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Possible disclosure of permanent session cookie due to missing Vary: Cookie header

High
davidism published GHSA-m2qf-hxjv-5gpq May 1, 2023

Package

pip flask (pip)

Affected versions

<2.3.1

Patched versions

2.2.5, 2.3.2

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.
  3. The application does not access or modify the session at any point during a request.
  4. 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.

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

CVE ID

CVE-2023-30861

Weaknesses

No CWEs

Credits