id: PYSEC-2026-1377
published: "2026-07-07T16:02:52.410162Z"
modified: "2026-07-07T16:02:52.410162Z"
aliases:
- CVE-2025-47278
- GHSA-4grg-w6v8-c28g
summary: Flask uses fallback key instead of current signing key
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.
affected:
- package:
    name: flask
    ecosystem: PyPI
    purl: pkg:pypi/flask
  ranges:
  - type: ECOSYSTEM
    events:
    - introduced: 3.1.0
    - fixed: 3.1.1
  versions:
  - 3.1.0
references:
- type: WEB
  url: https://github.com/pallets/flask/security/advisories/GHSA-4grg-w6v8-c28g
- type: ADVISORY
  url: https://nvd.nist.gov/vuln/detail/CVE-2025-47278
- type: FIX
  url: https://github.com/pallets/flask/commit/73d6504063bfa00666a92b07a28aaf906c532f09
- type: PACKAGE
  url: https://github.com/pallets/flask
- type: WEB
  url: https://github.com/pallets/flask/releases/tag/3.1.1
- type: PACKAGE
  url: https://pypi.org/project/flask
- type: ADVISORY
  url: https://github.com/advisories/GHSA-4grg-w6v8-c28g
severity:
- type: CVSS_V4
  score: 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