CVE-2025-47278 — flask
Fix: pallets/flask@73d6504CVE-2025-47278 is a CWE-683 vulnerability in flask. A fix is available for flask — see the affected versions and patch details below.
Flask uses fallback key instead of current signing key
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2025-47278.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
flaskReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Flask is a web server gateway interface (WSGI) web application framework. 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 care 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. Version 3.1.1 contains a patch for the issue.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | flask | ≥ 3.1.0&&< 3.1.1 | 3.1.1pip install --upgrade 'flask==3.1.1' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for flask, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update flask to 3.1.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-47278 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2025-47278 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2025-47278. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
Frequently Asked Questions
Is CVE-2025-47278 in your dependencies?
O3 Security finds CVE-2025-47278 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.