{"cve":"CVE-2026-27205","nvd":{"published":"2026-02-21T06:17:00.910","modified":"2026-06-17T10:26:50.700","status":"Analyzed","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_v4":{"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N","score":2.3},"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N","score":4.3},"cwes":["CWE-524"],"references":["https://github.com/pallets/flask/commit/089cb86dd22bff589a4eafb7ab8e42dc357623b4","https://github.com/pallets/flask/releases/tag/3.1.3","https://github.com/pallets/flask/security/advisories/GHSA-68rp-wp8r-4726"],"products":["palletsprojects:flask"],"source":"nvd","synced_at":"2026-10-19T22:24:17.937441719+00:00"},"epss":{"cve":"CVE-2026-27205","score":0.004,"percentile":0.34316,"date":"2026-10-31"},"kev":null,"osv":[{"id":"GHSA-68rp-wp8r-4726","url":"https://osv.dev/vulnerability/GHSA-68rp-wp8r-4726","summary":"Flask session does not add `Vary: Cookie` header when accessed in some ways","modified":"2026-09-10T03:49:54.354981590Z","packages":[{"ecosystem":"PyPI","name":"flask","purl":"pkg:pypi/flask","ranges":["<3.1.3"],"fixed":["3.1.3"]}]},{"id":"PYSEC-2026-2151","url":"https://osv.dev/vulnerability/PYSEC-2026-2151","summary":"","modified":"2026-07-13T07:15:25.033843440Z","packages":[{"ecosystem":"PyPI","name":"flask","purl":"pkg:pypi/flask","ranges":["<3.1.3"],"fixed":["3.1.3"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}