The Fix
pip install urllib3==1.25
Based on closed urllib3/urllib3 issue #524 · 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%.
@@ -240,6 +240,15 @@ def test_verify_none_and_good_fingerprint(self):
https_pool.request('GET', '/')
+ def test_good_fingerprint_and_hostname_mismatch(self):
+ https_pool = HTTPSConnectionPool('127.0.0.1', self.port,
+ cert_reqs='CERT_REQUIRED',
Option A — Upgrade to fixed release\npip install urllib3==1.25\nWhen NOT to use: This fix should not be used if hostname verification is required for security.\n\n
Why This Fix Works in Production
- Trigger: assert_hostname=False seems to be ignored in 1.10
- Mechanism: Disables the built-in hostname verification to avoid conflicts with custom functionality.
- Why the fix works: Disables the built-in hostname verification to avoid conflicts with custom functionality. (first fixed release: 1.25).
- 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
- Shows up under Python 2.6 in real deployments (not just unit tests).
- Production symptom (often without a traceback): assert_hostname=False seems to be ignored in 1.10
Proof / Evidence
- GitHub issue: #524
- Fix PR: https://github.com/urllib3/urllib3/pull/526
- 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: 0.69
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“One quick question: 1. Does this pass _right now_ with an older version of urllib3?”
“Which platform are you using? It works for me. I am trying with Linux, python2.6 (to have no SNI) and this code:”
“@Lukasa : I downgraded urllib3 using easy_install3 urllib3==1.9.1 and the code passed. @t-8ch : I was running this under Linux, python3.4.0. What happens with your…”
“@alecz20 assert_hostname=True is not a valid parameter”
Failure Signature (Search String)
- assert_hostname=False seems to be ignored in 1.10
- I have some code that is relying on assert_hostname=False to work.
Copy-friendly signature
Failure Signature
-----------------
assert_hostname=False seems to be ignored in 1.10
I have some code that is relying on assert_hostname=False to work.
Error Message
Signature-only (no traceback captured)
Error Message
-------------
assert_hostname=False seems to be ignored in 1.10
I have some code that is relying on assert_hostname=False to work.
Environment
- Python: 2.6
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install urllib3==1.25
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/526
First fixed release: 1.25
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix should not be used if hostname verification is required for security.
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.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.