urllib3 SSL cert digest check uses non-constant-time

The Fix

pip install urllib3==1.25

Based on closed urllib3/urllib3 issue #710 · 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

@@ -153,7 +153,7 @@ In chronological order:
 * Jon Wayne Parrott <[email protected]>
   * App Engine environment tests.
-  
+
 * John Krauss <https://github.com/talos>

fix.md

Option A — Upgrade to fixed release\npip install urllib3==1.25\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: SSL cert digest check uses non-constant-time comparison
Mechanism: The SSL certificate digest check used a non-constant-time comparison, making it vulnerable to timing attacks
Why the fix works: Implements a constant-time comparison for SSL certificate digest checks to prevent potential timing attacks. (first fixed release: 1.25).

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: 2.6 breaks; 1.25 is the first fixed release.
Shows up under Python 2.7.7 in real deployments (not just unit tests).
The SSL certificate digest check used a non-constant-time comparison, making it vulnerable to timing attacks
Production symptom (often without a traceback): SSL cert digest check uses non-constant-time comparison

Proof / Evidence

GitHub issue: #710
Fix PR: https://github.com/urllib3/urllib3/pull/711
First fixed release: 1.25
Affected versions: 2.6
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.82

Discussion

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

Jump to Sources Open on GitHub

“This is the problematic line: https://github.com/shazow/urllib3/blob/master/urllib3/util/ssl_.py#L137 The digests being compared are strings, and == is not constant-time, which could potentially allow a timing attack. While it may not immed”

Issue thread · issue description · source

Failure Signature (Search String)

SSL cert digest check uses non-constant-time comparison
https://github.com/shazow/urllib3/blob/master/urllib3/util/ssl_.py#L137

Copy-friendly signature

signature.txt

Failure Signature
-----------------
SSL cert digest check uses non-constant-time comparison
https://github.com/shazow/urllib3/blob/master/urllib3/util/ssl_.py#L137

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
SSL cert digest check uses non-constant-time comparison
https://github.com/shazow/urllib3/blob/master/urllib3/util/ssl_.py#L137

Environment

Python: 2.7.7

What Broke

Potential security vulnerability allowing timing attacks on SSL certificate digest checks.

Why It Broke

The SSL certificate digest check used a non-constant-time comparison, making it vulnerable to timing attacks

Fix Options (Details)

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

pip install urllib3==1.25

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/711

First fixed release: 1.25

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
2.6	Broken
1.25	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 SSL cert digest check uses non-constant-time comparison (Fix)