# CVE-2026-27205: flask: Flask is a web server gateway interface (WSGI) web application framework

CVE-2026-27205 is fixed in 4 products on 1 platform, first by python3-flask 1.1.1-2ubuntu0.1+esm1, released February 20, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-27205
- Priority: Routine
- CVSS: 4.3 (medium)
- EPSS: 0.0040
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Feb 20, 2026

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | flask Debian 13.5 (trixie) | 3.1.1-1+deb13u1 | [DEBIAN-CVE-2026-27205](https://patchblox.com/security/data/fix/linux/debian-cve-2026-27205.md) | Feb 21, 2026 |
| Linux | flask Debian 14 | 3.1.3-1 | [DEBIAN-CVE-2026-27205](https://patchblox.com/security/data/fix/linux/debian-cve-2026-27205.md) | Feb 21, 2026 |
| Linux | python-Flask opensuse-leap 15.6 | 2.3.2-150400.3.9.1 | [SUSE-SU-2026:0849-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-0849-1.md) | Mar 9, 2026 |
| Linux | python-Flask sles 15.7 | 2.3.2-150400.3.9.1 | [SUSE-SU-2026:0849-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-0849-1.md) | Mar 9, 2026 |
| Linux | python-Flask sles 16.0 | 3.1.1-160000.3.1 | [SUSE-SU-2026:22023-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-22023-1.md) | Jun 2, 2026 |
| Linux | python-flask-doc amzn 2023 | 1:1.1.4-5.amzn2023.0.1 | [ALAS2023-2026-1476](https://patchblox.com/security/data/fix/linux/alas2023-2026-1476.md) | Mar 16, 2026 |
| Linux | python3-flask Ubuntu 20.04 LTS (focal) | 1.1.1-2ubuntu0.1+esm1 | [python3-flask 1.1.1-2ubuntu0.1+esm1](https://patchblox.com/security/data/fix/linux/ubuntu-focal-python3-flask-1.1.1-2ubuntu0.1-esm1.md) | Feb 20, 2026 |
| Linux | python3-flask Ubuntu 22.04.1 LTS (jammy) | 2.0.1-2ubuntu1.2 | [python3-flask 2.0.1-2ubuntu1.2](https://patchblox.com/security/data/fix/linux/ubuntu-jammy-python3-flask-2.0.1-2ubuntu1.2.md) | Feb 20, 2026 |
| Linux | python3-flask Ubuntu 24.04.5 LTS (noble) | 3.0.2-1ubuntu1.1 | [python3-flask 3.0.2-1ubuntu1.1](https://patchblox.com/security/data/fix/linux/ubuntu-noble-python3-flask-3.0.2-1ubuntu1.1.md) | Feb 20, 2026 |
| Linux | python3-flask amzn 2023 | 1:1.1.4-5.amzn2023.0.1 | [ALAS2023-2026-1476](https://patchblox.com/security/data/fix/linux/alas2023-2026-1476.md) | Mar 16, 2026 |

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Data as of 2026-11-08T06:59:42Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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