urllib3 Unclosed socket warning after HTTP 407 response

The Fix

pip install urllib3==1.26.20

Based on closed urllib3/urllib3 issue #3374 · 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 ResourceWarning on CONNECT with Python < 3.11.4 by backporting https://github.com/python/cpython/issues/103472.
diff --git a/src/urllib3/connection.py b/src/urllib3/connection.py
index 9f47b1b25e..7cbef7d5a9 100644

repro.py

from http.server import BaseHTTPRequestHandler
from socketserver import ThreadingTCPServer


class HTTPConnectProxyHandler(BaseHTTPRequestHandler):
    protocol_version = 'HTTP/1.1'
    default_request_version = 'HTTP/1.1'

    def do_CONNECT(self):
        self.send_response(407)
        self.send_header('Proxy-Authenticate', 'Basic')
        self.end_headers()


ThreadingTCPServer(('127.0.0.1', 55556), HTTPConnectProxyHandler).serve_forever()

verify

Re-run: python3.10 -Werror test.py

fix.md

Option A — Upgrade to fixed release\npip install urllib3==1.26.20\nWhen NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.\n\n

Why This Fix Works in Production

Trigger: $ python3.10 -Werror test.py
Mechanism: ResourceWarnings were not properly handled for HTTP CONNECT responses with status 407 in urllib3
Why the fix works: Backports a fix for ResourceWarnings on CONNECT with Python versions lower than 3.11.4, addressing unclosed socket warnings after receiving a 407 response from an HTTP CONNECT proxy. (first fixed release: 1.26.20).

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 3.10 in real deployments (not just unit tests).
ResourceWarnings were not properly handled for HTTP CONNECT responses with status 407 in urllib3
Surfaces as: $ python3.10 -Werror test.py

Proof / Evidence

GitHub issue: #3374
Fix PR: https://github.com/urllib3/urllib3/pull/3252
First fixed release: 1.26.20
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.29

Discussion

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

Jump to Sources Open on GitHub

“Perhaps related to https://github.com/urllib3/urllib3/pull/3252?”

@coletdjnz · 2024-05-03 · source

“(I'm going to consider that #3252 fixed it until I hear otherwise, since this is the exact same issue.)”

@pquentin · 2024-08-13 · source

“> @coletdjnz can you repeat on 3.11.3? I don't have 3.11.3 on hand, but testing with 3.10.14 on main branch it appears to be fixed…”

@coletdjnz · 2024-09-08 · source

Failure Signature (Search String)

$ python3.10 -Werror test.py

Error Message

Stack trace

error.txt

Error Message
-------------
$ python3.10 -Werror test.py

Traceback (most recent call last):
  File "/tmp/venv/lib64/python3.10/site-packages/urllib3/connectionpool.py", line 779, in urlopen
    self._prepare_proxy(conn)
  File "/tmp/venv/lib64/python3.10/site-packages/urllib3/connectionpool.py", line 1048, in _prepare_proxy
    conn.connect()
  File "/tmp/venv/lib64/python3.10/site-packages/urllib3/connection.py", line 633, in connect
    self._tunnel()  # type: ignore[attr-defined]
  File "/usr/lib64/python3.10/http/client.py", line 925, in _tunnel
    raise OSError(f"Tunnel connection failed: {code} {message.strip()}")
OSError: Tunnel connection failed: 407 Proxy Authentication Required

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/tmp/test.py", line 3, in <module>
    pm.urlopen('GET', 'https://example.com')
  File "/tmp/venv/lib64/python3.10/site-packages/urllib3/poolmanager.py", line 634, in urlopen
    return super().urlopen(method, url, redirect=redirect, **kw)
  File "/tmp/venv/lib64/python3.10/site-packages/urllib3/poolmanager.py", line 444, in urlopen
    response = conn.urlopen(method, u.request_uri, **kw)
  File "/tmp/venv/lib64/python3.10/site-packages/urllib3/connectionpool.py", line 847, in urlopen
    retries = retries.increment(
  File "/tmp/venv/lib64/python3.10/site-packages/urllib3/util/retry.py", line 445, in increm
... (truncated) ...

Stack trace

error.txt

Error Message
-------------
$ python3.10 -Werror test.py

Traceback (most recent call last):
  File "/tmp/venv/lib64/python3.10/site-packages/urllib3/connectionpool.py", line 779, in urlopen
    self._prepare_proxy(conn)
  File "/tmp/venv/lib64/python3.10/site-packages/urllib3/connectionpool.py", line 1048, in _prepare_proxy
    conn.connect()
  File "/tmp/venv/lib64/python3.10/site-packages/urllib3/connection.py", line 633, in connect
    self._tunnel()  # type: ignore[attr-defined]
  File "/usr/lib64/python3.10/http/client.py", line 925, in _tunnel
    raise OSError(f"Tunnel connection failed: {code} {message.strip()}")
OSError: Tunnel connection failed: 407 Proxy Authentication Required

The above exception was the direct cause of the following exception:

urllib3.exceptions.ProxyError: ('Unable to connect to proxy', OSError('Tunnel connection failed: 407 Proxy Authentication Required'))

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/tmp/test.py", line 3, in <module>
    pm.urlopen('GET', 'https://example.com')
  File "/tmp/venv/lib64/python3.10/site-packages/urllib3/poolmanager.py", line 634, in urlopen
    return super().urlopen(method, url, redirect=redirect, **kw)
  File "/tmp/venv/lib64/python3.10/site-packages/urllib3/poolmanager.py", line 444, in urlopen
    response = conn.urlopen(method, u.request_uri, **kw)
  File "/tmp/venv/
... (truncated) ...

Minimal Reproduction

repro.py

from http.server import BaseHTTPRequestHandler
from socketserver import ThreadingTCPServer


class HTTPConnectProxyHandler(BaseHTTPRequestHandler):
    protocol_version = 'HTTP/1.1'
    default_request_version = 'HTTP/1.1'

    def do_CONNECT(self):
        self.send_response(407)
        self.send_header('Proxy-Authenticate', 'Basic')
        self.end_headers()


ThreadingTCPServer(('127.0.0.1', 55556), HTTPConnectProxyHandler).serve_forever()

Environment

Python: 3.10
urllib3: 2.2.1

What Broke

CI fails due to unclosed socket warnings when receiving a 407 response from an HTTP proxy.

Why It Broke

ResourceWarnings were not properly handled for HTTP CONNECT responses with status 407 in urllib3

Fix Options (Details)

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

pip install urllib3==1.26.20

When NOT to use: Do not use if it changes public behavior or if the failure cannot be reproduced.

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/3252

First fixed release: 1.26.20

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

When NOT to Use This Fix

Do not use if it changes public behavior or if the failure cannot be reproduced.

Verify Fix

verify

Re-run: python3.10 -Werror test.py

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
1.26.20	Fixed

Related Issues

No related fixes found.

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Related clusters: Data consistency SSL handshake Other

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

urllib3 Unclosed socket warning after HTTP 407 response from HTTP CONNECT proxy (Fix)