Techniques are described for use in inhibiting attempts to fraudulently obtain access to confidential information about users. In some situations, the techniques involve automatically analyzing at least some requests for information that are received by a Web site or other electronic information service, such as to determine whether they likely reflect fraudulent activities by the request senders or other parties that initiate the requests. For example, if a request is being made to a Web site based on a user's interaction with a third-party information source (e.g., another unaffiliated Web site) that is not authorized to initiate the request, the third-party information source may be a fraudulent phishing site or engaging in other types of fraudulent activity. If fraudulent activity is suspected based on analysis of one or more information requests, one or more actions may be taken to inhibit the fraudulent activity.