the process. The flaw is pre-authentication by design: the receive path decodes OPC UA headers and matches them against a configured DataSetReader before calling verifyAndDecrypt(), so the underflow is triggered before any signature validity is established. The affected code path is reachable via crafted UDP PubSub datagrams; source review by the researcher suggests the MQTT transport reaches the same path, though this was not independently tested.