CVE-2023-30861: Flask is a lightweight WSGI web application framework

This month

HIGH 7.5REMOTEUPGRADE

CVE-2023-30861 is a high-severity flaw (CVSS 7.5) affecting flask. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	1.2% (top 31%)
CISA exploited list	No
Weakness	CWE-539
Published	2023-05-02
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: palletsprojects

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2023-05-02 18:15 UTCPublished CVE record published
2026-06-17 05:55 UTCUpdated Record last modified

palletsprojects flask UPGRADE

Affected: < 2.2.5 · >= 2.3.0 and < 2.3.2

Fixed in: 2.2.5, 2.3.2

Safe to downgrade to: a release before 2.3.0

1. Check installed version

apt-cache policy flask || rpm -q flask || flask --version

2. UPGRADE to 2.2.5 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade flask

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade flask

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 2.3.0 (Debian / Ubuntu)

apt-cache madison flask sudo apt install flask=<VERSION_BEFORE_2.3.0> sudo apt-mark hold flask

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list flask sudo dnf downgrade flask-<VERSION_BEFORE_2.3.0> sudo dnf versionlock add flask

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2023-30861?

flask: upgrade to 2.2.5, 2.3.2. Exact commands for each OS are on this page.

Is CVE-2023-30861 being exploited?

CVE-2023-30861 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2023-30861 affect?

CVE-2023-30861 affects flask. Severity is high (CVSS 7.5). 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 cac

How urgent is CVE-2023-30861?

Priority 49/100. High severity; Remotely exploitable, no login, no user action

Software pages

Flask vulnerabilities

Related CVEs

CVE-2026-27205 MEDIUM 4.3 UPGRADE Flask is a web server gateway interface (WSGI) web application framework
CVE-2019-1010083 HIGH 7.5 UPGRADE The Pallets Project Flask before 1.0 is affected by: unexpected memory usage
CVE-2018-1000656 HIGH 7.5 UPGRADE MITIGATE The Pallets Project flask version Before 0.12.3 contains a CWE-20: Improper Input Validation vulnerability in

References

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