urllib3 Respect SSLContext.hostname_checks_common

The Fix

pip install urllib3==1.26.9

Based on closed urllib3/urllib3 issue #2192 · 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

@@ -40,6 +40,10 @@ jobs:
             experimental: false
             nox-session: test-pypy
+          - python-version: "pypy-3.8"
+            os: ubuntu-latest
+            experimental: false

fix.md

Option A — Upgrade to fixed release\npip install urllib3==1.26.9\nWhen NOT to use: Do not use this fix if your application relies on the previous behavior of ignoring commonName checks.\n\nOption C — Workaround\nfor previous OpenSSL versions, https://bugs.python.org/issue43522.\nWhen NOT to use: Do not use this fix if your application relies on the previous behavior of ignoring commonName checks.\n\n

Why This Fix Works in Production

Trigger: Respect SSLContext.hostname_checks_common_name set manually by users
Mechanism: Respects the SSLContext.hostname_checks_common_name setting, allowing users to enforce commonName checks.
Why the fix works: Respects the SSLContext.hostname_checks_common_name setting, allowing users to enforce commonName checks. (first fixed release: 1.26.9).

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

Triggered by an upgrade/regression window: 3.8.9–3.9.3 breaks; 1.26.9 is the first fixed release.
Shows up under Python 3.8.9 in real deployments (not just unit tests).
Production symptom (often without a traceback): Respect SSLContext.hostname_checks_common_name set manually by users

Proof / Evidence

GitHub issue: #2192
Fix PR: https://github.com/urllib3/urllib3/pull/2518
First fixed release: 1.26.9
Affected versions: 3.8.9–3.9.3
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.74

Discussion

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

Jump to Sources Open on GitHub

“The issue is now fixed in OpenSSL and Python has a workaround for previous OpenSSL versions, https://bugs.python.org/issue43522”

@tiran · 2021-04-17 · confirmation · source

“Adding Breaking Change label solely for being considered a blocker of v2.0”

@sethmlarson · 2021-03-21 · source

Failure Signature (Search String)

Respect SSLContext.hostname_checks_common_name set manually by users
If a user sets `SSLContext.hostname_checks_common_name` we should respect their request and check commonName. This provides a fallback we can point to in case users rely on

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Respect SSLContext.hostname_checks_common_name set manually by users
If a user sets `SSLContext.hostname_checks_common_name` we should respect their request and check commonName. This provides a fallback we can point to in case users rely on commonName support.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Respect SSLContext.hostname_checks_common_name set manually by users
If a user sets `SSLContext.hostname_checks_common_name` we should respect their request and check commonName. This provides a fallback we can point to in case users rely on commonName support.

Environment

Python: 3.8.9

What Broke

Users experienced SSL certificate verification failures due to ignored commonName checks.

Fix Options (Details)

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

pip install urllib3==1.26.9

When NOT to use: Do not use this fix if your application relies on the previous behavior of ignoring commonName checks.

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

Option C — Workaround Temporary workaround

for previous OpenSSL versions, https://bugs.python.org/issue43522.

When NOT to use: Do not use this fix if your application relies on the previous behavior of ignoring commonName checks.

Use only if you cannot change versions today. Treat this as a stopgap and remove once upgraded.

Fix reference: https://github.com/urllib3/urllib3/pull/2518

First fixed release: 1.26.9

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

When NOT to Use This Fix

Do not use this fix if your application relies on the previous behavior of ignoring commonName checks.

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
3.8.9	Broken
3.9.3	Broken
1.26.9	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 Respect SSLContext.hostname_checks_common_name set manually by users (Fix)