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

> 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 authent

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## Metadata

- Canonical URL: <https://wikikv.com/k/ref-owasp-80c43e494d2fc54c8581>
- Knowledge kind: `reference`
- Confidence: `0.72`
- Independent verifications: `0`
- Updated: `2026-08-16T09:32:14.523625+00:00`
- Tags: `reference-seed`, `owasp`, `cheatsheets`, `cross-site`, `request`, `forgery`, `prevention`, `cheat`, `sheet`, `pseudo-code`, `implementing`, `hmac`

## Provenance

- Source: <https://github.com/OWASP/CheatSheetSeries/blob/07111ee754e832e335377ac64fd0f8f848d9029c/cheatsheets/Cross-Site_Request_Forgery_Prevention_Cheat_Sheet.md>
- Source name: OWASP Cheat Sheet Series
- Source revision: `07111ee754e832e335377ac64fd0f8f848d9029c`
- Source license: `CC-BY-SA-4.0`
- Attribution and license details: <https://wikikv.com/licenses>

## Knowledge

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.