CVE-2026-27205
Flask session does not add `Vary: Cookie` header when accessed in some ways
Description
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.
INFO
Published Date :
Feb. 21, 2026, 6:17 a.m.
Last Modified :
Feb. 24, 2026, 9:59 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | MEDIUM | [email protected] | ||||
| CVSS 4.0 | LOW | [email protected] |
Solution
- Update Flask to version 3.1.3 or later.
- Ensure caches correctly handle Vary: Cookie headers.
- Configure caching proxies to ignore responses with cookies.
- Mark non-cacheable pages as private.
Public PoC/Exploit Available at Github
CVE-2026-27205 has a 6 public
PoC/Exploit available at Github.
Go to the Public Exploits tab to see the list.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-27205.
| URL | Resource |
|---|---|
| https://github.com/pallets/flask/commit/089cb86dd22bff589a4eafb7ab8e42dc357623b4 | Patch |
| https://github.com/pallets/flask/releases/tag/3.1.3 | Product Release Notes |
| https://github.com/pallets/flask/security/advisories/GHSA-68rp-wp8r-4726 | Vendor Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-27205 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-27205
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
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Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-27205 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-27205 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
Initial Analysis by [email protected]
Feb. 24, 2026
Action Type Old Value New Value Added CVSS V3.1 AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N Added CPE Configuration OR *cpe:2.3:a:palletsprojects:flask:*:*:*:*:*:*:*:* versions up to (excluding) 3.1.3 Added Reference Type GitHub, Inc.: https://github.com/pallets/flask/commit/089cb86dd22bff589a4eafb7ab8e42dc357623b4 Types: Patch Added Reference Type GitHub, Inc.: https://github.com/pallets/flask/releases/tag/3.1.3 Types: Product, Release Notes Added Reference Type GitHub, Inc.: https://github.com/pallets/flask/security/advisories/GHSA-68rp-wp8r-4726 Types: Vendor Advisory -
New CVE Received by [email protected]
Feb. 21, 2026
Action Type Old Value New Value Added Description 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. Added CVSS V4.0 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 Added CWE CWE-524 Added Reference https://github.com/pallets/flask/commit/089cb86dd22bff589a4eafb7ab8e42dc357623b4 Added Reference https://github.com/pallets/flask/releases/tag/3.1.3 Added Reference https://github.com/pallets/flask/security/advisories/GHSA-68rp-wp8r-4726