To identify whether a content item is prohibited, a content management system can generate a content item fingerprint for the content item and then compare the generated content item fingerprint to a blacklist of content item fingerprints for prohibited content items. If the generated content item fingerprint matches any of the content item fingerprints included in the blacklist, the content management system can determine that the content item is prohibited. The content management system can deny requests to share prohibited content items and/or requests to assign prohibited content items to a user account on the content management system. The content management system can generate the content item fingerprint using the content item as input in a fingerprinting algorithm that was used to generate the content item fingerprints on the blacklist.