The Fix
pip install urllib3==1.25
Based on closed urllib3/urllib3 issue #816 · 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%.
@@ -74,15 +74,18 @@ def add_stderr_logger(level=logging.DEBUG):
+# All warning filters *must* be appended unless you're really certain that they
+# shouldn't be: otherwise, it's very hard for users to use most Python
+# mechanisms to silence them.
Option A — Upgrade to fixed release\npip install urllib3==1.25\nWhen NOT to use: Do not apply this fix if you require SNIMissingWarning for debugging purposes.\n\n
Why This Fix Works in Production
- Trigger: Users see persistent SNIMissingWarning messages during HTTPS requests.
- Mechanism: SNIMissingWarning is not silenced due to improper warning filter configuration
- Why the fix works: Appends warning filters to ensure SNIMissingWarning can be silenced appropriately. (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 2.7 in real deployments (not just unit tests).
- SNIMissingWarning is not silenced due to improper warning filter configuration
- Production symptom (often without a traceback): Users see persistent SNIMissingWarning messages during HTTPS requests.
Proof / Evidence
- GitHub issue: #816
- Fix PR: https://github.com/urllib3/urllib3/pull/817
- 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: 0.79
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“I've not tried the patch file per se, but redid the same modification in place, and it worked. Thanks. Please tell me when there's a…”
“Arg, yes, good spot. Want to try the patch from #817 to see if that solves your problem?”
Failure Signature (Search String)
- Users see persistent SNIMissingWarning messages during HTTPS requests.
Copy-friendly signature
Failure Signature
-----------------
Users see persistent SNIMissingWarning messages during HTTPS requests.
Error Message
Signature-only (no traceback captured)
Error Message
-------------
Users see persistent SNIMissingWarning messages during HTTPS requests.
Environment
- Python: 2.7
What Broke
Users see persistent SNIMissingWarning messages during HTTPS requests.
Why It Broke
SNIMissingWarning is not silenced due to improper warning filter configuration
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/817
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 you require SNIMissingWarning for debugging purposes.
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.