Update dependency flask to v3.1.3 [SECURITY]

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
flask (changelog)	3.1.0 → 3.1.3

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Flask uses fallback key instead of current signing key

CVE-2025-47278 / GHSA-4grg-w6v8-c28g

More information

Details

In Flask 3.1.0, the way fallback key configuration was handled resulted in the last fallback key being used for signing, rather than the current signing key.

Signing is provided by the itsdangerous library. A list of keys can be passed, and it expects the last (top) key in the list to be the most recent key, and uses that for signing. Flask was incorrectly constructing that list in reverse, passing the signing key first.

Sites that have opted-in to use key rotation by setting SECRET_KEY_FALLBACKS are likely to unexpectedly be signing their sessions with stale keys, and their transition to fresher keys will be impeded. Sessions are still signed, so this would not cause any sort of data integrity loss.

Severity

• CVSS Score: 1.8 / 10 (Low)
• Vector String: CVSS:4.0/AV:L/AC:L/AT:P/PR:H/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N

References

• https://github.com/pallets/flask/security/advisories/GHSA-4grg-w6v8-c28g
• https://nvd.nist.gov/vuln/detail/CVE-2025-47278
• https://github.com/pallets/flask/commit/73d6504063bfa00666a92b07a28aaf906c532f09
• https://github.com/pallets/flask/releases/tag/3.1.1
• https://github.com/advisories/GHSA-4grg-w6v8-c28g

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

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

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

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

This data is provided by the GitHub 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

v3.1.1

Compare Source

Released 2025-05-13

• Fix signing key selection order when key rotation is enabled via
  SECRET_KEY_FALLBACKS. :ghsa:4grg-w6v8-c28g
• Fix type hint for cli_runner.invoke. :issue:5645
• flask --help loads the app and plugins first to make sure all commands
  are shown. :issue:5673
• Mark sans-io base class as being able to handle views that return
  AsyncIterable. This is not accurate for Flask, but makes typing easier
  for Quart. :pr:5659

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