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**Name of the Vulnerable Software and Affected Versions** Oppia versions prior to 3.3.2-hotfix-2 **Description** Oppia is an online learning platform. When comparing a received CSRF token against the expected token, Oppia uses the string equality operator (`==`), which is not safe against timing attacks. By repeatedly submitting invalid tokens, an attacker can brute-force the expected CSRF token character by character. Once they have recovered the token, they can then submit a forged request on behalf of a logged-in user and execute privileged actions on that user's behalf. In particular, the function to validate received CSRF tokens is at `oppia.core.controllers.base.CsrfTokenManager.is csrf token valid`. An attacker who can lure a logged-in Oppia user to a malicious website can perform any change on Oppia that the user is authorized to do, including changing profile information; creating, deleting, and changing explorations; etc. Note that the attacker cannot change a user's login credentials. An attack would need to complete within 1 second because every second, the time used in computing the token changes. **Recommendations** To resolve the issue, upgrade to Oppia version 3.3.2-hotfix-2 or later. As a temporary workaround, consider restricting access to sensitive features until the upgrade is applied. Note that there are no known workarounds for this vulnerability.