The Fix
pip install urllib3==1.25.9
Based on closed urllib3/urllib3 issue #1892 · 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%.
@@ -116,6 +116,11 @@ def certificate(self, request):
return Response(json.dumps(subject))
+ def alpn_protocol(self, request):
+ """Return the selected ALPN protocol."""
+ proto = request.connection.stream.socket.selected_alpn_protocol()
Option A — Upgrade to fixed release\npip install urllib3==1.25.9\nWhen NOT to use: Do not use this fix if the server does not support ALPN.\n\n
Why This Fix Works in Production
- Trigger: It ended up being a 200 lines fix, as we support PyOpenSSL, SecureTransport, and had to test this. But it's in the master branch now :)
- Mechanism: Some servers require ALPN extension to avoid forced HTTPS connection shutdowns
- Why the fix works: Added ALPN support to urllib3, allowing it to send the http/1.1 ALPN extension by default, which is required by some servers. (first fixed release: 1.25.9).
- 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
- Some servers require ALPN extension to avoid forced HTTPS connection shutdowns
- Production symptom (often without a traceback): It ended up being a 200 lines fix, as we support PyOpenSSL, SecureTransport, and had to test this. But it's in the master branch now :)
Proof / Evidence
- GitHub issue: #1892
- Fix PR: https://github.com/urllib3/urllib3/pull/1894
- First fixed release: 1.25.9
- Reproduced locally: No (not executed)
- Last verified: 2026-02-09
- Confidence: 0.95
- Did this fix it?: Yes (upstream fix exists)
- Own content ratio: 0.68
Discussion
High-signal excerpts from the issue thread (symptoms, repros, edge-cases).
“Thanks for opening this! I agree we should implement this.”
“> The fix is a trivial one-liner. It ended up being a 200 lines fix, as we support PyOpenSSL, SecureTransport, and had to test this.…”
“I should know better that anything with the TLS label should disqualify the Contributor Friendly label just based on what we've got going in the…”
Failure Signature (Search String)
- It ended up being a 200 lines fix, as we support PyOpenSSL, SecureTransport, and had to test this. But it's in the master branch now :)
- I should know better that anything with the `TLS` label should disqualify the `Contributor Friendly` label just based on what we've got going in the TLS department. :)
Copy-friendly signature
Failure Signature
-----------------
It ended up being a 200 lines fix, as we support PyOpenSSL, SecureTransport, and had to test this. But it's in the master branch now :)
I should know better that anything with the `TLS` label should disqualify the `Contributor Friendly` label just based on what we've got going in the TLS department. :)
Error Message
Signature-only (no traceback captured)
Error Message
-------------
It ended up being a 200 lines fix, as we support PyOpenSSL, SecureTransport, and had to test this. But it's in the master branch now :)
I should know better that anything with the `TLS` label should disqualify the `Contributor Friendly` label just based on what we've got going in the TLS department. :)
What Broke
Clients may experience forced connection shutdowns when ALPN is not sent.
Why It Broke
Some servers require ALPN extension to avoid forced HTTPS connection shutdowns
Fix Options (Details)
Option A — Upgrade to fixed release Safe default (recommended)
pip install urllib3==1.25.9
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/1894
First fixed release: 1.25.9
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- Do not use this fix if the server does not support ALPN.
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 |
|---|---|
| 1.25.9 | Fixed |
Related Issues
No related fixes found.
Sources
We don’t republish the full GitHub discussion text. Use the links above for context.