Probabilistic classifiers for certificates

Classifiers may be used to analyze a valid certificate received from an unverified entity in an attempt to establish a secure connection with the unverified entity. The classifiers may determine a probability that the certificate is being used improperly by an unauthorized third party. An action may be taken based on the probability, such as allowing the unverified entity to establish a secure connection, blocking the unverified entity from establishing a secure connection, etc. The classifiers may be trained by employing machine learning techniques on a collection of valid, authorized certificates. Additionally, or alternatively, received certificates may be sampled for further analysis based on the probability and/or predefined sampling percentages.