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TI-2026-21640 Aliases CVE-2026-27205GHSA-68rp-wp8r-4726

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

Priority LOW 12CWE-524Published · Updated

Key facts

Attack vector
Network
Privileges required
None
User interaction
Passive
Attack complexity
Low
Attack requirements
Present
Impact type
—
Confidentiality · integrity · availability
Low · None · None
Fix status
Partial Red Hat: 3 without a fix

Attack profile from CVSS 4.0 · CNA. Fix status from CNA, GitHub and Red Hat.

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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.004 34th percentileScored 2026-11-07
Fix Partial flask (pallets): 3.1.3flask (PyPI): 3.1.3Red Hat: 3 without a fix

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
None
Automatable
No
Technical impact
Partial

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
palletsflaskbefore 3.1.33.1.3Patch availableCNA
—flask (PyPI)before 3.1.33.1.3Patch availableGHSA-68rp-wp8r-4726
  • flask Vendorpallets Affected before 3.1.3 Fixed in 3.1.3 Patch available CNA
  • flask (PyPI) Vendor— Affected before 3.1.3 Fixed in 3.1.3 Patch available GHSA-68rp-wp8r-4726

CPE configurations NVD · 1

CPEVersionsRoleVendor
cpe:2.3:a:palletsprojects:flask:*:*:*:*:*:*:*:*before 3.1.3Vulnerablepallets
  • cpe:2.3:a:palletsprojects:flask:*:*:*:*:*:*:*:* Versions before 3.1.3 Vulnerable ·pallets

Remediation

  • Fix · CNA flask (pallets) Fixed in 3.1.3
  • Fix · GHSA-68rp-wp8r-4726 flask (PyPI) Fixed in 3.1.3 Advisory →

Vendor assessments

Red Hat

Red Hat rating: Moderate. Red Hat's page for this CVE →

ProductStatusPlan or fix
Red Hat OpenShift AI (RHOAI)AffectedNo fix planned: Out of support scope
Red Hat Quay 3AffectedNo fix yet: Fix deferred
Red Hat Satellite 6AffectedNo fix yet: Fix deferred

Timeline

  1. 2026-10-02 Threatist record created from CVE-2026-27205
  2. 2026-10-03 Alias added: GHSA-68rp-wp8r-4726

References

Patch 1

Advisory 2

Other 2

  • TI-2018-16009 The Pallets Project flask version Before 0.12.3 contains a CWE-20: Improper Input… CVSS 3.0HIGH 7.5
  • TI-2019-15971 Uncontrolled resource consumption in Pallets Project Flask CVSS 3.0HIGH 7.5
  • TI-2023-14640 Flask vulnerable to possible disclosure of permanent session cookie due to missing Vary: Cookie header CVSS 3.1HIGH 7.5

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences