GHSA-4grg-w6v8-c28g
Flask uses fallback key instead of current signing key
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.
Blast Radius
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
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 Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | flask | ≥ 3.1.0&&< 3.1.1 | 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. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update flask to 3.1.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-4grg-w6v8-c28g 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 pinpoints whether GHSA-4grg-w6v8-c28g is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-4grg-w6v8-c28g. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-4grg-w6v8-c28g in your dependencies?
O3 detects GHSA-4grg-w6v8-c28g across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.