CVE-2020-14422: Python ipaddress.py DoS Vulnerability

CVE-2020-14422 Overview

CVE-2020-14422 is an Algorithmic Complexity Attack vulnerability in Python's Lib/ipaddress.py module that improperly computes hash values in the IPv4Interface and IPv6Interface classes. This flaw might allow a remote attacker to cause a denial of service if an application is affected by the performance of a dictionary containing IPv4Interface or IPv6Interface objects, and the attacker can cause many dictionary entries to be created.

Critical Impact

Applications using Python's ipaddress module to process untrusted network interface data in dictionary structures may experience severe performance degradation or denial of service conditions when an attacker triggers hash collisions.

Affected Products

• Python versions through 3.8.3
• openSUSE Leap 15.1 and 15.2
• Fedora 31 and 32
• Oracle Enterprise Manager Ops Center 12.4.0.0

Discovery Timeline

• June 18, 2020 - CVE-2020-14422 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-14422

Vulnerability Analysis

The vulnerability exists in Python's ipaddress.py library, specifically within the hash computation methods for IPv4Interface and IPv6Interface classes. When these objects are used as dictionary keys, the improper hash value computation can lead to excessive hash collisions. In Python dictionaries, hash collisions cause lookups and insertions to degrade from O(1) average time complexity to O(n) worst-case complexity.

An attacker who can control the IP interface values being stored in a dictionary can craft inputs that all hash to the same or similar bucket values. As the number of collisions grows, dictionary operations become increasingly expensive, consuming CPU resources and potentially rendering the application unresponsive. This is classified under CWE-330 (Use of Insufficiently Random Values).

Root Cause

The root cause is the improper hash value calculation algorithm used in the IPv4Interface and IPv6Interface classes. The hash function did not produce sufficiently distributed hash values across the possible input space, making it trivial for an attacker to generate inputs that would collide in Python's internal hash table implementation. The fix, implemented in Python GitHub Pull Request #20956, addresses the hash computation to ensure proper distribution of hash values.

Attack Vector

The attack requires network access where an attacker can cause an application to create multiple IPv4Interface or IPv6Interface objects and store them in dictionary structures. This could occur in applications that:

• Process network configuration data from untrusted sources
• Handle IP address information in web services or APIs
• Parse network interface data from user-controlled input
• Maintain caches or lookups keyed by IP interface objects

The attacker sends specially crafted IP interface values designed to produce hash collisions. As the dictionary grows with colliding entries, normal dictionary operations become computationally expensive, degrading application performance. The attack complexity is high as it requires specific application behaviors and the ability to influence dictionary contents with controlled interface values.

Detection Methods for CVE-2020-14422

Indicators of Compromise

• Unusual CPU consumption spikes in Python processes handling network interface data
• Abnormally slow dictionary operations in applications using ipaddress module
• Large numbers of IPv4Interface or IPv6Interface objects being created in short time periods
• Application timeouts or unresponsiveness when processing IP interface data

Detection Strategies

• Monitor Python application performance metrics for degradation patterns consistent with hash collision attacks
• Implement application-level logging to track the creation rate of IPv4Interface and IPv6Interface objects
• Review application code for dictionary usage patterns involving IP interface objects from untrusted sources
• Deploy runtime analysis tools to identify excessive hash collisions in Python dictionaries

Monitoring Recommendations

• Set up alerts for abnormal CPU utilization in Python applications processing network data
• Implement rate limiting on endpoints that accept IP interface data from external sources
• Monitor application response times for gradual degradation indicative of algorithmic complexity attacks
• Track memory allocation patterns for dictionary growth anomalies

How to Mitigate CVE-2020-14422

Immediate Actions Required

• Upgrade Python to a patched version: v3.5.10, v3.6.12, v3.7.9, v3.8.4 or later, or v3.9.0 or later
• Audit applications for usage of IPv4Interface and IPv6Interface objects in dictionaries with untrusted input
• Implement input validation and rate limiting on endpoints processing IP interface data
• Consider using alternative data structures or IP address string representations as dictionary keys until patching is complete

Patch Information

The vulnerability is fixed in multiple Python versions. Apply the appropriate update based on your Python branch:

For detailed patch information, see the Python Issue Tracker Entry and the Python GitHub Pull Request.

Distribution-specific advisories are available from openSUSE, Debian LTS, and Gentoo GLSA.

Workarounds

• Avoid using IPv4Interface or IPv6Interface objects as dictionary keys when processing untrusted data
• Use string representations of IP addresses as dictionary keys instead of interface objects
• Implement application-level limits on the number of IP interface objects that can be created per request
• Consider using IPv4Address or IPv6Address objects instead of interface classes where network mask information is not required

# Check current Python version
python3 --version

# Update Python on Debian/Ubuntu systems
sudo apt update && sudo apt upgrade python3

# Update Python on Fedora systems
sudo dnf upgrade python3

# Update Python on openSUSE systems
sudo zypper update python3