buglwIP - A Lightweight TCP/IP stack - Bugs: bug #61479, MQTT: DoS caused by unsigned...


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bug #61479: MQTT: DoS caused by unsigned integer overflow

Submitted by:  Hyeonsu Kim <icekrim>
Submitted on:  Sun 14 Nov 2021 06:31:19 AM UTC
Category:  Security-related Severity:  3 - Normal
Item Group:  Crash Error Status:  None
Privacy:  Public Assigned to:  None
Open/Closed:  Open Planned Release:  None
lwIP version:  * git head
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Sun 14 Nov 2021 06:31:19 AM UTC, original submission:  

Integer overflow in mqtt_parse_incoming of src/apps/mqtt/mqtt.c allows attackers to perform a Denial of Service via MQTT packet with fixed header length of 256 bytes or more


mqtt.c 866

while (p->tot_len > in_offset) {
if ((fixed_hdr_len < 2) || ((b & 0x80) != 0)) {
  if (fixed_hdr_len < client->msg_idx) {
    b = client->rx_buffer[fixed_hdr_len];
  } else {
    b = pbuf_get_at(p, in_offset++);
    client->rx_buffer[client->msg_idx++] = b;
  if (fixed_hdr_len >= 2) {
    msg_rem_len |= (u32_t)(b & 0x7f) << ((fixed_hdr_len - 2) * 7);
    if ((b & 0x80) == 0) {


If it receives a packet of 256 bytes with b & 80 == 1, it goes into an infinite loop and the device stops. (like [0x80]*256)

This is because fixed_hdr_len, which is a u8 type, can never be larger than client->msg_idx (in this case 256), so in_offset cannot be increased.

I have completed the POC on the stm32 board I have personally, and I will prepare linux porting if necessary. Thank you.

Hyeonsu Kim <icekrim>


(Note: upload size limit is set to 16384 kB, after insertion of the required escape characters.)

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