Patent ID: 8626861

Claim:
A computer implemented method of detecting a compromised machine on a network, comprising: replicating an outgoing email traffic stream originated on the network; redirecting the replicated outgoing email traffic stream to the compromised machine detection system; receiving an email message from a machine on the network; classifying the email message as spam or non-spam; updating a probability ratio by applying a sequential probability ratio test, wherein the probability ratio is increased if the email message was classified as spam or decreased if the email message was classified as non-spam, wherein the probability ratio is updated according to the equation Λ n = ln ⁢ ⁢ θ 1 θ 0 , responsive to the email message being classified as spam; wherein Λ n represents the probability ratio, θ 0 represents the probability that an email message received from a machine on the network is spam given that the machine is not compromised, and θ 1 represents the probability that an email message received from a machine on the network is spam given that the machine is compromised; indicating the status of the machine as compromised, responsive to the probability ratio being greater than or equal to a first predetermined threshold (B); indicating the status of the machine as normal and resetting the probability ratio, responsive to the probability ratio being less than or equal to a second predetermined threshold (A); repeating, for a plurality of email messages, the operations of receiving an email message, classifying the email message, updating the probability ratio, and indicating the machine is normal or compromised until the probability ratio is greater than or equal to the first predetermined threshold, wherein the repeated operations are performed on each of the plurality of email messages one at a time as each of the plurality of email messages is received; and identifying the machine indicated as compromised to a system administrator whereby the compromised machine may be cleaned.