CVE-2019-17210 in Mbed OS
Summary
by MITRE
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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Analysis
by VulDB Data Team • 02/04/2024
The vulnerability described in CVE-2019-17210 represents a critical denial-of-service condition within the MQTT library implementation in Arm Mbed OS version 2017-11-02. This issue stems from improper input validation within the readMQTTLenString() function, which serves as a crucial component in the MQTT protocol parsing mechanism. The vulnerability specifically manifests when the MQTTDeserialize_publish() function invokes readMQTTLenString() to process MQTT topic names, creating a pathway for malicious manipulation of the parsing flow.
The technical flaw resides in the handling of user-controllable data within the readMQTTLenString() function where mqttstring->lenstring.len parameter directly reflects user input without adequate bounds checking or validation. This parameter, which should represent the length of the MQTT topic string, becomes a vector for attack when an adversary manipulates it to an excessively large value. The manipulation causes a logical bypass in the conditional statement that governs the processing flow, effectively allowing execution to skip critical validation steps that would normally ensure proper memory allocation and data handling.
The operational impact of this vulnerability extends beyond simple service disruption to encompass unpredictable program behavior that can compromise the entire system stability. When an attacker sets mqttstring->lenstring.len to an inflated value, the subsequent if statement validation fails, causing the program to proceed with mqttstring->lenstring.data defaulting to zero. This zero value creates a dangerous pointer dereference scenario when curn is subsequently accessed, as it points to the address mqttstring->lenstring.data, which in the context of Arm Cortex-M processors corresponds to address 0x0. This address typically contains the initial value of the MSP (Main Stack Pointer) register, making the system's behavior completely unpredictable and potentially leading to complete system crashes or erratic behavior.
This vulnerability aligns with CWE-129, which addresses improper validation of array indices, and more specifically relates to CWE-125, improper bounds checking in memory access operations. The attack pattern follows ATT&CK technique T1499.001, which involves network denial of service attacks through protocol manipulation. The issue demonstrates how seemingly minor input validation gaps can lead to catastrophic system failures in embedded environments where resource constraints and memory management are already critical concerns.
The mitigation strategy for this vulnerability requires immediate implementation of proper bounds checking within the readMQTTLenString() function to validate the mqttstring->lenstring.len parameter against reasonable maximum values. Additionally, defensive programming practices should be enforced to prevent null pointer dereferences and ensure that all user inputs are properly sanitized before processing. System designers should also consider implementing memory protection mechanisms and robust error handling to prevent the propagation of invalid data through the MQTT parsing stack. Regular security audits of embedded protocol implementations and adherence to secure coding standards are essential to prevent similar vulnerabilities in future deployments. The vulnerability underscores the importance of thorough input validation in embedded systems where the consequences of malformed data can extend far beyond simple application-level failures to encompass complete system compromise.