Vulnerability in Pallets Flask
CVE-2026-27205
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N.
EPSS exploit prediction
EPSS: 0.005 (40.8th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 41st percentile — 40.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.004 · newest: 0.005 · change: +0.001
Affected products
- Pallets Flask — versions < 3.1.3
- Palletsprojects Flask
Weakness classification (CWE)
References
- github.com/pallets/flask/security/advisories/GHSA-68rp-wp8r-4726 (Vendor Advisory)
- github.com/pallets/flask/commit/089cb86dd22bff589a4eafb7ab8e42dc357623b4 (Patch)
- github.com/pallets/flask/releases/tag/3.1.3 (Product, Release Notes)
Frequently asked questions
- What is CVE-2026-27205?
- CVE-2026-27205 is a medium-severity vulnerability in Pallets Flask, classified under Use of Cache Containing Sensitive Information. CVSS score: 4.3/10. Published 2026-02-21.
- How severe is CVE-2026-27205?
- Medium severity. CVSS v3 base score is 4.3 out of 10.