CVE-2019-9749 in Fluent Bit
Summary
by MITRE
An issue was discovered in the MQTT input plugin in Fluent Bit through 1.0.4. When this plugin acts as an MQTT broker (server), it mishandles incoming network messages. After processing a crafted packet, the plugin's mqtt_packet_drop function (in /plugins/in_mqtt/mqtt_prot.c) executes the memmove() function with a negative size parameter. That leads to a crash of the whole Fluent Bit server via a SIGSEGV signal.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 08/01/2023
The vulnerability identified as CVE-2019-9749 represents a critical memory corruption flaw within the MQTT input plugin of Fluent Bit version 1.0.4 and earlier. This issue manifests when the plugin operates in server mode, accepting incoming MQTT connections and processing messages from clients. The flaw stems from improper validation of network packet sizes during message processing, creating a scenario where maliciously crafted packets can trigger unintended behavior in the underlying memory management functions. The vulnerability specifically impacts the mqtt_packet_drop function located in the /plugins/in_mqtt/mqtt_prot.c source file, which handles the cleanup of malformed or dropped MQTT packets within the message processing pipeline.
The technical exploitation of this vulnerability occurs through a precise manipulation of network packet structures that causes the memmove() function to receive a negative size parameter. This memory management function is designed to copy data from one memory location to another, but when provided with a negative size value, it triggers undefined behavior that ultimately results in a segmentation fault. The memmove() function operates by copying a specified number of bytes from a source to a destination, and when this count becomes negative, the function's internal logic fails catastrophically, leading to memory corruption and subsequent process termination. The SIGSEGV signal represents the operating system's response to this memory violation, effectively crashing the entire Fluent Bit server instance.
The operational impact of CVE-2019-9749 extends beyond simple service disruption to potentially enable denial of service attacks against logging infrastructure. Since Fluent Bit serves as a critical component in many logging and monitoring systems, compromising its stability can lead to complete loss of log data collection capabilities. Attackers can exploit this vulnerability by sending specially crafted MQTT packets to the vulnerable server, causing immediate service termination without any recovery mechanism. This vulnerability directly maps to CWE-121, which describes stack-based buffer overflow conditions, and aligns with ATT&CK technique T1499.004 for network denial of service attacks. The flaw demonstrates poor input validation practices and inadequate bounds checking in network protocol handling code, creating a pathway for remote code execution or system compromise through service disruption.
Mitigation strategies for this vulnerability require immediate patching of Fluent Bit to version 1.0.5 or later, where the problematic memmove() call has been corrected to properly validate input parameters before execution. Organizations should implement network segmentation and access controls to limit exposure of the MQTT input plugin to untrusted networks, particularly when operating in server mode. Additionally, monitoring systems should be configured to detect unusual termination patterns or memory access violations that might indicate exploitation attempts. The fix typically involves adding proper parameter validation before calling memmove() and ensuring that packet size calculations cannot result in negative values. Security teams should also consider implementing intrusion detection systems that can identify and block malformed MQTT packets attempting to exploit this specific vulnerability pattern.