要点
- CVE-2026-27205 は flask, flask (palletsprojects) に影響する深刻度「中」 (CVSS 4.3) の脆弱性です。
- 今後30日間に悪用される確率（EPSS）：0.40%（全脆弱性の 34.3% より高い）。
- 対処：3.1.3 以降に更新してください。
| パッケージ | 影響を受けるバージョン | 修正済みバージョン |
|---|
| flask PyPI | < 3.1.3
| 3.1.3
|
| flask | < 3.1.3
| 3.1.3
|
概要
When the session object is accessed, Flask should set the Vary: Cookie header. This 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 depends on the application's use of the session, and the cache's behavior regarding cookies. The risk depends on all these conditions being met.
- The application must be hosted behind a caching proxy that does not ignore responses with cookies.
- The application does not set a
Cache-Control header to indicate that a page is private or should not be cached.
- The application accesses the session in a way that does not access the values, only the keys, and does not mutate the session.
NVD の説明
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.
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