How a Self-Signed Flask Cookie Led to a $90,000 RCE in Google
jedisct1 · x · 2026-10-14
Simon Koeck details a Google remote code execution bug that earned a $90,000 bounty:
- While mapping the shared OAuth flow on developers.google.com, he noticed an unusually large (1,200 byte) Flask session cookie;
- Flask cookies are signed, not encrypted — decoding revealed a pickle payload containing Google's OAuth clientid and clientsecret in plaintext, with the cookie signed by a secret it carried itself;
- With the secret extracted, a forged session became code execution across Google's developer sites.
Full decoding/repro code included — a textbook case of the cookie self-signing antipattern.
Related event: Researcher Earns $90K Google Bounty for Flask Cookie Key Leak RCE(3 posts)→
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