CVE-2026-27205: Flask session does not add `Vary: Cookie` header when accessed in some ways

Severity: medium (CVSS 4.3 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-02-21 · updated 2026-06-17
Weakness: CWE-524

Description

Flask is a web server gateway interface (WSGI) web application framework. In versions 3.1.2 and below, when the session object is accessed, Flask should set the Vary: Cookie header., resulting in a Use of Cache Containing Sensitive Information vulnerability. The logic 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 and risk depend on the application being hosted behind a caching proxy that doesn't ignore responses with cookies, not setting a Cache-Control header to mark pages as private or non-cacheable, and accessing the session in a way that only touches keys without reading values or mutating the session. The issue has been fixed in version 3.1.3.

Affected products

pallets flask: before 3.1.3 — fixed in 3.1.3
Pypi Flask: before 3.1.3; 0.1, 0.10, 0.10.1, 0.11, 0.11.1, 0.12 (+57) — fixed in 3.1.3

How to fix

Upgrade to a fixed version: pallets flask 3.1.3; Pypi Flask 3.1.3. Check the vendor advisory for mitigations.

References

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