Excessive hash collisions in IPv4Network and IPv6Network classes

[mssalvatore]

Bug report

Bug description:

While working with IPv4Network objects, @cakekoa and I discovered that hash collisions were common. Below is a small script that shows a few examples of different networks that have hash collisions.

from ipaddress import IPv4Network, IPv6Network


def test_hash_collision(network_1, network_2):
    # Shows that the networks are not equivalent.
    assert network_1 != network_2
    assert network_1.num_addresses != network_2.num_addresses

    # Shows a hash collision similar to CVE-2020-14422
    assert hash(network_1) == hash(network_2)


test_hash_collision(IPv4Network("192.168.1.255/32"), IPv4Network("192.168.1.0/24"))
test_hash_collision(IPv4Network("172.24.255.0/24"), IPv4Network("172.24.0.0/16"))
test_hash_collision(IPv4Network("192.168.1.87/32"), IPv4Network("192.168.1.86/31"))
test_hash_collision(
    IPv4Network("10.0.0.0/8"), IPv6Network("ffff:ffff:ffff:ffff:ffff:ffff:aff:0/112")
)

Upon investigating, we discovered CVE-2020-14422, which fixed a similar (albeit much more severe) hash collision in the IPv4Interface and IPv6Interface classes. This CVE was fixed in b98e779.

The implementation of _BaseNetwork.__hash() looks like this on the main branch:

    def __hash__(self):
        return hash(int(self.network_address) ^ int(self.netmask))

Based on the fix for CVE-2020-14422, the fix for the _BaseNetwork class would likely look like:

diff --git a/Lib/ipaddress.py b/Lib/ipaddress.py
index 703fa289dda..d8a84f33264 100644
--- a/Lib/ipaddress.py
+++ b/Lib/ipaddress.py
@@ -729,7 +729,7 @@ def __eq__(self, other):
             return NotImplemented
 
     def __hash__(self):
-        return hash(int(self.network_address) ^ int(self.netmask))
+        return hash((int(self.network_address), int(self.netmask)))
 
     def __contains__(self, other):
         # always false if one is v4 and the other is v6.

As this method produces far fewer collisions than what caused CVE-2020-14422, the security impact is likely negligible. @sethmlarson from the PSRT team has given us the green light to publicly submit a fix.

CPython versions tested on:

3.12

Operating systems tested on:

Linux, macOS

Linked PRs

• gh-134062: Fix hash collisions in IPv4Network and IPv6Network #134063
• [3.14] gh-134062: Fix hash collisions in IPv4Network and IPv6Network (GH-134063) #134476
• [3.13] gh-134062: Fix hash collisions in IPv4Network and IPv6Network (GH-134063) #134477
• [3.12] gh-134062: Fix hash collisions in IPv4Network and IPv6Network (GH-134063) #134478
• [3.11] gh-134062: Fix hash collisions in IPv4Network and IPv6Network (GH-134063) #134479
• [3.10] gh-134062: Fix hash collisions in IPv4Network and IPv6Network (GH-134063) #134480
• [3.9] gh-134062: Fix hash collisions in IPv4Network and IPv6Network (GH-134063) #134481