CVE-2022-48560: A Deep Dive into the Use-After-Free Vulnerability in Python up to Version 3.9 via heappushpop in heapq

Python's heapq module provides an implementation of the heap queue algorithm, also known as the priority queue algorithm. This module is popular among developers for its simplicity in managing items based on their priority. Recently, a use-after-free vulnerability, tracked as CVE-2022-48560, has been identified in Python versions up to 3.9 via the heappushpop function in the heapq module. This article aims to shed light on the technical details of this vulnerability, the code snippet in question, and potential exploitation techniques.

What is CVE-2022-48560?

CVE-2022-48560 is a use-after-free vulnerability that exists in Python up to version 3.9 via the heappushpop function in the heapq module. A use-after-free vulnerability occurs when a memory resource is accessed after it has been freed, which may lead to unpredictable results, crashes, or other security implications.

Code Snippet

Heappushpop is a function provided by Python's heapq module and offers efficient O(log n) performance. The function is defined as follows:

def heappushpop(heap, ele):
    """Push item onto heap, then pop and return the smallest item from the
    heap. The combined action runs more efficiently than heappush()
    followed by a separate call to heappop()."""
    if heap and heap[] < ele:
        ele, heap[] = heap[], ele
        _siftup(heap, )
    return ele

Links to Original References

1. Official Python heapq module documentation: https://docs.python.org/3/library/heapq.html
2. Python bug tracker report: https://bugs.python.org/issue45555
3. CWE-416: Use After Free - https://cwe.mitre.org/data/definitions/416.html

Exploit Details

To understand the exploitation potential for this use-after-free vulnerability, we need to delve into the workings of the heappushpop function.

The vulnerability arises because the heappushpop function does not properly manage the lifetime of the objects on the heap while performing a heap push and pop. If an object's lifetime is not carefully managed, it can be inadvertently freed by a garbage collector after it has been used in the heuristic calculation.

Consider the following code snippet

import heapq

class VulnerableObject:
    def __lt__(self, other):
        # Perform some operation that frees an object
        pass

    def __del__(self):
        # Code to execute when the object is garbage collected
        print("VulnerableObject collected")

# Prepare a heap with two VulnerableObject instances
heap = [VulnerableObject(), VulnerableObject()]

# Trigger the use-after-free vulnerability in heappushpop
heapq.heappushpop(heap, VulnerableObject())

In the code above, the __lt__ method of the VulnerableObject class is used for heapq comparisons. However, it also performs an operation that can potentially free an object. When executing the heappushpop function, the objects in the heap can be freed by the garbage collector, leading to a use-after-free scenario.

This use-after-free vulnerability can be exploited by an attacker to gain unauthorized access to memory, execute arbitrary code, or crash the Python application. To mitigate this vulnerability, developers should ensure that the objects' lifetimes are properly managed within the heapq module functions.

Conclusion

CVE-2022-48560 is a critical use-after-free vulnerability in Python up to version 3.9 via the heappushpop function in the heapq module. It is essential for developers using heapq to be aware of this vulnerability and take appropriate measures to ensure the safe management of objects' lifetimes in their implementations to mitigate potential exploitation risks.

Timeline

Published on: 08/22/2023 19:16:00 UTC
Last modified on: 08/26/2023 02:16:00 UTC