urllib3 [v2] Remove _const_compare

The Fix

pip install urllib3==1.26.3

Based on closed urllib3/urllib3 issue #2093 · 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%.

Open PR/Commit

@@ -19,21 +19,6 @@
 
 
-def _const_compare_digest_backport(a, b):
-    """
-    Compare two digests of equal length in constant time.

fix.md

Option A — Upgrade to fixed release\npip install urllib3==1.26.3\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: Within `urllib3.util.ssl_` we previously needed to backport `hmac.compare_digest()`, this is no longer the case in Python 3.6+
Mechanism: Removes the backport of _const_compare_digest in favor of hmac.compare_digest, addressing issue #2093.
Why the fix works: Removes the backport of _const_compare_digest in favor of hmac.compare_digest, addressing issue #2093. (first fixed release: 1.26.3).

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

Triggered by an upgrade/regression window: 3.6 breaks; 1.26.3 is the first fixed release.
Shows up under Python 3.6 in real deployments (not just unit tests).
Production symptom (often without a traceback): Within `urllib3.util.ssl_` we previously needed to backport `hmac.compare_digest()`, this is no longer the case in Python 3.6+

Proof / Evidence

GitHub issue: #2093
Fix PR: https://github.com/urllib3/urllib3/pull/2095
First fixed release: 1.26.3
Affected versions: 3.6
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.83

Discussion

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

Jump to Sources Open on GitHub

“Within urllib3.util.ssl_ we previously needed to backport hmac.compare_digest(), this is no longer the case in Python 3.6+”

Issue thread · issue description · source

Failure Signature (Search String)

Within `urllib3.util.ssl_` we previously needed to backport `hmac.compare_digest()`, this is no longer the case in Python 3.6+

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Within `urllib3.util.ssl_` we previously needed to backport `hmac.compare_digest()`, this is no longer the case in Python 3.6+

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Within `urllib3.util.ssl_` we previously needed to backport `hmac.compare_digest()`, this is no longer the case in Python 3.6+

Environment

Python: 3.6

What Broke

Potential security vulnerabilities due to outdated digest comparison methods.

Fix Options (Details)

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

pip install urllib3==1.26.3

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/2095

First fixed release: 1.26.3

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.

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
3.6	Broken
1.26.3	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 [v2] Remove _const_compare_digest() in favor of hmac.compare_digest() (Fix)