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.
This vulnerability carries a MEDIUM severity rating with a CVSS v3.1 score of 4.3, indicating it can be exploited remotely over the network with relatively low complexity though user interaction is required and does not require pre-existing privileges . The vulnerability impacts limited data confidentiality, for affected systems. Impacting 1 product from palletsprojects organizations running these solutions should prioritize assessment and patching.
Reported in 2026, this vulnerability emerged during an era marked by increased sophistication in supply chain attacks, cloud infrastructure vulnerabilities, and software-as-a-service (SaaS) security challenges. Security practices during this period emphasized zero-trust architectures, container security, and API protection.
2026-02-21T06:17:00.910
2026-02-24T21:59:52.183
Analyzed
CVSSv3.1: 4.3 (MEDIUM)
| Type | Vendor | Product | Version/Range | Vulnerable? |
|---|---|---|---|---|
| Application | palletsprojects | flask | < 3.1.3 | Yes |
SecUtils normalizes and enriches National Vulnerability Database (NVD) records by standardizing vendor and product identifiers, aggregating vulnerability metadata from both NVD and MITRE sources, and providing structured context for security teams. For palletsprojects's affected products, we extract Common Platform Enumeration (CPE) data, Common Weakness Enumeration (CWE) classifications, CVSS severity metrics, and reference data to enable rapid vulnerability prioritization and asset correlation. This record contains no exploit code, proof-of-concept instructions, or attack methodologies—only defensive intelligence necessary for patch management, risk assessment, and security operations.