urllib3 Lean on SSLContext to do HTTPS hostname

The Fix

pip install urllib3==1.26.5

Based on closed urllib3/urllib3 issue #517 · 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

@@ -30,7 +30,7 @@ class BaseSSLError(BaseException):
 )
 from .packages.ssl_match_hostname import CertificateError, match_hostname
-from .util import SKIP_HEADER, SKIPPABLE_HEADERS, connection
+from .util import SKIP_HEADER, SKIPPABLE_HEADERS, connection, ssl_
 from .util.ssl_ import (

repro.py

def __init__(self, poolmanager_server_hostname, *args, **kwargs):                                                                                                                                                                                                                                                                             
    self.poolmanager_server_hostname = poolmanager_server_hostname                                                                                                                                                                                                                                                                            
                                                                                                                                                                                                                                                                                                                                              
    requests.adapters.HTTPAdapter.__init__(self, *args, **kwargs)                                                                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                                                                                              
def init_poolmanager(self, *args, **kwargs):                                                                                                                                                                                                                                                                                                  
    self.poolmanager = requests.adapters.PoolManager(                                                                                                                                                                                                                                                                                         
        *args,                                                                                                                                                                                                                                                                                                                                
        server_hostname=self.poolmanager_server_hostname,                                                                                                                                                                                                                                                                                     
        **kwargs)

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==1.26.5\nWhen NOT to use: This fix should not be used if compatibility with older Python versions is required.\n\n

Why This Fix Works in Production

Trigger: ... (truncated) ...
Mechanism: urllib3 did not utilize SSLContext's check_hostname feature for hostname verification
Why the fix works: Modifies urllib3 to lean on SSLContext for hostname verification when possible, addressing issue #517. (first fixed release: 1.26.5).

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 3.4 in real deployments (not just unit tests).
urllib3 did not utilize SSLContext's check_hostname feature for hostname verification
Surfaces as: Traceback (most recent call last):\n File "/usr/local/lib/python2.7/site-packages/pip-6.0.7-py2.7.egg/pip/basecommand.py", line 232, in main\n status = self.run(options, args)\n…

Proof / Evidence

GitHub issue: #517
Fix PR: https://github.com/urllib3/urllib3/pull/2178
First fixed release: 1.26.5
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.32

Discussion

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

Jump to Sources Open on GitHub

“Currently, it appears that because urllib3 **doesn't** use this, an exception is raised. I haven't had a chance to isolate this yet, but a pip…”

@mjpieters · 2015-02-04 · source

“Followup: This is pip 6.0.7 (obvious from the traceback), which uses requests 2.5.1 (see pypa/pip@ec51b69cd73125e8bb37029d936c57f29b1a00fa) which in turn uses a urllib3 checkout with SHA @a27758625e4169330fcf965652b1093faf5aaaa2…”

@mjpieters · 2015-02-04 · source

“1”

@sethmlarson · 2019-01-21 · source

“It's a new ticket, and it's what we should have opened when I closed kennethreitz/requests#2435.”

@Lukasa · 2015-02-04 · source

Failure Signature (Search String)

... (truncated) ...

Error Message

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):\n  File "/usr/local/lib/python2.7/site-packages/pip-6.0.7-py2.7.egg/pip/basecommand.py", line 232, in main\n    status = self.run(options, args)\n  File "/usr/local/lib/python2.7/site-packages/pip-6.0.7-py2.7.egg/pip/commands/install.py", line 339, in run\n    requirement_set.prepare_files(finder)\n  File "/usr/local/lib/python2.7/site-packages/pip-6.0.7-py2.7.egg/pip/req/req_set.py", line 333, in prepare_files\n    upgrade=self.upgrade,\n  File "/usr/local/lib/python2.7/site-packages/pip-6.0.7-py2.7.egg/pip/index.py", line 305, in find_requirement\n    page = self._get_page(main_index_url, req)\n  File "/usr/local/lib/python2.7/site-packages/pip-6.0.7-py2.7.egg/pip/index.py", line 783, in _get_page\n    return HTMLPage.get_page(link, req, session=self.session)\n  File "/usr/local/lib/python2.7/site-packages/pip-6.0.7-py2.7.egg/pip/index.py", line 872, in get_page\n    "Cache-Control": "max-age=600",\n  File "/usr/local/lib/python2.7/site-packages/pip-6.0.7-py2.7.egg/pip/_vendor/requests/sessions.py", line 473, in get\n    return self.request('GET', url, **kwargs)\n  File "/usr/local/lib/python2.7/site-packages/pip-6.0.7-py2.7.egg/pip/download.py", line 365, in request\n    return super(PipSession, self).request(method, url, *args, **kwargs)\n  File "/usr/local/lib/python2.7/site-packages/pip-6.0.7-py2.7.egg/pip/_vendor/requests/sessions.py",
... (truncated) ...

Minimal Reproduction

repro.py

def __init__(self, poolmanager_server_hostname, *args, **kwargs):                                                                                                                                                                                                                                                                             
    self.poolmanager_server_hostname = poolmanager_server_hostname                                                                                                                                                                                                                                                                            
                                                                                                                                                                                                                                                                                                                                              
    requests.adapters.HTTPAdapter.__init__(self, *args, **kwargs)                                                                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                                                                                                              
def init_poolmanager(self, *args, **kwargs):                                                                                                                                                                                                                                                                                                  
    self.poolmanager = requests.adapters.PoolManager(                                                                                                                                                                                                                                                                                         
        *args,                                                                                                                                                                                                                                                                                                                                
        server_hostname=self.poolmanager_server_hostname,                                                                                                                                                                                                                                                                                     
        **kwargs)

Environment

Python: 3.4

What Broke

Users experienced exceptions during HTTPS requests due to hostname verification failures.

Why It Broke

urllib3 did not utilize SSLContext's check_hostname feature for hostname verification

Fix Options (Details)

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

pip install urllib3==1.26.5

When NOT to use: This fix should not be used if compatibility with older Python versions is required.

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/2178

First fixed release: 1.26.5

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

When NOT to Use This Fix

This fix should not be used if compatibility with older Python versions is required.

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
1.26.5	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 Lean on SSLContext to do HTTPS hostname verification (Fix)