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).

v3.1.3

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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

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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

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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

v3.1.0

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Released 2024-11-13

• Drop support for Python 3.8. :pr:5623
• Update minimum dependency versions to latest feature releases.
  Werkzeug >= 3.1, ItsDangerous >= 2.2, Blinker >= 1.9. :pr:5624,5633
• Provide a configuration option to control automatic option
  responses. :pr:5496
• Flask.open_resource/open_instance_resource and
  Blueprint.open_resource take an encoding parameter to use when
  opening in text mode. It defaults to utf-8. :issue:5504
• Request.max_content_length can be customized per-request instead of only
  through the MAX_CONTENT_LENGTH config. Added
  MAX_FORM_MEMORY_SIZE and MAX_FORM_PARTS config. Added documentation
  about resource limits to the security page. :issue:5625
• Add support for the Partitioned cookie attribute (CHIPS), with the
  SESSION_COOKIE_PARTITIONED config. :issue:5472
• -e path takes precedence over default .env and .flaskenv files.
  load_dotenv loads default files in addition to a path unless
  load_defaults=False is passed. :issue:5628
• Support key rotation with the SECRET_KEY_FALLBACKS config, a list of old
  secret keys that can still be used for unsigning. Extensions will need to
  add support. :issue:5621
• Fix how setting host_matching=True or subdomain_matching=False
  interacts with SERVER_NAME. Setting SERVER_NAME no longer restricts
  requests to only that domain. :issue:5553
• Request.trusted_hosts is checked during routing, and can be set through
  the TRUSTED_HOSTS config. :issue:5636

v3.0.3

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Released 2024-04-07

• The default hashlib.sha1 may not be available in FIPS builds. Don't
  access it at import time so the developer has time to change the default.
  :issue:5448
• Don't initialize the cli attribute in the sansio scaffold, but rather in
  the Flask concrete class. :pr:5270

v3.0.2

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Released 2024-02-03

• Correct type for jinja_loader property. :issue:5388
• Fix error with --extra-files and --exclude-patterns CLI options.
  :issue:5391

v3.0.1

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Released 2024-01-18

• Correct type for path argument to send_file. :issue:5336
• Fix a typo in an error message for the flask run --key option. :pr:5344
• Session data is untagged without relying on the built-in json.loads
object_hook. This allows other JSON providers that don't implement that.
  :issue:5381
• Address more type findings when using mypy strict mode. :pr:5383

v3.0.0

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Released 2023-09-30

• Remove previously deprecated code. :pr:5223
• Deprecate the __version__ attribute. Use feature detection, or
  importlib.metadata.version("flask"), instead. :issue:5230
• Restructure the code such that the Flask (app) and Blueprint
  classes have Sans-IO bases. :pr:5127
• Allow self as an argument to url_for. :pr:5264
• Require Werkzeug >= 3.0.0.