The US told users of a secret file-sharing network that new technology made stealing information easier [EPA]

The secret computer network used by the US government to share classified information was vulnerable to leaks that have been made "easier" by new technology, according to an official user-guide to the system.

The document, posted on the Defense Personnel Security Research Center's website, is intended for new users to the SIPRNET system, a classified version of the internet from which millions of documents are believed to have been taken and handed to the whistle-blower organisation WikiLeaks.

"Technological advances in storage devices are making it easier for classified information to be removed from secure areas," the document says, warning that any device connected to an SIPRNET-enabled machine automatically becomes considered secret government material. Unauthorised connections are considered a "serious security violation," the document says.  

Access to the SIPRNET network is controlled by a hierachical system of security clearances issued to individuals on a "need to know" basis. Authorities believe that the fact there are no documents marked top secret amongst the leaked files suggests the documents orginated from SIPRNET, which is used by both the US department of defence and department of state to transmit classified information.

Whoever leaked the files would have needed to circumvent a strict security system in order take copies of the information. SIPRNET is based on the same technology as the internet, but uses dedicated and encrypted lines and servers that are held in isolation from all other communications networks, and the user-guide describes some of the strict measures in place to protect the information it contains.

All users of the system must be cleared to at least secret level, and are individually approved and registered by US authorities before being given access. They are issued complex passwords made up of a string of letters, numbers and special characters that are changed every 150 days, the document says.

Once a password is issued, cleared individuals can only use specially enabled computers to access SIPRNET. The computers, and any associated hard drives, must be stored in approved secure location, where they must never be linked to the civilian internet, nor to any other storage device without prior approval.

"Once any media storage device such as a CD, floppy disk, or memory stick has been connected to a computer with access to the SIPRNET, it becomes classified at the secret level," the guide states.

Before using SIPRNET, users are screened for electronic devices as a safeguard against wireless data transfer.

"Data-storage devices such as personal digital assistants (PDA), key-chain drives, memory watches etc, should not be allowed in an environment where classified information is processed because of their infrared and similar recording capabilities," the document says.

The guide suggests disabling any infrared port on SIPRNET-enabled machines, or as a last resort, employees are advised to "cover the IR port with magnetic tape."

Every session a user spends on the SIPRNET system is carefully logged, with login/off times user identities and all activities while on the network, recorded in an "audit trail of all users". But the guide admits that even with these precautions in place, the system is not foolproof.  "Technological advances in storage devices are making it easier for classified information to be removed from secure areas," it says.

It seems that with the latest huge leak of diplomatic cables, the system administrators' worst nightmares have been confirmed.  The US says it has known for some time that WikiLeaks has held the diplomatic cables.

No one has been charged with passing them to the website, but suspicion focuses on US Army Pfc. Bradley Manning, an intelligence analyst arrested in Iraq in June and charged over the earlier leak of a video showing civilians being killed during a US helicopter attack in Iraq.