pallets flask up to 3.1.2 Setting cache containing sensitive information

Summaryinfo

A vulnerability marked as problematic has been reported in pallets flask up to 3.1.2. Affected by this vulnerability is an unknown functionality of the component Setting Handler. This manipulation causes use of cache containing sensitive information. This vulnerability is handled as CVE-2026-27205. The attack can be initiated remotely. There is not any exploit available. It is suggested to upgrade the affected component.

Detailsinfo

A vulnerability was found in pallets flask up to 3.1.2. It has been declared as problematic. Affected by this vulnerability is an unknown code of the component Setting Handler. The manipulation with an unknown input leads to a use of cache containing sensitive information vulnerability. The CWE definition for the vulnerability is CWE-524. The code uses a cache that contains sensitive information, but the cache can be read by an actor outside of the intended control sphere. As an impact it is known to affect confidentiality. The summary by CVE is:

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.

The advisory is shared at github.com. This vulnerability is known as CVE-2026-27205 since 02/18/2026. The exploitation appears to be easy. The attack can be launched remotely. Neither technical details nor an exploit are publicly available. MITRE ATT&CK project uses the attack technique T1592 for this issue.

The vulnerability scanner Nessus provides a plugin with the ID 299724 (Linux Distros Unpatched Vulnerability : CVE-2026-27205), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 3.1.3 eliminates this vulnerability. The upgrade is hosted for download at github.com. Applying the patch 089cb86dd22bff589a4eafb7ab8e42dc357623b4 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the vulnerability database at Tenable (299724). Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Vendor

• pallets

Name

• flask

Version

• 3.1.0
• 3.1.1
• 3.1.2

License

• open-source

Website

• Product: https://github.com/pallets/flask/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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