CVE-2023-34488 - Heap Buffer Overflow Vulnerability in NanoMQ .17.5's conn_handler function

A recent vulnerability, identified as CVE-2023-34488, has been discovered in NanoMQ .17.5, a lightweight MQTT message broker. The vulnerability is due to a heap-buffer-overflow that occurs in the conn_handler function within the mqtt_parser.c file. Attackers can exploit this vulnerability by sending a specially crafted, malformed message to the broker and causing a heap-buffer-overflow, potentially leading to denial-of-service, remote code execution, or other severe consequences.

Affected Versions

NanoMQ .17.5 and potentially earlier versions are affected by this vulnerability.

Exploit Details

The heap-buffer-overflow vulnerability resides in the conn_handler function of the mqtt_parser.c file. When a malformed message is sent to the MQTT broker, the function fails to properly validate the length of the message. As a result, the heap buffer gets overflowed, potentially allowing attackers to exploit this vulnerability.

The following code snippet demonstrates the point in the conn_handler function where the issue might occur:

/* File: mqtt_parser.c */
...
static void conn_handler(struct conn *s, char *buf)
{
...
    msg_len = buf[] << 24 | buf[1] << 16 | buf[2] << 8 | buf[3];
    if (msg_len > s->buf_len)
    {
        /* Prepare a buffer of appropriate size */
        s->buf_needs_resize = true;
        return;
    }
...
}

In the code snippet above, the msg_len variable is calculated based on the first four bytes from the incoming message buffer (buf). However, there's no proper validation to ensure that msg_len does not exceed the maximum accepted length. Subsequently, the heap-buffer-overflow might occur and allow malicious actors to exploit this deficiency.

Original References

1. Original advisory by the researcher

2. CVE Details page for CVE-2023-34488

Mitigation

At the time of writing this post, there's no official patch available to remediate this vulnerability in NanoMQ .17.5. However, users can mitigate the risk by applying the following temporary fix:

1. Add proper validation to check if msg_len exceeds the maximum accepted length, and return an error if the validation fails.

The following is a modified version of the conn_handler function with the appropriate safety check

/* File: mqtt_parser.c */
...
static void conn_handler(struct conn *s, char *buf)
{
...
    msg_len = buf[] << 24 | buf[1] << 16 | buf[2] << 8 | buf[3];
    if (msg_len > s->buf_len || msg_len > MAX_ACCEPTED_LEN) // Added validation for maximum length
    {
        /* Prepare a buffer of appropriate size */
        s->buf_needs_resize = true;
        return;
    }
...
}

Replace MAX_ACCEPTED_LEN with the maximum acceptable length for a message in your specific use case.

Conclusion

CVE-2023-34488 is a critical vulnerability within NanoMQ .17.5, which could potentially be exploited by attackers sending a specially crafted, malformed message to the MQTT broker. Applying the suggested mitigation - adding proper validation to check if msg_len exceeds the maximum accepted length - could effectively address this issue when dealing with potentially untrusted messages.

Stay vigilant by updating to the latest versions of software and keeping an eye out for official patches that may become available in the future.

Timeline

Published on: 06/12/2023 14:15:00 UTC
Last modified on: 06/16/2023 03:56:00 UTC