Anti-CSRF

Cross-site request forgery (CSRF) attacks trick a user’s browser into submitting an authenticated request to your Cloud Page without their knowledge. The standard defence is a server-generated, single-use token that is tied to the user’s session and verified on submission.

Pattern Overview

Render phase — generate a random token, store it alongside the subscriber key in a DE, embed it in the form as a hidden field.
Submit phase — read the submitted token, look it up in the DE, verify it matches the expected subscriber key, then delete it so it cannot be replayed.

Setup: Token Storage DE

Create a Data Extension named CSRFTokens with these fields:

Field	Type	Is Primary Key
`Token`	Text (50)	Yes
`SubscriberKey`	Text (254)	No
`CreatedAt`	Date	No

Step 1 — Render the Form

Platform.Load("core", "1.1.5");

var subKey = Platform.Variable.GetValue("@SubscriberKey");
if (!subKey) { subKey = "anon_" + Platform.Function.GUID(); }

// Generate a random token and store it
var token = Platform.Function.GUID();
Platform.Function.InsertData(
    "CSRFTokens",
    ["Token", "SubscriberKey", "CreatedAt"],
    [token, subKey, Platform.Function.Now()]
);

Embed the token in your form:

<form method="post" action="%%=CloudPagesURL(123)=%%">
  <input type="hidden" name="csrfToken" value="%%=v(@token)=%%">
  <!-- other form fields -->
  <button type="submit">Submit</button>
</form>

Note: The form value is set via AMPscript’s %%=v(@token)=%% after assigning the SSJS variable to an AMPscript variable with Variable.SetValue("@token", token).

Platform.Variable.SetValue("@token", token);

Step 2 — Validate on Submission

Platform.Load("core", "1.1.5");

var submittedToken = Platform.Request.GetFormField("csrfToken");
var subKey = Platform.Variable.GetValue("@SubscriberKey");

if (!submittedToken) {
    Platform.Response.SetResponseCode(403, "Forbidden");
    Write("Missing security token.");
    Platform.Function.RaiseError("Missing CSRF token", true);
}

// Look up the token
var storedKey = Platform.Function.Lookup(
    "CSRFTokens", "SubscriberKey", "Token", submittedToken
);

if (!storedKey || storedKey !== subKey) {
    Platform.Response.SetResponseCode(403, "Forbidden");
    Write("Invalid or expired security token.");
    Platform.Function.RaiseError("CSRF token mismatch", true);
}

// Token is valid — delete it immediately (single-use)
Platform.Function.DeleteData("CSRFTokens", ["Token"], [submittedToken]);

// Continue with form processing...

Cleanup: Expire Old Tokens

Tokens should not accumulate. Run this in an Automation on a schedule (e.g. hourly) to remove tokens older than 30 minutes:

var proxy = new Script.Util.WSProxy();
var cutoff = Platform.Function.DateAdd(Platform.Function.Now(), -30, "MI");
var filter = {
    Property: "CreatedAt",
    SimpleOperator: "lessThan",
    Value: cutoff
};
var stale = proxy.retrieve("DataExtensionObject[CSRFTokens]", ["Token"], filter);
for (var i = 0; i < stale.Results.length; i++) {
    Platform.Function.DeleteData("CSRFTokens", ["Token"], [stale.Results[i].Token]);
}