CVE-2026-27205

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.

Read the notes from the security team

Why is this CVE low priority?

Per flask developers, this is a low severity issue

Learn more about Ubuntu priority

Status

Show unmaintained releases

Package	Ubuntu Release	Status
flask	25.10 questing	Vulnerable
	24.04 LTS noble	Fixed 3.0.2-1ubuntu1.1
	22.04 LTS jammy	Fixed 2.0.1-2ubuntu1.2
	20.04 LTS focal	Fixed 1.1.1-2ubuntu0.1+esm1
	18.04 LTS bionic	Ignored code not present
	16.04 LTS xenial	Ignored code not present

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Notes

shishirsub10

version in bionic and xenial do not have functionality to to set the vary cookie header as it was only introduced in upstream in version 1.0

Patch details

For informational purposes only. We recommend not to cherry-pick updates. How can I get the fixes?

Package	Patch details
flask	Upstream: 089cb86

References

Related Ubuntu Security Notices (USN)

USN-8104-1
Flask vulnerability
18 March 2026