Flask uses fallback key instead of current signing key
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 software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | flask | >= 3.1.0, < 3.1.1 | 3.1.1 |
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
- https://github.com/pallets/flask/releases/tag/3.1.1
- https://pypi.org/project/flask
- https://github.com/advisories/GHSA-4grg-w6v8-c28g
- CVSS vector
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- Weakness (CWE)
CWE-683- Platform
- Python (PyPI)
- Package
flask- Disclosed
- 13.05.2025
- Updated
- 11.10.2026 15:17
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2026-1377 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-4grg-w6v8-c28g - CC BY 4.0
Vulnerability history: flask
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 5
- First → latest
- 08.2018 02.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 3
- Low 2
Disclosures per year
- 2018: 1
- 2019: 1
- 2020: 0
- 2021: 0
- 2022: 0
- 2023: 1
- 2024: 0
- 2025: 1
- 2026: 1
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