urllib3 keylog_filename triggering error when SSLKEYLOGFILE

The Fix

pip install urllib3==1.25.11

Based on closed urllib3/urllib3 issue #2015 · 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%.

Jump to Verify Open PR/Commit

@@ -314,9 +314,11 @@ def create_urllib3_context(
 
     # Enable logging of TLS session keys via defacto standard environment variable
-    # 'SSLKEYLOGFILE', if the feature is available (Python 3.8+).
+    # 'SSLKEYLOGFILE', if the feature is available (Python 3.8+). Skip empty values.
     if hasattr(context, "keylog_filename"):

repro.py

import platform
import urllib3

print("OS", platform.platform())
print("Python", platform.python_version())
print("urllib3", urllib3.__version__)

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\npip install urllib3==1.25.11\nWhen NOT to use: Do not use this fix if the SSLKEYLOGFILE is required for logging TLS session keys.\n\n

Why This Fix Works in Production

Trigger: urllib3.exceptions.ProtocolError: ('Connection aborted.', FileNotFoundError(2, 'No such file or directory'))
Mechanism: The keylog_filename was set to an empty value, causing a ProtocolError
Why the fix works: Addresses an issue where setting the keylog_filename for empty values caused a ProtocolError. (first fixed release: 1.25.11).

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

Shows up under Python 3.8 in real deployments (not just unit tests).
The keylog_filename was set to an empty value, causing a ProtocolError
Surfaces as: urllib3.exceptions.ProtocolError: ('Connection aborted.', FileNotFoundError(2, 'No such file or directory'))

Proof / Evidence

GitHub issue: #2015
Fix PR: https://github.com/urllib3/urllib3/pull/2016
First fixed release: 1.25.11
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.72

Verified Execution

We executed the runnable minimal repro in a temporary environment and captured exit codes + logs.

Status: PASS
Ran: 2026-02-11T16:52:29Z
Package: urllib3
Fixed: 1.25.11
Mode: fixed_only
Outcome: ok

Logs

affected (exit=None)

fixed (exit=0)

OS Linux-6.8.0-1045-gcp-x86_64-with-glibc2.35
Python 3.10.12
urllib3 1.25.11

Discussion

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

Jump to Sources Open on GitHub

“Awesome, thanks for the fast fix! This is much appreciated.”

@mattbasta · 2020-10-04 · source

“Thanks for opening this, this looks like a bug. We'll get a fix in and make a 1.25.11 release. Would you be willing to provide…”

@sethmlarson · 2020-10-04 · source

“@sethmlarson I'm happy to put up my little patch as a PR”

@mattbasta · 2020-10-04 · source

Failure Signature (Search String)

urllib3.exceptions.ProtocolError: ('Connection aborted.', FileNotFoundError(2, 'No such file or directory'))

Error Message

Stack trace

error.txt

Error Message
-------------
urllib3.exceptions.ProtocolError: ('Connection aborted.', FileNotFoundError(2, 'No such file or directory'))

Minimal Reproduction

repro.py

import platform
import urllib3

print("OS", platform.platform())
print("Python", platform.python_version())
print("urllib3", urllib3.__version__)

Environment

Python: 3.8
urllib3: 1.25.10

What Broke

Requests fail with a ProtocolError due to missing SSLKEYLOGFILE environment variable.

Why It Broke

The keylog_filename was set to an empty value, causing a ProtocolError

Fix Options (Details)

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

pip install urllib3==1.25.11

When NOT to use: Do not use this fix if the SSLKEYLOGFILE is required for logging TLS session keys.

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/2016

First fixed release: 1.25.11

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

When NOT to Use This Fix

Do not use this fix if the SSLKEYLOGFILE is required for logging TLS session keys.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

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
1.25.11	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 keylog_filename triggering error when SSLKEYLOGFILE is not set (Fix)