At an advertising server: adding tracking code to advertisements served by the advertising server, wherein the tracking code is configured to cause web browsers displaying the served advertisements to transmit their contents to a security server. At the security server: scanning the received advertisements to detect presence of malicious code, and storing results of the scanning in a database. At the advertising server: prior to serving a new advertisement that has won in RTB, querying the database for scan results associated with the new advertisement. When the scan results indicate a malicious advertisement, preventing a serving of the new advertisement. When the scan results indicate a safe advertisement, allowing a serving the new advertisement. When no scan results are available for the new advertisement, adding the tracking code to the new advertisement and serving it, such that its contents are scanned by the security server.