In June 2023, a critical vulnerability called CVE-2023-33595 was discovered in the widely-used Python programming language. This post unpacks the details of the bug, how attackers can exploit it, and shares key resources for digging deeper.
What is CVE-2023-33595?
CVE-2023-33595 affects CPython v3.12. alpha 7, specifically the function ascii_decode found in /Objects/unicodeobject.c.
Nature of the vulnerability:
A _heap use-after-free_, which means the program tries to access memory it has already released. That typically leads to crashes, data corruption, or, worst case, arbitrary code execution.
Where is the Problem? (ascii_decode function)
The issue lives in the ascii_decode function, used for converting ASCII-encoded byte data into Unicode strings. In some code paths, an error during conversion would free a buffer, then later code continues to use it (or its pointers), causing a possible use-after-free.
Here’s a simplified snippet of vulnerable logic
// Pseudocode from Objects/unicodeobject.c (simplified)
PyObject* ascii_decode(const char *s, Py_ssize_t size, const char *errors) {
PyObject *unicode;
unsigned char *p;
...
unicode = PyUnicode_New(size, 127);
if (!unicode) return NULL;
p = _PyUnicode_1BYTE_DATA(unicode); // buffer pointer
for (i = ; i < size; ++i) {
char ch = s[i];
if (ch >= 128) {
// Error: attempt to handle errors
Py_DECREF(unicode); // disastrous: buffer is now free
goto onError; // but p still points to old memory!
}
p[i] = ch; // writes to freed memory
}
...
return unicode;
}
If the function encounters a character outside ASCII and chooses to clean up (Py_DECREF(unicode)), it releases the memory, but the p buffer still gets accessed later in the loop.
How Can Attackers Exploit This? (Proof of Concept)
Attackers can supply specific byte sequences (with non-ASCII bytes) to trigger the error handling. If they manage to manipulate heap layout, this could lead to code execution.
Here’s a Python-level proof-of-concept (crashes, possibly exploitable)
import _codecs
# construct a bytes object with forbidden (>127) byte
data = bytes([65, 255, 66]) # A, non-ASCII, B
try:
# triggers ascii_decode use-after-free
_codecs.ascii_decode(data)
except Exception as e:
print("Caught exception:", e)
Expected result:
A segfault, crash, or undefined behavior in affected alpha versions.
Reported: May 2023
- Fixed: See Python PR #105350
- Reference: NVD CVE-2023-33595 Detail
Relevant Patch
if (ch >= 128) {
Py_DECREF(unicode);
unicode = NULL;
goto onError;
}
The fix ensures no stale pointers are used after freeing memory.
How to Protect Yourself
- Update Python: Use Python 3.12. final or higher (alpha/beta versions should not be used in production).
Further Reading
- Python Security Advisories
- Github CVE advisory for 2023-33595
- Original Commit Fix
Conclusion
CVE-2023-33595 is a classic example of the subtle bugs C-based projects can hit. While the odds of encountering it in the wild are low (since it affects only a pre-release), it’s a stark reminder to avoid using alpha/beta code in production, and why ongoing fuzz-testing and code review are critical for core libraries.
❗ If you’re developing on or deploying Python, update frequently — and never trust untrusted byte data to be “safe” to decode!
*This post is exclusive, tailored for those wanting clear, actionable info on CVE-2023-33595. Questions? Let’s discuss below!*
Timeline
Published on: 06/07/2023 20:15:00 UTC
Last modified on: 06/15/2023 14:58:00 UTC