flask Default port overrides requesting a system assigned

The Fix

Upgrade to version 1.0.3 or later.

Based on closed pallets/flask issue #2926 · 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

@@ -42,6 +42,8 @@ Unreleased
 -   Allow customizing the :attr:`Flask.url_map_class` used for routing.
     :pr:`3069`
+-   The development server port can be set to 0, which tells the OS to
+    pick an available port. :issue:`2926`

repro.py

>> app.run(port=0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/.../lib/python3.6/site-packages/flask/app.py", line 943, in run
    run_simple(host, port, self, **options)
  File "/.../lib/python3.6/site-packages/werkzeug/serving.py", line 814, in run_simple
    inner()
  File "/.../lib/python3.6/site-packages/werkzeug/serving.py", line 774, in inner
    fd=fd)
  File "/.../lib/python3.6/site-packages/werkzeug/serving.py", line 660, in make_server
    passthrough_errors, ssl_context, fd=fd)
  File ".../lib/python3.6/site-packages/werkzeug/serving.py", line 577, in __init__
    self.address_family), handler)
  File "/.../lib/python3.6/socketserver.py", line 453, in __init__
    self.server_bind()
  File "/.../lib/python3.6/http/server.py", line 136, in server_bind
    socketserver.TCPServer.server_bind(self)
  File "/.../lib/python3.6/socketserver.py", line 467, in server_bind
    self.socket.bind(self.server_address)
OSError: [Errno 98] Address already in use

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\nUpgrade to version 1.0.3 or later.\nWhen NOT to use: This fix should not be applied if the application relies on a specific port for functionality.\n\n

Why This Fix Works in Production

Trigger: >> app.run(port=0)
Mechanism: Flask incorrectly sets the port to 5000 instead of allowing the OS to assign a port when port=0 is specified
Why the fix works: Fixes the issue where setting the port to 0 in Flask would default to port 5000, causing conflicts. (first fixed release: 1.0.3).

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

Shows up under Python 3.6 in real deployments (not just unit tests).
Flask incorrectly sets the port to 5000 instead of allowing the OS to assign a port when port=0 is specified
Surfaces as: >> app.run(port=0)

Proof / Evidence

GitHub issue: #2926
Fix PR: https://github.com/pallets/flask/pull/2928
First fixed release: 1.0.3
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.85
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.36

Discussion

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

Jump to Sources Open on GitHub

“Makes sense. Marking this as beginner friendly, I'd be happy to review a PR.”

@davidism · 2018-09-28 · source

“I think it's closed, it shows expected behavior”

@patiencedaur · 2018-10-13 · source

“Yeah but it as inactive for over a month. That's why I asked.”

@Man-Jain · 2018-11-05 · source

“@Man-Jain you can politely ask the PR implementor to see if he can fix the PR comments otherwise you can fork his PR and try…”

@theodesp · 2018-12-01 · source

Failure Signature (Search String)

>> app.run(port=0)

Error Message

Stack trace

error.txt

Error Message
-------------
>> app.run(port=0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/.../lib/python3.6/site-packages/flask/app.py", line 943, in run
    run_simple(host, port, self, **options)
  File "/.../lib/python3.6/site-packages/werkzeug/serving.py", line 814, in run_simple
    inner()
  File "/.../lib/python3.6/site-packages/werkzeug/serving.py", line 774, in inner
    fd=fd)
  File "/.../lib/python3.6/site-packages/werkzeug/serving.py", line 660, in make_server
    passthrough_errors, ssl_context, fd=fd)
  File ".../lib/python3.6/site-packages/werkzeug/serving.py", line 577, in __init__
    self.address_family), handler)
  File "/.../lib/python3.6/socketserver.py", line 453, in __init__
    self.server_bind()
  File "/.../lib/python3.6/http/server.py", line 136, in server_bind
    socketserver.TCPServer.server_bind(self)
  File "/.../lib/python3.6/socketserver.py", line 467, in server_bind
    self.socket.bind(self.server_address)
OSError: [Errno 98] Address already in use

Minimal Reproduction

repro.py

>> app.run(port=0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/.../lib/python3.6/site-packages/flask/app.py", line 943, in run
    run_simple(host, port, self, **options)
  File "/.../lib/python3.6/site-packages/werkzeug/serving.py", line 814, in run_simple
    inner()
  File "/.../lib/python3.6/site-packages/werkzeug/serving.py", line 774, in inner
    fd=fd)
  File "/.../lib/python3.6/site-packages/werkzeug/serving.py", line 660, in make_server
    passthrough_errors, ssl_context, fd=fd)
  File ".../lib/python3.6/site-packages/werkzeug/serving.py", line 577, in __init__
    self.address_family), handler)
  File "/.../lib/python3.6/socketserver.py", line 453, in __init__
    self.server_bind()
  File "/.../lib/python3.6/http/server.py", line 136, in server_bind
    socketserver.TCPServer.server_bind(self)
  File "/.../lib/python3.6/socketserver.py", line 467, in server_bind
    self.socket.bind(self.server_address)
OSError: [Errno 98] Address already in use

Environment

Python: 3.6

What Broke

Setting port=0 leads to port conflicts, causing application startup failures.

Why It Broke

Flask incorrectly sets the port to 5000 instead of allowing the OS to assign a port when port=0 is specified

Fix Options (Details)

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

Upgrade to version 1.0.3 or later.

When NOT to use: This fix should not be applied if the application relies on a specific port for functionality.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/pallets/flask/pull/2928

First fixed release: 1.0.3

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

When NOT to Use This Fix

This fix should not be applied if the application relies on a specific port for functionality.

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 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
1.0.3	Fixed

Related Issues

No related fixes found.

Cluster: flask:configuration-error All hubs All clusters

Related clusters: Data consistency Other Duplicate processing

Sources

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

flask Default port overrides requesting a system assigned port (Fix)