An authentication vulnerability was discovered in Jira Service Management Server and Data Center which allows an attacker to impersonate another user and gain access to a Jira Service Management instance under certain circumstances_._ With write access to a User Directory and outgoing email enabled on a Jira Service Management instance, an attacker could gain access to signup tokens sent to users with accounts that have never been logged into. Access to these tokens can be obtained in two cases: * If the attacker is included on Jira issues or requests with these users, or * If the attacker is forwarded or otherwise gains access to emails containing a “View Request” link from these users. Bot accounts are particularly susceptible to this scenario. On instances with single sign-on, external customer accounts can be affected in projects where anyone can create their own account.