urllib3 Enable TLS 1.3 post-handshake authentication (Fix)

The Fix

pip install urllib3==1.25.4

Based on closed urllib3/urllib3 issue #1634 · 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

@@ -271,6 +271,13 @@ def create_urllib3_context(
     context.options |= options
 
+    # Enable post-handshake authentication for TLS 1.3, see GH #1634. PHA is
+    # necessary for conditional client cert authentication with TLS 1.3.
+    # The attribute is None for OpenSSL <= 1.1.0 or does not exist in older

fix.md

Option A — Upgrade to fixed release\npip install urllib3==1.25.4\nWhen NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.\n\n

Why This Fix Works in Production

Trigger: Enable TLS 1.3 post-handshake authentication
Mechanism: Enables post-handshake authentication for TLS 1.3 in urllib3, addressing issue #1634.
Why the fix works: Enables post-handshake authentication for TLS 1.3 in urllib3, addressing issue #1634. (first fixed release: 1.25.4).

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

Production symptom (often without a traceback): Enable TLS 1.3 post-handshake authentication

Proof / Evidence

GitHub issue: #1634
Fix PR: https://github.com/urllib3/urllib3/pull/1635
First fixed release: 1.25.4
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.70

Discussion

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

Jump to Sources Open on GitHub

“urllib3 should enable post-handshake authentication by default: * https://docs.python.org/3/library/ssl.html#ssl.SSLContext.post_handshake_auth * https://tools.ietf.org/html/rfc8446#section-4.6.2 PHA is well defined for HTTPS and necessary”

Issue thread · issue description · source

Failure Signature (Search String)

Enable TLS 1.3 post-handshake authentication
urllib3 should enable post-handshake authentication by default:

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Enable TLS 1.3 post-handshake authentication
urllib3 should enable post-handshake authentication by default:

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Enable TLS 1.3 post-handshake authentication
urllib3 should enable post-handshake authentication by default:

What Broke

Clients could not authenticate using client certificates based on HTTP request parameters.

Fix Options (Details)

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

pip install urllib3==1.25.4

When NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.

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/1635

First fixed release: 1.25.4

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

When NOT to Use This Fix

Do not use if it changes public behavior or if the failure cannot be reproduced.

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
1.25.4	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.