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Fencer Vulns
medium 4.3

CVE-2026-27205 json

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

published 2026-02-19 8 mo ago modified 2026-09-10 59 d ago also EUVD-2026-7731 , GHSA-68rp-wp8r-4726 , PYSEC-2026-2151

cvss
4.3
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 34%
public exploits
0
none known

Summary

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 operator were overlooked. The severity and risk depend on the application being hosted behind a caching proxy that doesn't ignore responses with cookies, not setting a Cache-Control header to mark pages as private or non-cacheable, and accessing the session in a way that only touches keys without reading values or mutating the session. The issue has been fixed in version 3.1.3.

Patch / fix

  • pypi flask 3.1.3
  • palletsprojects flask 3.1.3
  • flask Debian 13 3.1.1-1+deb13u1
  • flask Debian 14 3.1.3-1

debian advisories DEBIAN-CVE-2026-27205

  • flask Ubuntu 22.04 LTS 2.0.1-2ubuntu1.2
  • flask Ubuntu 24.04 LTS 3.0.2-1ubuntu1.1
  • flask Ubuntu Pro 20.04 LTS 1.1.1-2ubuntu0.1+esm1

ubuntu advisories UBUNTU-CVE-2026-27205

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • python-Flask SLES for SAP applications 16.0, SLES 16.0 3.1.1-160000.3.1
  • python-Flask SLE Module for Python 3 15 SP7, openSUSE Leap 15.6 2.3.2-150400.3.9.1

suse advisories SUSE-SU-2026:22023-1SUSE-SU-2026:0849-1

  • python-Flask openSUSE Tumbleweed 3.1.3-1.1

opensuse advisories openSUSE-SU-2026:10264-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-27205 updated cvss 4.3

flask: Flask: Information disclosure via improper caching of session data

  • Red Hat OpenShift AI (RHOAI) rhoai/odh-notebook-controller-rhel8, rhoai/odh-kf-notebook-controller-rhel8
    affected: no fix planned “Out of support scope”
  • Red Hat Quay 3 quay/quay-rhel8, quay/quay-rhel9
    affected: no fix available, no fix planned “Out of support scope”
  • Red Hat Satellite 6 satellite/iop-advisor-engine-rhel9, satellite/iop-host-inventory-rhel9, satellite/iop-insights-engine-rhel9, satellite/iop-vulnerability-engine-rhel9
    affected: no fix available “Fix deferred”

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
8 mo ago

Exploit prediction

0.40% chance of exploitation in the next 30 days, higher than 34% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 4.0 2.3 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
CNA 4.0 2.3 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
NVD 3.1 4.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N
OSV 3.1 4.3 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N

Affected packages

ecosystem package affected fixed in
pypi flask
<3.1.3
3.1.3

Affected products

  • pallets flask

Classification

Weakness
  • CWE-524 Use of Cache Containing Sensitive Information
Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

The maintainers of pallets/flask published their own advisory on GitHub. Reviewed sources take precedence over it.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.