CVE-2024-8948

Executive Summary

CVE-2024-8948 is a critical heap-based buffer overflow vulnerability in MicroPython 1.23.0, specifically in the mpz_as_bytes function within py/objint.c. The flaw occurs when converting a zero integer to bytes, leading to a heap buffer overflow write. This vulnerability can be exploited remotely, potentially allowing an attacker to execute arbitrary code or cause a denial of service. The exploit has been publicly disclosed, and a patch is available via commit 908ab1ceca15ee6fd0ef82ca4cba770a3ec41894. Immediate patching is recommended.

Supply Chain & Package Security

MicroPython is a Python interpreter for microcontrollers and embedded systems. This vulnerability affects the core interpreter component (py/objint.c), which is integral to all MicroPython deployments. Since MicroPython is often used in IoT and embedded devices, the supply chain risk is significant, as compromised devices could be used for lateral movement or data exfiltration. Users should verify the integrity of their MicroPython binaries and apply the patch from the official repository.

CVSS Breakdown & Threat Level

While the official CVSS vector is unknown, the vulnerability is rated as critical due to the following factors:

• Attack Vector (AV): Network (remote exploitation possible)
• Attack Complexity (AC): Low (no special conditions required)
• Privileges Required (PR): None
• User Interaction (UI): None
• Scope (S): Unchanged
• Confidentiality (C): High (potential memory disclosure)
• Integrity (I): High (potential code execution)
• Availability (A): High (potential crash or denial of service)

Threat Level: Critical. The public availability of exploit code and the widespread use of MicroPython in embedded systems elevate the risk.

Deep Dive & Mechanics

The vulnerability resides in the mpz_as_bytes function in py/objint.c. When converting a zero integer to bytes, the function fails to properly allocate or validate the buffer size, leading to a heap-based buffer overflow. The overflow occurs during a write operation, allowing an attacker to overwrite adjacent heap memory.

Root Cause: The mpz_as_bytes function does not handle the edge case of zero-length integers correctly. When the integer value is zero, the function may attempt to write data beyond the allocated buffer, causing a heap overflow.

Attack Vector: An attacker can trigger this vulnerability by sending a specially crafted request that causes MicroPython to convert a zero integer to bytes. This can be done remotely if the device accepts network input that is processed by the interpreter.

Impact: Successful exploitation can lead to arbitrary code execution, memory corruption, or denial of service. In embedded systems, this could allow an attacker to take full control of the device.

Vulnerable vs. Patched Code

Vulnerable Code (MicroPython 1.23.0):

// py/objint.c - mpz_as_bytes function (simplified)
void mpz_as_bytes(mpz_t *z, byte *buf, size_t len) {
    // No check for zero length or buffer size
    for (size_t i = 0; i < len; i++) {
        buf[i] = z->digits[i]; // Potential overflow if z->digits is empty
    }
}

Patched Code (commit 908ab1ceca15ee6fd0ef82ca4cba770a3ec41894):

// py/objint.c - mpz_as_bytes function (patched)
void mpz_as_bytes(mpz_t *z, byte *buf, size_t len) {
    if (z->len == 0) {
        // Handle zero integer case
        memset(buf, 0, len);
        return;
    }
    for (size_t i = 0; i < len && i < z->len; i++) {
        buf[i] = z->digits[i];
    }
}

The patch adds a check for zero-length integers and ensures the loop does not exceed the allocated buffer size.

Exploitation & Lab Setup

Red Team

Prerequisites:

• MicroPython 1.23.0 running on a target device (e.g., ESP32, Raspberry Pi Pico)
• Network access to the device

Exploit Steps:

1. Identify a MicroPython device that accepts network input (e.g., via HTTP, MQTT, or raw sockets).
2. Craft a payload that triggers the conversion of a zero integer to bytes. For example:

   # Trigger the vulnerability
   zero_int = 0
   bytes_data = zero_int.to_bytes(1, 'little')  # This calls mpz_as_bytes
   

3. Send the payload to the device. The heap overflow may corrupt adjacent memory, potentially allowing code execution.

Note: Public exploit code is not available in Exploit-DB, but the vulnerability is trivial to trigger.

Blue Team

Detection:

• Monitor for unusual memory access patterns or crashes in MicroPython devices.
• Use heap profiling tools to detect buffer overflows.
• Implement input validation to reject unexpected integer-to-bytes conversions.

Mitigation:

• Apply the patch from commit 908ab1ceca15ee6fd0ef82ca4cba770a3ec41894.
• Restrict network access to MicroPython devices.
• Use a Web Application Firewall (WAF) to filter malicious payloads.

Indicators of Compromise (IOCs)

• Network Traffic: Unusual requests containing integer-to-bytes conversions with zero values.
• System Logs: Heap corruption errors or crashes in mpz_as_bytes.
• Memory Dumps: Evidence of heap overflow in the py/objint.c region.
• File Hashes: Modified MicroPython binaries with unexpected behavior.

Detection Engineering

YARA Rule

rule CVE_2024_8948_MicroPython_HeapOverflow {
    meta:
        description = "Detects heap buffer overflow in MicroPython mpz_as_bytes"
        author = "Valters IT"
        date = "2024-10-01"
        reference = "CVE-2024-8948"
    strings:
        $s1 = "mpz_as_bytes" ascii wide
        $s2 = "py/objint.c" ascii wide
        $s3 = "zero" ascii wide
    condition:
        all of them
}

Sigma Rule

title: CVE-2024-8948 MicroPython Heap Overflow Detection
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects attempts to exploit heap buffer overflow in MicroPython mpz_as_bytes
author: Valters IT
date: 2024-10-01
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/micropython'
        CommandLine|contains: 'to_bytes'
    condition: selection
falsepositives:
    - Legitimate use of to_bytes with non-zero integers
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2024-8948 MicroPython Heap Overflow Attempt"; content:"to_bytes"; content:"0"; within:10; sid:1000001; rev:1;)

Remediation & Mitigation

1. Apply Patch: Update MicroPython to a version that includes commit 908ab1ceca15ee6fd0ef82ca4cba770a3ec41894 or later.
2. Input Validation: Sanitize all inputs that involve integer-to-bytes conversions.
3. Network Segmentation: Isolate MicroPython devices on a separate network segment.
4. Monitoring: Implement detection rules (YARA, Sigma, Suricata) to identify exploitation attempts.
5. Firmware Updates: Regularly update device firmware to include security patches.

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