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

Flask vulnerable to possible disclosure of permanent session cookie due to missing Vary: Cookie header (CVE-2023-30861) | HOL Guard CVE

Why this deserves its current priority

CVSS is 7.5. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps flask (pypi), flask (pypi). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
flaskpypi	>=2.3.0 <2.3.2	2.3.2
flaskpypi	>=0 <2.2.5	2.2.5

Recommended response

Check inventory. Check lockfiles and deployed manifests for flask, flask.
Review the reported fix. Update flask to 2.3.2; flask to 2.2.5 if you use the affected versions. Test the change in a non-production environment first.

Evidence timeline and material changes

Published upstream
May 1, 2023
Evidence: source:osv:source_dates:source-dates:record
Source modified
Sep 10, 2026
Evidence: source:osv:source_dates:source-dates:record
First seen by HOL
Sep 10, 2026

Sources and claim methodology

Upstream source description

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.

Quoted source text, attributed separately from HOL analysis.