urllib3 HTTPSConnection fails when assert

The Fix

pip install urllib3==1.26.4

Based on closed urllib3/urllib3 issue #2126 · 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

@@ -232,14 +232,13 @@ you could connect to a server by IP using HTTPS like so::
     >>> import urllib3
     >>> pool = urllib3.HTTPSConnectionPool(
-    ...     "10.0.0.10",
-    ...     assert_hostname="example.org",
-    ...     server_hostname="example.org"

repro.py

import urllib3

manager = urllib3.PoolManager(assert_hostname=True)
r = manager.request('GET', 'https://google.com/')
print(r.status)

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==1.26.4\nWhen NOT to use: This fix is not applicable if the hostname validation is intentionally bypassed.\n\n

Why This Fix Works in Production

Trigger: chunked=chunked,
Mechanism: HTTPSConnection fails to validate hostnames when assert_hostname is set to True
Why the fix works: Improves documentation for `assert_hostname` and clarifies its usage in HTTPS connections. (first fixed release: 1.26.4).

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

Shows up under Python 3.6.12 in real deployments (not just unit tests).
HTTPSConnection fails to validate hostnames when assert_hostname is set to True
Surfaces as: Traceback (most recent call last):

Proof / Evidence

GitHub issue: #2126
Fix PR: https://github.com/urllib3/urllib3/pull/2169
First fixed release: 1.26.4
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.41

Discussion

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

Jump to Sources Open on GitHub

“As said in the PR: we currently accept False, None and an actual hostname. We could document assert_hostname as an Optional[str], but would still have…”

@pquentin · 2021-02-23 · source

Failure Signature (Search String)

chunked=chunked,

Error Message

Stack trace

error.txt

Error Message
-------------
Traceback (most recent call last):
  File "<redacted>/lib/python3.6/site-packages/urllib3/connectionpool.py", line 677, in urlopen
    chunked=chunked,
  File "<redacted>/lib/python3.6/site-packages/urllib3/connectionpool.py", line 381, in _make_request
    self._validate_conn(conn)
  File "<redacted>/lib/python3.6/site-packages/urllib3/connectionpool.py", line 978, in _validate_conn
    conn.connect()
  File "<redacted>/lib/python3.6/site-packages/urllib3/connection.py", line 397, in connect
    _match_hostname(cert, self.assert_hostname or server_hostname)
  File "<redacted>/lib/python3.6/site-packages/urllib3/connection.py", line 407, in _match_hostname
    match_hostname(cert, asserted_hostname)
  File "<redacted>/lib/python3.6/ssl.py", line 327, in match_hostname
    % (hostname, ', '.join(map(repr, dnsnames))))
ssl.CertificateError: hostname True doesn't match either of '*.google.com', '*.android.com', '*.appengine.google.com', '*.bdn.dev', '*.cloud.google.com', '*.crowdsource.google.com', '*.datacompute.google.com', '*.g.co', '*.gcp.gvt2.com', '*.gcpcdn.gvt1.com', '*.ggpht.cn', '*.gkecnapps.cn', '*.google-analytics.com', '*.google.ca', '*.google.cl', '*.google.co.in', '*.google.co.jp', '*.google.co.uk', '*.google.com.ar', '*.google.com.au', '*.google.com.br', '*.google.com.co', '*.google.com.mx', '*.google.com.tr', '*.google.com.vn', '*.google.de', '*.google.es', '*.goog
... (truncated) ...

Minimal Reproduction

repro.py

import urllib3

manager = urllib3.PoolManager(assert_hostname=True)
r = manager.request('GET', 'https://google.com/')
print(r.status)

Environment

Python: 3.6.12
urllib3: 1.25.11

What Broke

Users experience SSL certificate errors when connecting to HTTPS servers.

Why It Broke

HTTPSConnection fails to validate hostnames when assert_hostname is set to True

Fix Options (Details)

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

pip install urllib3==1.26.4

When NOT to use: This fix is not applicable if the hostname validation is intentionally bypassed.

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/2169

First fixed release: 1.26.4

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

When NOT to Use This Fix

This fix is not applicable if the hostname validation is intentionally bypassed.

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
1.26.4	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.

urllib3 HTTPSConnection fails when assert_hostname is True (Fix)