Vulnerability 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.
CVSS Score
MEDIUM
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Palletsprojects | Flask | < 3.1.3 |
Related Weaknesses (CWE)
References
- https://github.com/pallets/flask/commit/089cb86dd22bff589a4eafb7ab8e42dc357623b4Patch
- https://github.com/pallets/flask/releases/tag/3.1.3ProductRelease Notes
- https://github.com/pallets/flask/security/advisories/GHSA-68rp-wp8r-4726Vendor Advisory
FAQ
What is CVE-2026-27205?
CVE-2026-27205 is a vulnerability with a CVSS score of 4.3 (MEDIUM). 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 Us...
How severe is CVE-2026-27205?
CVE-2026-27205 has been rated MEDIUM with a CVSS base score of 4.3/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-27205?
Check the references section above for vendor advisories and patch information. Affected products include: Palletsprojects Flask.