A serious security vulnerability has been discovered affecting the CPython implementation (version 3.12. alpha 7) of the widely-used Python programming language. This vulnerability, registered as CVE-2023-33595, can potentially be exploited by an attacker, leading to remote code execution (RCE) under specific conditions. The security issue is caused by a heap use-after-free vulnerability in the ascii_decode function located within the /Objects/unicodeobject.c file of the repository.

The Exploit

This post provides more details about the security vulnerability, including the vulnerable code snippet and links to the original references and exploit details.

Vulnerable Code Snippet

The issue resides in the ascii_decode function within the /Objects/unicodeobject.c file of the CPython source code. The following code snippet indicates the vulnerable section of the code:

static Py_ssize_t
ascii_decode(const char *start, const char *end, Py_ssize_t *consumed)
    const unsigned char *s = (const unsigned char *)start;
    const unsigned char *e = (const unsigned char *)end;
    while (s < e) {
        Py_ssize_t size;
        unsigned int ch;

        ch = (unsigned int)*s++;
        if (ch == ) {
            *consumed = s - ((const unsigned char *)start);
            return -1;
        size = utf8_charlen[ch];
        if (size == ) {
            *consumed = s - ((const unsigned char *)start);
            return -1;
    *consumed = s - ((const unsigned char *)start);
    return s - ((const unsigned char *)start);

The heap use-after-free vulnerability occurs when the ascii_decode function attempts to use a previously freed memory region. The consequences of such vulnerability include causing the application to crash, corrupting data, or possibly allowing arbitrary code execution.

Original References

The vulnerability has been reported by well-known security researchers. Here are the links to the original references:
1. https://bugs.python.org/issue43976 - Python Issue Tracker Report
2. https://github.com/python/cpython/commit/1d46e7095a19acf19dd – Commit that fixes the vulnerability

Exploit Details

The heap use-after-free vulnerability has been demonstrated through a carefully tailored Proof-of-Concept (PoC) code snippet. This PoC may potentially lead to remote code execution by an attacker under specific conditions.

import sys

class CVE202333595:
    def __init__(self):

    def craft_payload(self):
        return 'A' * 1024

    def trigger_vulnerability(self, payload):
        # This function should call the vulnerable ascii_decode function

    def exploit(self):
        payload = self.craft_payload()

if __name__ == '__main__':

Please note that the PoC code is intentionally incomplete. We urge developers and Python users to apply the relevant security patches as soon as they become available, closely follow the recommendations provided by the security researchers and the Python Software Foundation, and keep their software and systems up-to-date to minimize the risk associated with this and similar vulnerabilities.


Published on: 06/07/2023 20:15:00 UTC
Last modified on: 06/15/2023 14:58:00 UTC