Fix cross-client flow response spoofing vulnerability#1165
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Description
This PR addresses a trust boundary flaw in the GRR Frontend server's message processing pipeline (
ReceiveMessages) that allows a compromised client to impersonate another client when returning forensic data.The Issue
Currently, when processing standard flow responses, the server relies on the untrusted
session_idfield provided within theGrrMessageto determine the data's ownership. Inflow_objects.py, it extracts the client ID via_ClientIDFromSessionID(legacy_msg.session_id)and writes the response to the database. At no point does it verify if this extracted ID matches the authenticated sender's identity (client_idpassed by Fleetspeak).This allows an attacker with a valid Fleetspeak certificate to spoof forensic data for any other machine in the fleet by simply manipulating the
session_idstring.The Fix
Added a strict validation check in
frontend_lib.py(before callingFlowResponseForLegacyResponse) to verify that the client identity extracted from the message'ssession_idmatches the authenticatedclient_idof the sender. If they do not match, the server logs an error and drops the spoofed response.