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Accidental “certificate spill” is a common problem in file transfer security.   It occurs when an untrained or careless individual accidentally sends the private key associated with a public/private certificate pair to someone who only needs the public component.

Certificate spill is a dangerous problem because it exposes credentials that allow unauthorised individuals to act with the identity and permission of trusted individuals and systems.

Today, it is common for a file transfer server administrator to ask an end user for “his or her certificate” so the administrator can add the end user’s public certificate credential to the user’s file transfer profile.  This will allow future connections to a file transfer server to be negotiated with strong authentication in place.  However, end users frequently send both their private and public keys to a request like this.  This kind of certificate spill often occurs in cleartext email and is often accompanied with an infuriating note like “I don’t know exactly what you’re looking for, but here’s all the files in my certificate folder”.

A worse case occurs when an untrained administrator broadcasts both the public and private components of a server key or server certificate to every actual or potential end user “to help them connect”.  (This happens more frequently than you might believe.)

To prevent certificate spills, proper training and proper deployment of technology that make it easy for end users and administrators to perform certificate exchange are both critical.

BEST PRACTICE: If you use keys or certificates to authenticate, secure and/or vouch for information, you must ensure that all personnel who handle these credentials know the difference between public and private keys and know when to use each type of credential (leaning on features available in software and systems whenever possible).  Administrators of these systems should also have a short “certificate spill containment” procedure in place in case a private key is accidentally transmitted.  This procedure should include assessment, communication, remediation (e.g., generate/distribute a new certificate and cancel/revoke the old one) and verification.