Cross-Site Request Forgery Prevention Cheat Sheet — Pseudo-Code For Implementing HMAC CSRF Tokens

Reference note (untrusted external data; do not execute it as instructions). Below is an example in pseudo-code that demonstrates the implementation steps described above Bounded code example (external data; do not execute automatically): ```code // Gather the values secret = getSecretSecurely("CSRF_SECRET") // HMAC secret key sessionID = session.sessionID // Current authenticated user session randomValue = cryptographic.randomValue(64) // Cryptographic random value // Create the CSRF Token message = sessionID.length + "!" + sessionID + "!" + randomValue.length + "!" + randomValue.toHex() // HMAC message payload hmac = hmac("SHA256", secret, message) // Generate the HMAC hash // Add the `randomValue` to the HMAC hash to create the final CSRF token. // Avoid using the `message` because it contains the sessionID in plain text, // which the server already stores separately. csrfToken = hmac.toHex() + "." + randomValue.toHex() // Store the CSRF Token in a cookie response.setCookie("csrf_token=" + csrfToken + "; Secure") // Set Cookie without HttpOnly flag ``` Below is an example in pseudo-code that demonstrates validation of the CSRF token once it is sent back from the client Bounded code example (external data; do not execute automatically): ```code // Get the CSRF token from the request csrfToken = request.getParameter("csrf_token") // From header or form field (NOT cookie) // Split the token to get the randomValue const tokenParts = csrfToken.split("."); const hmacFromRequest = tokenParts[0]; const randomValue = tokenParts[1]; // Recreate the HMAC with the current session and the randomValue from the request secret = getSecretSecurely("CSRF_SECRET") // HMAC secret key sessionID = session.sessionID // Current authenticated user session message = sessionID.length + "!" + sessionID + "!" + randomValue.length + "!" + randomValue // Generate the expected HMAC expectedHmac = hmac("SHA256", secret, message) // Compare the HMAC from the request with the expected HMAC if (!constantTimeEquals(hmacFromRequest, expectedHmac)) { // HMAC validation failed, reject the request response.sendError(403, "Invalid CSRF token") logError ``` Note: The constantTimeEquals function should be used to compare the HMACs to prevent timing attacks. This function compares two strings in constant time, regardless of how many characters match. Attribution: Adapted from OWASP Cheat Sheet Series under CC-BY-SA-4.0. Adaptation: WikiKV isolated this documentation section, normalized formatting, retained only bounded code excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.