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A denial-of-service issue was discovered in the MQTT library in Arm Mbed OS 2017-11-02. The function readMQTTLenString() is called by the function MQTTDeserialize_publish() to get the length and content of the MQTT topic name. In the function readMQTTLenString(), mqttstring->lenstring.len is a part of user input, which can be manipulated. An attacker can simply change it to a larger value to invalidate the if statement so that the statements inside the if statement are skipped, letting the value of mqttstring->lenstring.data default to zero. Later, curn is accessed, which points to mqttstring->lenstring.data. On an Arm Cortex-M chip, the value at address 0x0 is actually the initialization value for the MSP register. It is highly dependent on the actual firmware. Therefore, the behavior of the program is unpredictable from this time on.

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ARM Mbed OS MQTT library 输入验证错误漏洞

ARM Mbed OS是英国ARM公司的一套专用于物联网的开源嵌入式操作系统。MQTT library是其中的一个MQTT（消息队列遥测传输协议）库。 Arm Mbed OS 2017-11-02版本中的MQTT library存在安全漏洞。攻击者可通过将mqttstring->lenstring.len更改成较大值利用该漏洞造成拒绝服务。

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