Flask vulnerable to possible disclosure of permanent session cookie due to missing Vary: Cookie header
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.
- The application must be hosted behind a caching proxy that does not strip cookies or ignore responses with cookies.
- The application sets
session.permanent = True. - The application does not access or modify the session at any point during a request.
SESSION_REFRESH_EACH_REQUESTis enabled (the default).- The application does not set a
Cache-Controlheader 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 software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | flask | >= 2.3.0, < 2.3.2, < 2.2.5 | 2.3.2, 2.2.5 |
References
- https://github.com/pallets/flask/commit/70f906c51ce49c485f1d355703e9cc3386b1cc2b
- https://github.com/pallets/flask/releases/tag/2.3.2
- https://github.com/pallets/flask/releases/tag/2.2.5
- https://github.com/pallets/flask/security/advisories/GHSA-m2qf-hxjv-5gpq
- https://github.com/pallets/flask/commit/afd63b16170b7c047f5758eb910c416511e9c965
- https://nvd.nist.gov/vuln/detail/CVE-2023-30861
- https://github.com/pallets/flask
- https://github.com/pypa/advisory-database/tree/main/vulns/flask/PYSEC-2023-62.yaml
- https://lists.debian.org/debian-lts-announce/2023/08/msg00024.html
- https://security.netapp.com/advisory/ntap-20230818-0006
- https://www.debian.org/security/2023/dsa-5442
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N- Weakness (CWE)
CWE-539- Platform
- Python (PyPI)
- Package
flask- Disclosed
- 01.05.2023
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2023-62 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-m2qf-hxjv-5gpq - CC BY 4.0
Vulnerability history: flask
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 5
- First → latest
- 08.2018 02.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 3
- Low 2
Disclosures per year
- 2018: 1
- 2019: 1
- 2020: 0
- 2021: 0
- 2022: 0
- 2023: 1
- 2024: 0
- 2025: 1
- 2026: 1
-
Flask vulnerable to possible disclosure of permanent session cookie due to missing Vary: Cookie header this entry
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