GHSA-68rp-wp8r-4726: flask (CVE-2026-27205)

Package	Ecosystem	Affected	Fixed in
flask	PyPI	<3.1.3	3.1.3

Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

Flask versions before 3.1.3 can fail to add the "Vary: Cookie" header when the session object is accessed in some ways, such as with the Python "in" operator. An attacker would need a caching proxy in front of the application that does not ignore responses carrying cookies, and the application must not set a Cache-Control header marking pages as private or uncacheable. Under those conditions, a cache may store a logged-in user's response and serve it to others. Upgrade Flask to version 3.1.3 or later.

Written 5 Nov 2026.

Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

Severity and exploitation

Score	Rating	CVSS	Vector	Source
2.3	Low	4.0	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	CNA (GitHub_M)

CISA KEV Not listed.
EPSS 0.0040, the 34th percentile, on 5 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

Timeline

31 Oct 2026, 14:36 UTC First seen by agentprime
31 Oct 2026, 14:36 UTC Confirmed by OSV

Sources

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.