An online advertising company has fixed a vulnerability in its platform that could have allowed hackers to steal information from a large number of users.The cross-site scripting (XSS) flaw in the platform of PublicityClerks\u00a0was found by a U.K. security researcher who goes by the handle CEHSecurity on Twitter.Cross-site scripting flaws are one of the most common faults in websites. They allow an attacker to inject malicious code into a website, which then can be used to steal data and for other attacks."As soon as we were aware of the issue, we fixed it ASAP and ensured our advertisers and publishers were not affected," said James Hakim, PublicityClerks' founder and CEO.The vulnerability was specifically in a form used by advertisers to upload their ads to PublicityClerk's platform. A field in the form did not properly filter out malicious code.CEHSecurity said in an interview over instant messaging that he would have been able to create an advertisement that, when displayed on a victim's computer, would steal a user's cookies.Cookies are small data files created by online trackers that are stored within a person's Web browser, recording information such as a person's browsing history.Vulnerabilities in ad platforms can be particularly far reaching. Since online advertising companies can serve ads to many websites with a high numbers of visitors, malicious ads can appear on a large numbers of computers in a short time.The technique is known as malvertising, and online advertising companies have been working to shore up their systems to keep harmful ads out.PublicityClerks says it has 10,000 registered websites and advertisers and serves about 15 million ad impressions monthly.The mistake by PublicityClerks could be easily fixed, CEHSecurity said.\u00a0He also found he could game PublicityClerks' billing system. By modifying a script, he could cause PublicityClerks' system to think an ad impression had not been delivered and issue a refund.The process could happen repeatedly, causing much more to be issued in a refund than a person spent placing an ad, he said.