Description

A weakness has been identified in Cesanta Mongoose up to 7.21. Affected by this vulnerability is the function fn of the file tutorials/mqtt/mqtt-server/main.c of the component MQTT Broker. Executing a manipulation can lead to stack-based buffer overflow. The attack can be launched remotely. The exploit has been made available to the public and could be used for attacks. Upgrading to version 7.22 addresses this issue. This patch is called a9df523f76f43a38bd53b4232b9cfd4c16869e71. Upgrading the affected component is advised.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.3",
  "pubDate": "2026-09-28T06:16:31.310Z",
  "pubdate": "2026-09-28T06:16:31.310Z",
  "executiveSummary": "A critical stack-based buffer overflow vulnerability has been identified in the MQTT Broker component of Cesanta Mongoose up to version 7.21.\nThe vulnerability resides within the 'fn' function located in 'tutorials/mqtt/mqtt-server/main.c', which fails to properly validate input length before performing memory operations.\nThe flaw allows remote, unauthenticated attackers to trigger a stack-based buffer overflow by sending specifically crafted payloads to the MQTT service.\nSuccessful exploitation may lead to arbitrary code execution, denial of service (system crash), or unauthorized memory corruption, posing a severe risk to system integrity and availability.\nDue to the public availability of an exploit, this vulnerability is considered high-risk, necessitating immediate remediation.\nThe issue is addressed in Mongoose version 7.22, incorporating the security patch identified as a9df523f76f43a38bd53b4232b9cfd4c16869e71.",
  "technicalDetails": "The vulnerability is a classic stack-based buffer overflow occurring within the 'fn' function in 'tutorials/mqtt/mqtt-server/main.c'. This function, serving as a callback or handler within the MQTT Broker implementation, lacks sufficient bounds checking when processing incoming MQTT packets or message data.\nThe root cause involves the application copying user-supplied input into a fixed-size stack buffer without verifying that the source data size is less than or equal to the destination buffer capacity. Because the application logic fails to implement secure length checks during the parsing of the MQTT protocol payload, an attacker can provide an input string or data structure that exceeds the intended memory allocation of the stack frame.\nThe attack flow begins with the attacker establishing a network connection to the Mongoose MQTT Broker. The attacker then constructs a malicious MQTT packet containing a payload designed to overflow the stack buffer during the execution of the 'fn' function. Upon receipt, the Mongoose server parses this packet. When the 'fn' function processes the over-sized input, the memory copy operation (such as memcpy or similar buffer-filling routines) extends beyond the designated stack buffer boundary.\nThis overflow allows for the overwriting of critical stack data, including the function's return address and saved frame pointer. By precisely controlling the overflow content, an attacker can hijack the program's control flow, redirecting execution to arbitrary instructions, such as shellcode injected into the payload or existing Return-Oriented Programming (ROP) gadgets.\nThis vulnerability is remotely exploitable without requiring prior authentication or specific user privileges, as the MQTT Broker typically listens for incoming connections over the network. The impact is significant: successful exploitation can result in full remote code execution under the process's privilege level, or alternatively, cause the application to crash, resulting in a denial-of-service condition. Given that the exploit code has been publicly released, the threat of active exploitation is elevated, necessitating prompt upgrades to version 7.22 or the application of the specific patch a9df523f76f43a38bd53b4232b9cfd4c16869e71."
}