CVE-2026-27205: Flask is a web server gateway interface (WSGI) web application framework

Q: How do I fix CVE-2026-27205?

flask: upgrade to 3.1.3. Exact commands for each OS are on this page.

Q: Is CVE-2026-27205 being exploited?

CVE-2026-27205 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-27205 affect?

CVE-2026-27205 affects flask. Severity is medium (CVSS 4.3). 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 Informat

Q: How urgent is CVE-2026-27205?

Priority 24/100. No elevated risk signals yet.

Routine

MEDIUM 4.3UPGRADE

CVE-2026-27205 is a medium-severity flaw (CVSS 4.3) affecting flask. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

No elevated risk signals yet

A fixed release or distro patch exists.

Risk facts

CVSS score	4.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.4% (top 66%)
CISA exploited list	No
Weakness	CWE-524
Published	2026-02-21
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

2026-02-21 06:17 UTCPublished CVE record published
2026-06-17 10:26 UTCUpdated Record last modified

palletsprojects flask UPGRADE

Affected: < 3.1.3

Fixed in: 3.1.3

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 3.1.3 (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.

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-2026-27205?

flask: upgrade to 3.1.3. Exact commands for each OS are on this page.

Is CVE-2026-27205 being exploited?

CVE-2026-27205 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-2026-27205 affect?

CVE-2026-27205 affects flask. Severity is medium (CVSS 4.3). 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 Informat

How urgent is CVE-2026-27205?

Priority 24/100. No elevated risk signals yet.

Software pages

Python vulnerabilities · Flask vulnerabilities

Related CVEs

CVE-2023-30861 HIGH 7.5 UPGRADE Flask is a lightweight 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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