urllib3 Resolve environment variables (and home directory?)

The Fix

pip install urllib3==2.6.0

Based on closed urllib3/urllib3 issue #3700 · 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

@@ -0,0 +1 @@
@@ -0,0 +1 @@
+Fixed set ``ssl.SSLContext.keylog_filename`` with expanded environment variable.
diff --git a/src/urllib3/util/ssl_.py b/src/urllib3/util/ssl_.py
index 37794d213a..56fe9093ad 100644

repro.py

import platform
import ssl
import urllib3

print("OS", platform.platform())
print("Python", platform.python_version())
print(ssl.OPENSSL_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==2.6.0\nWhen NOT to use: This fix is not applicable if the environment variable is not needed.\n\n

Why This Fix Works in Production

Trigger: context.keylog_filename = sslkeylogfile
Mechanism: Environment variable SSLKEYLOGFILE was not resolving nested variables correctly on Windows
Why the fix works: Expands the environment variable of SSLKEYLOGFILE to resolve paths correctly on Windows. (first fixed release: 2.6.0).

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 3.12 in real deployments (not just unit tests).
Environment variable SSLKEYLOGFILE was not resolving nested variables correctly on Windows
Surfaces as: File "C:\Program Files\Python312\Lib\site-packages\urllib3\util\ssl_.py", line 369, in create_urllib3_context

Proof / Evidence

GitHub issue: #3700
Fix PR: https://github.com/urllib3/urllib3/pull/3705
First fixed release: 2.6.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.66

Discussion

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

Jump to Sources Open on GitHub

“We should use an already implemented function for resolving the environment variables, like os.path.expandvars()?”

@sethmlarson · 2025-10-22 · source

“I have searched a little deeper and made tests using the ExpandEnvironmentStrings* functions from userenv.h and those functions are not recursive”

@mbouat · 2025-10-22 · source

Failure Signature (Search String)

context.keylog_filename = sslkeylogfile

Error Message

Stack trace

error.txt

Error Message
-------------
File "C:\Program Files\Python312\Lib\site-packages\urllib3\util\ssl_.py", line 369, in create_urllib3_context
    context.keylog_filename = sslkeylogfile
    ^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '%USERPROFILE%\\AppData\\ssl-keys.log'

Minimal Reproduction

repro.py

import platform
import ssl
import urllib3

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

Environment

Python: 3.12
urllib3: 2.5.0

What Broke

Requests fail with FileNotFoundError due to unresolved SSLKEYLOGFILE path.

Why It Broke

Environment variable SSLKEYLOGFILE was not resolving nested variables correctly on Windows

Fix Options (Details)

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

pip install urllib3==2.6.0

When NOT to use: This fix is not applicable if the environment variable is not needed.

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/3705

First fixed release: 2.6.0

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

When NOT to Use This Fix

This fix is not applicable if the environment variable is not needed.

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
2.6.0	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 Resolve environment variables (and home directory?) within SSLKEYLOGFILE path (Fix)