urllib3 SecureTransport does not close unverified

The Fix

pip install urllib3==1.26.0

Based on closed urllib3/urllib3 issue #1976 · 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

@@ -13,6 +13,7 @@
 import re
 import ssl
+import struct
 import tempfile

repro.py

import logging
import socket
import ssl
import threading

import trustme
import urllib3
from urllib3.contrib.securetransport import inject_into_urllib3, extract_from_urllib3

CERT_PATH = "/tmp/ca.pem"
SSL_CTX2 = ssl.SSLContext()
NUM_CONNECTIONS = 3

def server(s):
    for i in range(NUM_CONNECTIONS):
        c, _ = s.accept()
        try:
            logging.critical("Server iter")
            c = SSL_CTX2.wrap_socket(c, server_side=True)
            c.recv(1024)
            c.sendall(b"HTTP/1.1 404 Not Found\r\nConnection: close\r\n\r\n")
            c.close()
        except Exception as e:
            logging.critical("Server ex: %r" % (e,))

def connect_and_fail(port):
    with urllib3.HTTPSConnectionPool("localhost", port, cert_reqs="REQUIRED", ca_certs=CERT_PATH) as p:
        for i in range(NUM_CONNECTIONS):
            logging.critical("Client iter")
            try:
                p.request("GET", "/", retries=False)
            except urllib3.exceptions.SSLError as e:
                logging.critical("Client ex: %r" % (e,))

def main():
    logging.basicConfig(level=logging.INFO)
    # client's chain - ca
    ca = trustme.CA()
    ca.cert_pem.write_to_path(CERT_PATH)
    # server's chain - ca2
    ca2 = trustme.CA()
    server2_cert = ca2.issue_cert(u"localhost")
    server2_cert.configure_cert(SSL_CTX2)

    # start an SSL server with ca chain
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.bind(('', 0))
    port = s.getsockname()[1]
    s.listen(10)
    th = threading.Thread(target=server, args=(s, ))
    th.start()
    
    # inject SecureTransport and try to connect with ca2 chain
    inject_into_urllib3()
    connect_and_fail(port)
    extract_from_urllib3()
    th.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==1.26.0\nWhen NOT to use: Do not use this fix if custom verification is not enabled.\n\n

Why This Fix Works in Production

Trigger: SecureTransport does not close unverified connections correctly
Mechanism: SecureTransport does not terminate the socket on SSL custom-verification failure, leading to socket leaks
Why the fix works: Terminates the connection when custom verification fails in SecureTransport, addressing the issue of leaking sockets. (first fixed release: 1.26.0).

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

SecureTransport does not terminate the socket on SSL custom-verification failure, leading to socket leaks
Production symptom (often without a traceback): SecureTransport does not close unverified connections correctly

Proof / Evidence

GitHub issue: #1976
Fix PR: https://github.com/urllib3/urllib3/pull/1977
First fixed release: 1.26.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.95
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.43

Discussion

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

Jump to Sources Open on GitHub

“SecureTransport does not handle SSL custom-verification failure correctly. Instead of sending an alert and terminating the socket, the client socket leaks thus hanging the server socket. #### Expected behavior When custom-verification is en”

Issue thread · issue description · source

Failure Signature (Search String)

SecureTransport does not close unverified connections correctly
SecureTransport does not handle SSL custom-verification failure correctly. Instead of sending an alert and terminating the socket, the client socket leaks thus hanging the server

Copy-friendly signature

signature.txt

Failure Signature
-----------------
SecureTransport does not close unverified connections correctly
SecureTransport does not handle SSL custom-verification failure correctly. Instead of sending an alert and terminating the socket, the client socket leaks thus hanging the server socket.

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
SecureTransport does not close unverified connections correctly
SecureTransport does not handle SSL custom-verification failure correctly. Instead of sending an alert and terminating the socket, the client socket leaks thus hanging the server socket.

Minimal Reproduction

repro.py

import logging
import socket
import ssl
import threading

import trustme
import urllib3
from urllib3.contrib.securetransport import inject_into_urllib3, extract_from_urllib3

CERT_PATH = "/tmp/ca.pem"
SSL_CTX2 = ssl.SSLContext()
NUM_CONNECTIONS = 3

def server(s):
    for i in range(NUM_CONNECTIONS):
        c, _ = s.accept()
        try:
            logging.critical("Server iter")
            c = SSL_CTX2.wrap_socket(c, server_side=True)
            c.recv(1024)
            c.sendall(b"HTTP/1.1 404 Not Found\r\nConnection: close\r\n\r\n")
            c.close()
        except Exception as e:
            logging.critical("Server ex: %r" % (e,))

def connect_and_fail(port):
    with urllib3.HTTPSConnectionPool("localhost", port, cert_reqs="REQUIRED", ca_certs=CERT_PATH) as p:
        for i in range(NUM_CONNECTIONS):
            logging.critical("Client iter")
            try:
                p.request("GET", "/", retries=False)
            except urllib3.exceptions.SSLError as e:
                logging.critical("Client ex: %r" % (e,))

def main():
    logging.basicConfig(level=logging.INFO)
    # client's chain - ca
    ca = trustme.CA()
    ca.cert_pem.write_to_path(CERT_PATH)
    # server's chain - ca2
    ca2 = trustme.CA()
    server2_cert = ca2.issue_cert(u"localhost")
    server2_cert.configure_cert(SSL_CTX2)

    # start an SSL server with ca chain
    s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    s.bind(('', 0))
    port = s.getsockname()[1]
    s.listen(10)
    th = threading.Thread(target=server, args=(s, ))
    th.start()
    
    # inject SecureTransport and try to connect with ca2 chain
    inject_into_urllib3()
    connect_and_fail(port)
    extract_from_urllib3()
    th.join()

What Broke

The server socket hangs due to leaking client sockets after SSL verification failure.

Why It Broke

SecureTransport does not terminate the socket on SSL custom-verification failure, leading to socket leaks

Fix Options (Details)

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

pip install urllib3==1.26.0

When NOT to use: Do not use this fix if custom verification is not enabled.

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/1977

First fixed release: 1.26.0

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

When NOT to Use This Fix

Do not use this fix if custom verification is not enabled.

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
1.26.0	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 SecureTransport does not close unverified connections correctly (Fix)