urllib3 ResourceWarning due to

The Fix

pip install urllib3==2.2.0

Based on closed urllib3/urllib3 issue #2991 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -348,7 +348,8 @@ async def pyodide_upload() -> ResponseTypes:
     if file.filename != "lolcat.txt":
         return await make_response(f"File name incorrect {file.name}", 404)
-    data = file.read().decode("utf-8")
+    with contextlib.closing(file):
+        data = file.read().decode("utf-8")

repro.py

import sys
import urllib3
import pytest
import gc


def main():
    with urllib3.HTTPSConnectionPool(
        "google.com",
        443,
        assert_fingerprint=(
            "AA:AA:AA:AA:AA:AAAA:AA:AAAA:AA:AA:AA:AA:AA:AA:AA:AA:AA:AA"
        ),
    ) as https_pool:
        with pytest.raises(urllib3.exceptions.MaxRetryError):
            https_pool.request("GET", "/", retries=0)


if __name__ == "__main__":
    sys.exit(main())

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==2.2.0\nWhen NOT to use: Do not use this fix if the application relies on keeping sockets open for reuse.\n\n

Why This Fix Works in Production

Trigger: ResourceWarning due to _ssl_wrap_socket_and_match_hostname assert_fingerprint or assert_hostname failures not closing socket
Mechanism: The SSL socket was not closed properly on assertion failures, leading to ResourceWarnings
Why the fix works: Ensures that the SSL socket is closed if the assert_fingerprint or assert_hostname checks fail, preventing ResourceWarnings. (first fixed release: 2.2.0).

Production impact:

If left unfixed, this can cause silent data inconsistencies that propagate (bad cache entries, incorrect downstream decisions).

Why This Breaks in Prod

The SSL socket was not closed properly on assertion failures, leading to ResourceWarnings
Production symptom (often without a traceback): ResourceWarning due to _ssl_wrap_socket_and_match_hostname assert_fingerprint or assert_hostname failures not closing socket

Proof / Evidence

GitHub issue: #2991
Fix PR: https://github.com/urllib3/urllib3/pull/2842
First fixed release: 2.2.0
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.75
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.61

Discussion

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

Jump to Sources Open on GitHub

“### Subject ResourceWarning due to _ssl_wrap_socket_and_match_hostname assert_fingerprint or assert_hostname failures not closing socket discovered using https://github.com/urllib3/urllib3/pull/2842 ### Environment Describe your environment”

Issue thread · issue description · source

Failure Signature (Search String)

ResourceWarning due to _ssl_wrap_socket_and_match_hostname assert_fingerprint or assert_hostname failures not closing socket
assert_fingerprint=(

Copy-friendly signature

signature.txt

Failure Signature
-----------------
ResourceWarning due to _ssl_wrap_socket_and_match_hostname assert_fingerprint or assert_hostname failures not closing socket
assert_fingerprint=(

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
ResourceWarning due to _ssl_wrap_socket_and_match_hostname assert_fingerprint or assert_hostname failures not closing socket
assert_fingerprint=(

Minimal Reproduction

repro.py

import sys
import urllib3
import pytest
import gc


def main():
    with urllib3.HTTPSConnectionPool(
        "google.com",
        443,
        assert_fingerprint=(
            "AA:AA:AA:AA:AA:AAAA:AA:AAAA:AA:AA:AA:AA:AA:AA:AA:AA:AA:AA"
        ),
    ) as https_pool:
        with pytest.raises(urllib3.exceptions.MaxRetryError):
            https_pool.request("GET", "/", retries=0)


if __name__ == "__main__":
    sys.exit(main())

What Broke

ResourceWarnings were raised due to unclosed SSL sockets during connection failures.

Why It Broke

The SSL socket was not closed properly on assertion failures, leading to ResourceWarnings

Fix Options (Details)

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

pip install urllib3==2.2.0

When NOT to use: Do not use this fix if the application relies on keeping sockets open for reuse.

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/2842

First fixed release: 2.2.0

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

When NOT to Use This Fix

Do not use this fix if the application relies on keeping sockets open for reuse.

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
2.2.0	Fixed

Related Issues

No related fixes found.

Cluster: urllib3:data-consistency urllib3 hub urllib3 best practices All hubs All clusters

Related clusters: Configuration error SSL handshake Other

Sources

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

urllib3 ResourceWarning due to _ssl_wrap_socket_and_match_hostname assert_fingerprint or (Fix)