---
id: CVE-2023-30861
aliases:
  - GHSA-m2qf-hxjv-5gpq
  - PYSEC-2023-62
title: >-
  Flask vulnerable to possible disclosure of permanent session cookie due to
  missing Vary: Cookie header
summary: >-
  Flask vulnerable to possible disclosure of permanent session cookie due to
  missing Vary: Cookie header
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N'
vendor: flask
product: flask
ecosystem: pip
affected:
  - 'flask >= 2.3.0, < 2.3.2'
  - flask < 2.2.5
patched:
  - flask 2.3.2
  - flask 2.2.5
published: '2023-05-01'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:50:01.763387498Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-m2qf-hxjv-5gpq'
references:
  - url: 'https://github.com/pallets/flask/security/advisories/GHSA-m2qf-hxjv-5gpq'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2023-30861'
  - url: >-
      https://github.com/pallets/flask/commit/70f906c51ce49c485f1d355703e9cc3386b1cc2b
  - url: >-
      https://github.com/pallets/flask/commit/afd63b16170b7c047f5758eb910c416511e9c965
  - url: 'https://github.com/pallets/flask'
  - url: 'https://github.com/pallets/flask/releases/tag/2.2.5'
  - url: 'https://github.com/pallets/flask/releases/tag/2.3.2'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/flask/PYSEC-2023-62.yaml
  - url: 'https://lists.debian.org/debian-lts-announce/2023/08/msg00024.html'
  - url: 'https://security.netapp.com/advisory/ntap-20230818-0006'
  - url: 'https://www.debian.org/security/2023/dsa-5442'
tags:
  - osv
  - pip
  - exploit-available
epss: 0.01261
epssPercentile: 0.68376
exploits:
  github: 1
  githubRepos:
    - 'https://github.com/fromitive/cve-2023-30861-poc'
  checkedAt: '2026-09-26T09:05:29.794Z'
exploitAvailable: true
ingestedAt: '2026-09-12T03:13:01.708Z'
---

## Overview

When all of the following conditions are met, a response containing data intended for one client may be cached and subsequently sent by a proxy to other clients. If the proxy also caches `Set-Cookie` headers, it may send one client's `session` cookie to other clients. The severity depends on the application's use of the session, and the proxy's behavior regarding cookies. The risk depends on _all_ these conditions being met.

1. The application must be hosted behind a caching proxy that does not strip cookies or ignore responses with cookies.
2. The application sets [`session.permanent = True`](https://flask.palletsprojects.com/en/2.3.x/api/#flask.session.permanent).
2. The application does not access or modify the session at any point during a request.
4. [`SESSION_REFRESH_EACH_REQUEST`](https://flask.palletsprojects.com/en/2.3.x/config/#SESSION_REFRESH_EACH_REQUEST) is enabled (the default).
5. The application does not set a `Cache-Control` header to indicate that a page is private or should not be cached.

This happens because vulnerable versions of Flask only set the `Vary: Cookie` header when the session is accessed or modified, not when it is refreshed (re-sent to update the expiration) without being accessed or modified.

## Affected packages

- `flask >= 2.3.0, < 2.3.2`
- `flask < 2.2.5`

## Remediation

Upgrade to a patched release:

- `flask 2.3.2`
- `flask 2.2.5`