urllib3 Must remove square brackets from IPv6 addresses

The Fix

pip install urllib3==1.25

Based on closed urllib3/urllib3 issue #760 · PR/commit linked

Production note: Most teams hit this during upgrades or environment changes. Roll out with a canary and smoke critical endpoints (health, OpenAPI/docs) before 100%.

Open PR/Commit

@@ -0,0 +1,16 @@
@@ -0,0 +1,16 @@
+-----BEGIN CERTIFICATE-----
+MIIClTCCAX2gAwIBAgIBATANBgkqhkiG9w0BAQsFADAOMQwwCgYDVQQDDAM6OjEw
+HhcNMTUxMjE1MTY0NjQxWhcNMjEwODAyMDAwMDAwWjAOMQwwCgYDVQQDDAM6OjEw

fix.md

Option A — Upgrade to fixed release\npip install urllib3==1.25\nWhen NOT to use: Do not apply this fix if the hostname is not an IPv6 address.\n\n

Why This Fix Works in Production

Trigger: Apparently, while `httplib` itself doesn't care about the square brackets, the `match_hostname` function needs them gone. I suggest we move the stripping down…
Mechanism: The match_hostname function fails to handle IPv6 addresses with square brackets
Why the fix works: Strips brackets from hostnames before validating, resolving issue #760. (first fixed release: 1.25).

Production impact:

If left unfixed, the same config can fail only in production (env differences), causing startup failures or partial feature outages.

Why This Breaks in Prod

The match_hostname function fails to handle IPv6 addresses with square brackets
Production symptom (often without a traceback): Apparently, while `httplib` itself doesn't care about the square brackets, the `match_hostname` function needs them gone. I suggest we move the stripping down into our `ssl.py` module.

Proof / Evidence

GitHub issue: #760
Fix PR: https://github.com/urllib3/urllib3/pull/761
First fixed release: 1.25
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.76

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“This is the openstack bug, FYI: https://bugs.launchpad.net/nova/+bug/1526413”

@mriedem · 2015-12-15 · source

“@Lukasa seems to be working on this so I'm labelling it in progress and assigning him :-P”

@sigmavirus24 · 2015-12-15 · source

Failure Signature (Search String)

Apparently, while `httplib` itself doesn't care about the square brackets, the `match_hostname` function needs them gone. I suggest we move the stripping down into our `ssl.py`

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Apparently, while `httplib` itself doesn't care about the square brackets, the `match_hostname` function needs them gone. I suggest we move the stripping down into our `ssl.py` module.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Apparently, while `httplib` itself doesn't care about the square brackets, the `match_hostname` function needs them gone. I suggest we move the stripping down into our `ssl.py` module.

What Broke

Users experience SSL validation failures with IPv6 addresses due to incorrect formatting.

Why It Broke

The match_hostname function fails to handle IPv6 addresses with square brackets

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

pip install urllib3==1.25

When NOT to use: Do not apply this fix if the hostname is not an IPv6 address.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/urllib3/urllib3/pull/761

First fixed release: 1.25

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

Do not apply this fix if the hostname is not an IPv6 address.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Add a CI check that diffs key outputs after upgrades (OpenAPI schema snapshots, JSON payload shapes, CLI output).
Upgrade behind a canary and run integration tests against the canary before 100% rollout.
Add a TLS smoke test that performs a real handshake in CI (include CA bundle validation and hostname checks).
Alert on handshake failures by error string and endpoint to catch cert/CA changes quickly.

Version Compatibility Table

Version	Status
1.25	Fixed

Related Issues

No related fixes found.

Cluster: urllib3:configuration-error urllib3 hub urllib3 best practices All hubs All clusters

Related clusters: Data consistency SSL handshake Other

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

urllib3 Must remove square brackets from IPv6 addresses when using match_hostname (Fix)