CVE-2026-27205: pallets flask

Vulnerability summary

CVE-2026-27205 is a medium (CVSS 4.3) vulnerability in pallets flask. EPSS estimates a 0.43% chance of exploitation in the next 30 days, higher than 36% of scored vulnerabilities.

Timeline

1. 21-02-2026
   PublishedVulnerability first disclosed.
2. 24-02-2026
   Last modifiedVulnerability information updated.

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.

• CWE-524Use of Cache Containing Sensitive InformationThe code uses a cache that contains sensitive information, but the cache can be read by an actor outside of the intended control sphere.