Flask Session Cookie Decoder

Anatomy of a Flask session cookie

Flask uses itsdangerous to serialize session data into a single cookie value. The format is three base64url-encoded parts joined by .: payload.timestamp.signature. The payload is JSON; if it would be larger than the encoded original it gets prefixed with a literal . and zlib-compressed before encoding. The timestamp is Unix seconds (itsdangerous 2.x, which is what current Flask ships; the 1.x line offset from a 2011-01-01 epoch instead). The signature is HMAC-SHA1 over payload.timestamp using a key derived from the app’s SECRET_KEY and the salt cookie-session.

For CTF challenges the typical workflow is:

1. Decode the cookie to read what role / user is currently set (often something like {"is_admin": false}).
2. Find the SECRET_KEY (leaked source, debug page, environment variable, weak guess).
3. Verify the signature here to confirm the secret is correct.
4. Forge a new cookie with flask-unsign on the command line: flask-unsign --sign --cookie "{'is_admin': True}" --secret '...'.

When the cookie includes auth-relevant fields (admin role, user id, CSRF token), look at related tools: the JWT Decoder for token-based auth, URL Encoder for cookies that wrap percent-encoded data, and the Checksum Calculator if the secret is leaked as a hash.