CVE-2023-30861: Flask session cookie caching
CVE-2023-30861 (Flask session cookie caching) is a sensitive information disclosure vulnerability in flask (PyPI). As of 2026-06-18 it carries a 1.2% probability of exploitation in the next 30 days (EPSS). It is fixed in 2.3.2 / 2.2.5 — if you depend on an earlier version, upgrade.
| Severity | Medium |
|---|---|
| CVSS vector | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N |
| EPSS (exploit probability, 30d) | 1.2% · 66th percentile |
| Vulnerability type | Sensitive Information Disclosure |
| Affected package | flask (PyPI) |
| Fixed in | 2.3.2 / 2.2.5 |
What is CVE-2023-30861?
Flask before 2.3.2/2.2.5, under a specific proxy/caching setup, can cache a response containing a session cookie and serve it to other clients, leaking session data between users.
Is CVE-2023-30861 exploitable?
CVE-2023-30861 has an EPSS score of 1.2% (66th percentile), meaning a 1.2% probability of exploitation in the next 30 days. Its CVSS vector is CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N, which scores as Medium severity. It is a sensitive information disclosure issue.
What is the EPSS score for CVE-2023-30861?
As of 2026-06-18, the EPSS exploit-prediction score for CVE-2023-30861 is 1.2% (66th percentile). EPSS estimates the probability that a vulnerability will be exploited in the wild within the next 30 days and is recomputed daily by FIRST.org.
How do I fix CVE-2023-30861?
Upgrade flask to 2.3.2 / 2.2.5 or later. Flask before 2.3.2/2.2.5, under a specific proxy/caching setup, can cache a response containing a session cookie and serve it to other clients, leaking session data between users.
Am I affected by CVE-2023-30861?
You are affected if your project (directly or transitively) depends on a vulnerable version of flask in the PyPI ecosystem. Check your lockfile for the resolved version, or scan automatically with VulnFeed.
Check your own project automatically
Don't eyeball every dependency by hand. VulnFeed reads your lockfile, checks it against the same advisory data, and ranks findings by EPSS — free, no signup (10 scans/day):
curl -s https://vulnfeed-api.novadyne.ai/vulnscan/query \
-H 'content-type: application/json' \
-d '{"ecosystem":"npm","package":"PKG","version":"VERSION"}'
Or run it inside Claude Code with no API key — uvx vulnfeed-mcp — and just ask "scan this project for vulnerabilities."
Data sources: vulnerability metadata from OSV.dev and the National Vulnerability Database; exploit-probability (EPSS) from FIRST.org (recomputed daily; EPSS data shown here as of 2026-06-18). Page generated 2026-06-18.