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

Advisory record · `GHSA-68rp-wp8r-4726`

- **Identifiers:** `GHSA-68rp-wp8r-4726` · `CVE-2026-27205` · `PYSEC-2026-2151`
- **State:** Confirmed
- **Severity:** CVSS 2.3 Low
- **Critical:** No
- **Weakness (CWE):** [CWE-524](https://cwe.mitre.org/data/definitions/524.html)
- **Tags:** [CWE-524](https://agentprime.io/advisories/tag/cwe-524.md)
- **Published:** 19 Feb 2026
- **Updated:** 10 Sept 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 19 Feb 2026, 20:45 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `flask` (PyPI): affected `<3.1.3`; fixed in `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

- 2.3 Low, CVSS 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`, from 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](https://osv.dev/vulnerability/GHSA-68rp-wp8r-4726)

## Sources

- [GitHub Advisory Database: GHSA-68rp-wp8r-4726](https://github.com/advisories/GHSA-68rp-wp8r-4726)
- [OSV: GHSA-68rp-wp8r-4726](https://osv.dev/vulnerability/GHSA-68rp-wp8r-4726)
- <https://github.com/pallets/flask/security/advisories/GHSA-68rp-wp8r-4726>
- <https://nvd.nist.gov/vuln/detail/CVE-2026-27205>
- <https://github.com/pallets/flask/commit/089cb86dd22bff589a4eafb7ab8e42dc357623b4>
- <https://github.com/pallets/flask>
- <https://github.com/pallets/flask/releases/tag/3.1.3>

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.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-68rp-wp8r-4726>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-68rp-wp8r-4726/>. Everything for agents: <https://agentprime.io/llms.txt>.