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CVE-2023-30861 is an information-disclosure flaw in Flask's handling of refreshed permanent sessions. When Flask refreshes a permanent session cookie without the application accessing or modifying the session, vulnerable versions resend the cookie but do not add the HTTP Vary: Cookie response header. A shared caching proxy can therefore treat responses for clients with different cookies as equivalent and cache a response intended for one client for delivery to another. The issue is fixed in Flask 2.2.5 and 2.3.2.
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Set-Cookie headers, it can expose one client's Flask session cookie to another client. The resulting impact depends on the application's session design and the proxy's cookie-caching behavior, but may include session hijacking and unauthorized access under the affected user's session.If you can’t patch tonight, do this now.
Set-Cookie headers. Mark relevant responses with restrictive Cache-Control directives, such as private or no-store, as appropriate. Where operationally acceptable, avoid permanent sessions or disable SESSION_REFRESH_EACH_REQUEST.Patch, then assume compromise.
1 valid exploit after Mallory filtered fakes, detection scripts, and README-only repos.
This repository is a Proof of Concept (PoC) for CVE-2023-30861, a vulnerability in Flask applications when used behind a reverse proxy cache (such as nginx) that ignores Set-Cookie headers. The PoC demonstrates how improper session handling between Flask and the proxy can lead to security issues, such as session confusion or bypass. The repository includes a simple Flask app (app/app.py) with login and session functionality, a Dockerfile and docker-compose.yml for easy setup, and an nginx configuration (nginx.conf) that intentionally ignores Set-Cookie headers and caches responses. The test.http file provides example HTTP requests to interact with the app via the nginx proxy on port 8080. The main attack vector is network-based, targeting the HTTP endpoints exposed by the reverse proxy. The PoC is intended for local, educational use only and should not be exposed to the public internet.
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2 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
A Flask vulnerability that can disclose a permanent session cookie when the Vary: Cookie response header is missing. Red Hat fixed it in the OpenShift Container Platform 4.13.3 security update.
A Flask vulnerability that may disclose a permanent session cookie because the Vary: Cookie response header is missing.
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