CVE-2023-30861

Flask vulnerable to possible disclosure of permanent session cookie due to missing Vary: Cookie header

Published May 2, 2023 CVSS 3.1 · 7.5 High EPSS 1.2% KEV · Not listed
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.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Debian DLA Debian DLA DLA-3536-1 flask security update
Debian DSA Debian DSA DSA-5442-1 flask security update
EUVD EUVD EUVD-2023-0080 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.
Github GHSA Github GHSA GHSA-m2qf-hxjv-5gpq Flask vulnerable to possible disclosure of permanent session cookie due to missing Vary: Cookie header
Ubuntu USN Ubuntu USN USN-6111-1 Flask vulnerability
History
Type Values Removed Values Added
Metrics epss

{'score': 0.00199}

epss

{'score': 0.00205}

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
GitHub_M
Published
Updated
Feb 13, 2025, 4:49 PM
Reserved
Apr 18, 2023, 4:13 PM
Status
Modified
Published
May 2, 2023, 6:15 PM
Modified
Jun 17, 2026, 5:55 AM
Severity
Important
Public date
May 5, 2023, 12:00 AM