Update dependency Flask to v3.1.3 [SECURITY]

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
Flask (changelog)	==3.1.1 → ==3.1.3

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

CVE-2026-27205 / GHSA-68rp-wp8r-4726 / PYSEC-2026-2151

More information

Details

When the session object is accessed, Flask should set the Vary: Cookie header. This 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 depends on the application's use of the session, and the cache'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 ignore responses with cookies.
2. The application does not set a Cache-Control header to indicate that a page is private or should not be cached.
3. The application accesses the session in a way that does not access the values, only the keys, and does not mutate the session.

Severity

• CVSS Score: 2.3 / 10 (Low)
• Vector String: 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

References

• https://github.com/pallets/flask/security/advisories/GHSA-68rp-wp8r-4726
• https://nvd.nist.gov/vuln/detail/CVE-2026-27205
• https://github.com/pallets/flask/commit/089cb86dd22bff589a4eafb7ab8e42dc357623b4
• https://github.com/pallets/flask
• https://github.com/pallets/flask/releases/tag/3.1.3

This data is provided by OSV and the GitHub Advisory Database (CC-BY 4.0).

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CVE-2026-27205 / GHSA-68rp-wp8r-4726 / PYSEC-2026-2151

More information

Details

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.

Severity

• CVSS Score: 4.3 / 10 (Medium)
• Vector String: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:N

References

• https://github.com/pallets/flask/releases/tag/3.1.3
• https://github.com/pallets/flask/security/advisories/GHSA-68rp-wp8r-4726
• https://github.com/pallets/flask/commit/089cb86dd22bff589a4eafb7ab8e42dc357623b4

This data is provided by OSV and the PyPI Advisory Database (CC-BY 4.0).

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Release Notes

pallets/flask (Flask)

v3.1.3

Compare Source

Released 2026-02-18

• The session is marked as accessed for operations that only access the keys
  but not the values, such as in and len. :ghsa:68rp-wp8r-4726

v3.1.2

Compare Source

Released 2025-08-19

• stream_with_context does not fail inside async views. :issue:5774
• When using follow_redirects in the test client, the final state
  of session is correct. :issue:5786
• Relax type hint for passing bytes IO to send_file. :issue:5776

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