CVE-2026-27205

2.3 LOW 0.4% exploit odds Fix listed Other Web app

What it 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.

At a glance

Exploitability: Can be exploited over the network without logging in, but needs a user to act. Limited direct impact.
Exploit odds: 0.4% chance of exploitation in the next 30 days (EPSS).
Fix: A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

2026-02-21 06:17 UTC PublishedDisclosed publicly on CVE.org and NVD
2026-06-17 10:26 UTC Last updatedMost recent change to the record

Why it ranks here

Exploited in the wild: no, CISA does not list it.
Exploit odds: 0.4% in the next 30 days (EPSS).
Severity: 2.3 LOW (CVSS).
Fix: a patch or advisory is linked.

Details

Published: 2026-02-21 06:17 UTC
Last modified: 2026-06-17 10:26 UTC
Exploit probability: 0.4% · EPSS, next 30 days
Weakness: CWE-524
Record status: Analyzed

Affected

Palletsprojects Flask < 3.1.3

Attack profile CVSS 4.0 · 2.3

Attack vector: Network
Complexity: Low
Attack requirements: Present
Privileges needed: None
User interaction: Passive
Confidentiality: Low
Integrity: None
Availability: None

Raw 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2026-27205.md and follow it. Check whether this project is affected by CVE-2026-27205. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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Fix and advisories

References

NVD record · CVE.org record