Fax machines have proliferated to almost every business and many homes. This method of communications is one of the most easily and successfully targeted. The means for intercepting fax signals are the same as for telephones, as discussed under Telephones. Countermeasures for protecting the security of fax transmissions are the same as the countermeasures for protecting telephone conversations.
Stand-alone and PC-based fax and data intercept equipment is available that will covertly intercept and capture fax transmissions from any make or model machine, regardless of its "handshake" (the signal that two machines will emit to get synchronized to send the message). It will also decipher nonstandard fax protocols and perform automatic intercept and storage. 1
Some fax machines have another vulnerability -- remote access to stored fax images. For example, the hidden mailbox feature is used to reserve fax documents for privileged users. The privileged user can retrieve the document(s) by inputting a personal identification number (PIN) just like your automatic teller machine (ATM) cards. The invasion of one's fax could occur when an unauthorized user has access to your PIN. Like other code numbers and passwords, the PIN can often be guessed or broken. 2
2. Paul F. Barry & Charles L. Wilkinson (Trident Data Systems), "Invasion of Privacy and 902 Technologies," Security Awareness Bulletin 2-96. Department of Defense Security Institute, August 1996.