Flask uses fallback key instead of current signing key

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

1.8 · 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 · score source

EPSS

• CVE-2025-47278: 0.19% estimated exploitation activity in the next 30 days; percentile 8.89 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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 SECRETKEYFALLBACKS 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.

Published by the source

13 May 2025, 20:25 UTC

Last modified by the source

10 Sep 2026, 03:50 UTC

Source last checked

26 Oct 2026, 13:32 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-4grg-w6v8-c28g

2. Revision 2 · amended · 25 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-4grg-w6v8-c28g

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# Patch Next advisory brief: CVE-2025-47278

Advisory: Flask uses fallback key instead of current signing key
IDs: CVE-2025-47278, GHSA-4grg-w6v8-c28g
Record: https://patchnext.com/advisories/cve-2025-47278 (revision 3, source checked 26 Oct 2026, 13:32 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 1.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: flask 3.1.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI flask: affected >= 3.1.0, <= 3.1.0; fixed in 3.1.1

## Suggested inspection (read-only)
- `python -m pip show flask`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on flask.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-4grg-w6v8-c28g (OSV record GHSA-4grg-w6v8-c28g lists affected ranges: PyPI flask >=3.1.0 <3.1.1 (fixed))
- Verified: https://github.com/advisories/GHSA-4grg-w6v8-c28g (GitHub-reviewed advisory GHSA-4grg-w6v8-c28g lists affected ranges: PyPI flask >=3.1.0 <=3.1.0)

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