Flask session does not add `Vary: Cookie` header when accessed in some ways

CVE-2026-27205 → View on GitHub ↗ View on CVE.org ↗ View on NVD ↗

Description

When the session object is accessed, Flask should set the Vary: Cookie header. This instructs caches not to cache the response, as it may contain information specific to a logged in user. This is handled in most cases, but some forms of access such as the Python in operator were overlooked.

The severity depends on the application's use of the session, and the cache'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 ignore responses with cookies.
  2. The application does not set a Cache-Control header to indicate that a page is private or should not be cached.
  3. The application accesses the session in a way that does not access the values, only the keys, and does not mutate the session.

Basic information

Type
reviewed
Severity
low
Advisory on GitHub
Open advisory ↗
Repository advisory
Open repository advisory ↗
Source code
Browse source ↗
Published (advisory)
2026-02-19 20:45:41 UTC
Updated
2026-02-23 22:28:06 UTC
GitHub reviewed
2026-02-19 20:45:41 UTC
NVD published
2026-02-21

EPSS Score

Score Percentile
0.01% 1.35%

CVSS Scores

Base score Version Severity Vector
2.3 4.0 —
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network.
Attack complexity (AC:L)
Exploitation conditions are straightforward and stable.
Attack requirements (AT:P)
Additional preconditions must be present for exploitation.
Privileges required (PR:N)
No privileges are required.
User interaction (UI:P)
A user has to participate (for example click/open/approve).
Vulnerable system confidentiality impact (VC:L)
Limited confidentiality impact on the vulnerable system.
Vulnerable system integrity impact (VI:N)
No integrity impact on the vulnerable system.
Vulnerable system availability impact (VA:N)
No availability impact on the vulnerable system.
Subsequent system confidentiality impact (SC:N)
No confidentiality impact on subsequent systems.
Subsequent system integrity impact (SI:N)
No integrity impact on subsequent systems.
Subsequent system availability impact (SA:N)
No availability impact on subsequent systems.

Identifiers

Type Value
GHSA GHSA-68rp-wp8r-4726 ↗
CVE CVE-2026-27205 ↗

CWEs

CWE id Name
CWE-524 Use of Cache Containing Sensitive Information

Credits

  • shouryaj98 (reporter)

Affected packages (1)

Vulnerable version ranges and first patched releases as published by GitHub.

Ecosystem Package Vulnerable range First patched Vulnerable functions
pip flask < 3.1.3 3.1.3 —

References

cvelogic Threat Intelligence