The Fix
pip install urllib3==1.25
Based on closed urllib3/urllib3 issue #572 · 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%.
@@ -38,8 +38,6 @@
:var DEFAULT_SSL_CIPHER_LIST: The list of supported SSL/TLS cipher suites.
- Default: ``ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:
- ECDH+3DES:DH+3DES:RSA+AESGCM:RSA+AES:RSA+3DES:!aNULL:!MD5:!DSS``
Option A — Upgrade to fixed release\npip install urllib3==1.25\nWhen NOT to use: This fix is not suitable if you require the latest ciphers from the standard library.\n\n
Why This Fix Works in Production
- Trigger: except ImportError:
- Mechanism: urllib3 was using the standard library's default ciphers instead of its own
- Why the fix works: Ignores the standard library's default ciphers and sets its own default ciphers for urllib3. (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
- urllib3 was using the standard library's default ciphers instead of its own
- Production symptom (often without a traceback): from ssl import _DEFAULT_CIPHERS
Proof / Evidence
- GitHub issue: #572
- Fix PR: https://github.com/urllib3/urllib3/pull/588
- 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 feel like the risk here is pretty small, all things considered.”
“We should also reuse this in contrib/pyopenssl.py”
“I almost think that trying to do something like that is getting too clever for our own good”
“So I have the change here https://github.com/shazow/urllib3/compare/master...sigmavirus24:patch-1 The thing I'm concerned about is the following situation (which I'm uncertain of the likelihood)”
Failure Signature (Search String)
- except ImportError:
Copy-friendly signature
Failure Signature
-----------------
from ssl import _DEFAULT_CIPHERS
except ImportError:
Error Message
Signature-only (no traceback captured)
Error Message
-------------
from ssl import _DEFAULT_CIPHERS
except ImportError:
What Broke
This could lead to less secure ciphers being used in production environments.
Why It Broke
urllib3 was using the standard library's default ciphers instead of its own
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/588
First fixed release: 1.25
Last verified: 2026-02-09. Validate in your environment.
When NOT to Use This Fix
- This fix is not suitable if you require the latest ciphers from the standard library.
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.