CVE-2026-54412

LiamBindle MQTT-C through version 1.1.6 contains a heap-based out-of-bounds read and integer underflow in the mqttunpackpublishresponse() function in src/mqtt.c that allows a remote unauthenticated attacker controlling an MQTT broker - or able to inject MQTT traffic into an unencrypted session - to crash a subscribed MQTT-C client and potentially disclose adjacent heap memory by sending a single crafted PUBLISH packet. The function validates only that the fixed-header remaininglength is at least 4, then reads the 16-bit topicnamesize field from the broker-controlled packet and advances the parse pointer by that value without verifying that topicnamesize plus the surrounding overhead fits within remaininglength; it subsequently computes applicationmessagesize as remaininglength - topicnamesize - 2 (QoS 0) or - 4 (QoS greater than 0) in unsigned arithmetic, producing an integer underflow that is then passed to memmove(). A PUBLISH packet with topicnamesize = 0xFFFF and remaininglength = 7 advances the parse pointer 65535 bytes past the receive buffer (out-of-bounds read) and causes an applicationmessage_size near 2^32, crashing the process when the resulting memmove() is executed.