urllib3 Unnecessary SNI warning with SecureTransport (Fix)

The Fix

pip install urllib3==1.25.11

Based on closed urllib3/urllib3 issue #1902 · 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

@@ -375,14 +375,13 @@ def ssl_wrap_socket(
     # If we detect server_hostname is an IP address then the SNI
     # extension should not be used according to RFC3546 Section 3.1
-    # We shouldn't warn the user if SNI isn't available but we would
-    # not be using SNI anyways due to IP address for server_hostname.
-    if (

repro.py

import socket
import ssl
import threading
# comment these 2 lines
from urllib3.contrib import securetransport
securetransport.inject_into_urllib3()
from urllib3 import util
import trustme

ca = trustme.CA()
cert = ca.issue_cert(u"localhost")
ca.cert_pem.write_to_path("/tmp/ca.pem")
cert.private_key_pem.write_to_path("/tmp/cert.key")
cert.cert_chain_pems[0].write_to_path("/tmp/cert.pem")
server_up = threading.Event()
c = socket.socket()
l = socket.socket()


def client():
    c.connect(('127.0.0.1', l.getsockname()[1]))
    ssl_sock = util.ssl_wrap_socket(c, ca_certs="/tmp/ca.pem")
    ssl_sock.close()


def server():
    l.bind(('127.0.0.1', 0))
    l.listen(1)
    server_up.set()
    s = l.accept()[0]
    ssl_sock = ssl.wrap_socket(s, server_side=True, certfile="/tmp/cert.pem", keyfile="/tmp/cert.key")
    ssl_sock.close()


client_thread = threading.Thread(target=client)
server_thread = threading.Thread(target=server)
server_thread.start()
assert server_up.wait(5)
client_thread.start()
server_thread.join()
client_thread.join()

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==1.25.11\nWhen NOT to use: This fix should not be applied if SNI warnings are required for other backends.\n\n

Why This Fix Works in Production

Trigger: ssl_sock = util.ssl_wrap_socket(c, ca_certs="/tmp/ca.pem")
Mechanism: The SecureTransport backend incorrectly issues an SNI warning when server_hostname is None
Why the fix works: Fixes an unnecessary SNI warning that was issued when using the SecureTransport backend without a server_hostname. (first fixed release: 1.25.11).

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

The SecureTransport backend incorrectly issues an SNI warning when server_hostname is None
Production symptom (often without a traceback): ssl_sock = util.ssl_wrap_socket(c, ca_certs="/tmp/ca.pem")

Proof / Evidence

GitHub issue: #1902
Fix PR: https://github.com/urllib3/urllib3/pull/1903
First fixed release: 1.25.11
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.49

Discussion

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

Jump to Sources Open on GitHub

“SNI is always enabled with the SecureTransport backend. If a platform lacks SNI support and otherwise SNI would have been used (a valid is given) - we issue an SNI warning. With the SecureTransport backend, if one wraps a socket with no , a”

Issue thread · issue description · source

Failure Signature (Search String)

ssl_sock = util.ssl_wrap_socket(c, ca_certs="/tmp/ca.pem")
ssl_sock.close()

Copy-friendly signature

signature.txt

Failure Signature
-----------------
ssl_sock = util.ssl_wrap_socket(c, ca_certs="/tmp/ca.pem")
ssl_sock.close()

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
ssl_sock = util.ssl_wrap_socket(c, ca_certs="/tmp/ca.pem")
ssl_sock.close()

Minimal Reproduction

repro.py

import socket
import ssl
import threading
# comment these 2 lines
from urllib3.contrib import securetransport
securetransport.inject_into_urllib3()
from urllib3 import util
import trustme

ca = trustme.CA()
cert = ca.issue_cert(u"localhost")
ca.cert_pem.write_to_path("/tmp/ca.pem")
cert.private_key_pem.write_to_path("/tmp/cert.key")
cert.cert_chain_pems[0].write_to_path("/tmp/cert.pem")
server_up = threading.Event()
c = socket.socket()
l = socket.socket()


def client():
    c.connect(('127.0.0.1', l.getsockname()[1]))
    ssl_sock = util.ssl_wrap_socket(c, ca_certs="/tmp/ca.pem")
    ssl_sock.close()


def server():
    l.bind(('127.0.0.1', 0))
    l.listen(1)
    server_up.set()
    s = l.accept()[0]
    ssl_sock = ssl.wrap_socket(s, server_side=True, certfile="/tmp/cert.pem", keyfile="/tmp/cert.key")
    ssl_sock.close()


client_thread = threading.Thread(target=client)
server_thread = threading.Thread(target=server)
server_thread.start()
assert server_up.wait(5)
client_thread.start()
server_thread.join()
client_thread.join()

What Broke

Unnecessary SNI warnings clutter logs, causing confusion during SSL connections.

Why It Broke

The SecureTransport backend incorrectly issues an SNI warning when server_hostname is None

Fix Options (Details)

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

pip install urllib3==1.25.11

When NOT to use: This fix should not be applied if SNI warnings are required for other backends.

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/1903

First fixed release: 1.25.11

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

When NOT to Use This Fix

This fix should not be applied if SNI warnings are required for other backends.

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
1.25.11	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.