urllib3 Issue with Parsing URIs - Breaks Security Tools

The Fix

pip install urllib3==1.25.9

Based on closed urllib3/urllib3 issue #1790 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -93,7 +93,7 @@ def _call_method(self):
             path = urlsplit(path).path
 
-        target = path[1:].replace("/", "_")
+        target = path[1:].split("/", 1)[0]
         method = getattr(self, target, self.index)

repro.py

import urllib3

pool = urllib3.HTTPConnectionPool("localhost", 8000)
r = pool.urlopen("GET", "/../../../../doing/certain/check")
print(r.status)

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==1.25.9\nWhen NOT to use: This fix should not be used if path normalization is required for security compliance.\n\n

Why This Fix Works in Production

Trigger: Issue with Parsing URIs - Breaks Security Tools when testing for Path Traversal
Mechanism: Normalization of path segments in URI parsing was introduced, altering expected behavior
Why the fix works: Adds a test to ensure that the ConnectionPool preserves path dot segments, addressing the issue with URI parsing that breaks security tools. (first fixed release: 1.25.9).

Production impact:

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

Why This Breaks in Prod

Normalization of path segments in URI parsing was introduced, altering expected behavior
Production symptom (often without a traceback): Issue with Parsing URIs - Breaks Security Tools when testing for Path Traversal

Proof / Evidence

GitHub issue: #1790
Fix PR: https://github.com/urllib3/urllib3/pull/1828
First fixed release: 1.25.9
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.67

Discussion

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

Jump to Sources Open on GitHub

“Hi all, I have documented the alternative solutions here: https://mazinahmed.net/blog/testing-for-path-traversal-with-python/ Thanks again!”

@mazen160 · 2020-04-12 · source

“Might be worth documenting it as an escape hatch as well?”

@sigmavirus24 · 2020-02-12 · source

“Yeah I agree we should keep the previous behavior. My thoughts are that we should only do path normalization on redirects. Are you able to…”

@sethmlarson · 2020-01-24 · source

“I think we should be good with those alternatives. Can we add a simple test case to ensure we preserve the non-normalizing behavior of using…”

@sethmlarson · 2020-02-12 · source

Failure Signature (Search String)

Issue with Parsing URIs - Breaks Security Tools when testing for Path Traversal
I have noticed an odd behaviour in requests module, which uses urllib3. I inspected the root cause via regression testing and I found that the root cause of the issue is a change

Copy-friendly signature

signature.txt

Failure Signature
-----------------
Issue with Parsing URIs - Breaks Security Tools when testing for Path Traversal
I have noticed an odd behaviour in requests module, which uses urllib3. I inspected the root cause via regression testing and I found that the root cause of the issue is a change that was introduced in urlib3

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
Issue with Parsing URIs - Breaks Security Tools when testing for Path Traversal
I have noticed an odd behaviour in requests module, which uses urllib3. I inspected the root cause via regression testing and I found that the root cause of the issue is a change that was introduced in urlib3

Minimal Reproduction

repro.py

import urllib3

pool = urllib3.HTTPConnectionPool("localhost", 8000)
r = pool.urlopen("GET", "/../../../../doing/certain/check")
print(r.status)

What Broke

Security tools fail to detect path traversal vulnerabilities due to altered URI paths.

Why It Broke

Normalization of path segments in URI parsing was introduced, altering expected behavior

Fix Options (Details)

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

pip install urllib3==1.25.9

When NOT to use: This fix should not be used if path normalization is required for security compliance.

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/1828

First fixed release: 1.25.9

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

When NOT to Use This Fix

This fix should not be used if path normalization is required for security compliance.

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.
Track RSS + object counts after deployments; alert on monotonic growth and GC pressure.
Add a long-running test that repeats the failing call path and asserts stable memory.

Version Compatibility Table

Version	Status
1.25.9	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 Issue with Parsing URIs - Breaks Security Tools when testing for Path Traversal (Fix)