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Not in CISA KEV
HIGH severity

CVE-2023-30861 — flask

HIGHFix: pallets/flask@70f906c

CVE-2023-30861 is a high-severity (CVSS 7.5) CWE-539 vulnerability in flask. A fix is available for flask — see the affected versions and patch details below.

Flask vulnerable to possible disclosure of permanent session cookie due to missing Vary: Cookie header

Also known asGHSA-m2qf-hxjv-5gpqPYSEC-2023-62
Published
Updated
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed
Exploitation data as of Nov 4, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2023-30861.

EPSS Exploitation Probability

via FIRST.org ↗
1.3%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs69th percentile — riskier than 69% of all scored CVEsHighest risk
0.00%0.59%1.17%1.76%0.2%1.3%May 26Sep 26Nov 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

2 pkgs affected
🐍flask🐍flask

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Flask is a lightweight WSGI web application framework. When all of the following conditions are met, a response containing data intended for one client may be cached and subsequently sent by the proxy to other clients. If the proxy also caches Set-Cookie headers, it may send one client's session cookie to other clients. The severity depends on the application's use of the session and the proxy's behavior regarding cookies. The risk depends on all these conditions being met.

  1. The application must be hosted behind a caching proxy that does not strip cookies or ignore responses with cookies.
  2. The application sets session.permanent = True
  3. The application does not access or modify the session at any point during a request.
  4. SESSION_REFRESH_EACH_REQUEST enabled (the default).
  5. The application does not set a Cache-Control header to indicate that a page is private or should not be cached.

This happens because vulnerable versions of Flask only set the Vary: Cookie header when the session is accessed or modified, not when it is refreshed (re-sent to update the expiration) without being accessed or modified. This issue has been fixed in versions 2.3.2 and 2.2.5.

Affected Packages

2 total 2 fixed
EcosystemPackageVulnerable rangeFix
🐍PyPIflask≥ 2.3.0&&< 2.3.22.3.2pip install --upgrade 'flask==2.3.2'
🐍PyPIflaskall versions2.2.5pip install --upgrade 'flask==2.2.5'

Affected Products

1 product · 2 configurations
Application
flaskpalletsprojects
≥ 2.3.0 && < 2.3.2
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for flask, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update flask to 2.3.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2023-30861 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant
ProductFixed inAdvisory
Ironic content for Red Hat OpenShift Container Platform 4.11python-flask-1:1.1.2-6.el8RHSA-2023:3541
Ironic content for Red Hat OpenShift Container Platform 4.12python-flask-1:2.0.1-3.el9RHSA-2023:3545
Ironic content for Red Hat OpenShift Container Platform 4.13python-flask-2:2.0.1-4.el9.2RHSA-2023:3536
Red Hat Enterprise Linux 7 Extraspython-flask-1:0.10.1-7.el7_9RHSA-2023:3525
Red Hat OpenStack Platform 16.1python-flask-1:1.0.2-8.el8ostRHSA-2023:3446
Red Hat OpenStack Platform 16.2python-flask-1:1.0.2-8.el8ostRHSA-2023:3444
Red Hat OpenStack Platform 17.0python-flask-1:1.1.2-6.el9ostRHSA-2023:3440
Red Hat Quay 3quay/quay-rhel8:v3.10.0-150RHSA-2023:7341

Frequently Asked Questions

Flask is a lightweight WSGI web application framework. When all of the following conditions are met, a response containing data intended for one client may be cached and subsequently sent by the proxy to other clients. If the proxy also caches `Set-Cookie` headers, it may send one client's `session` cookie to other clients. The severity depends on the application's use of the session and the proxy's behavior regarding cookies. The risk depends on all these conditions being met. 1. The application must be hosted behind a caching proxy that does not strip cookies or ignore responses with cookie
O3 Security · Impact-Aware SCA

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CVE-2023-30861: flask — Fixed in 2.3.2