CVE-2026-27205: Flask session does not add `Vary: Cookie` header when accessed in some ways
CVE-2026-27205 · Severity: medium · CVSS 4 · Published 2026-02-21
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.
No. Junglewise Threat Intelligence has no report of CVE-2026-27205 being exploited in the wild.
How severe is CVE-2026-27205?
Medium severity, with a CVSS score of 4, published 21 February 2026.
Which products and versions are affected?
flask (PyPI) ECOSYSTEM:introduced=0,fixed=3.1.3.
Is there a fix for CVE-2026-27205?
Yes. flask (PyPI): upgrade to 3.1.3 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
Am I affected by CVE-2026-27205?
Check whether you run flask (PyPI) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.