What Is CSRF? A Practical Guide to Cross-Site Request Forgery

CSRF gets confused with XSS constantly, but the two work in almost opposite ways. Cross-Site Scripting is about getting your malicious code to run onthe victim's site. Cross-Site Request Forgery doesn't inject anything anywhere - it tricks the victim's own browser into sending a real, valid request to a site they're already logged into, on the attacker's behalf.

How a CSRF Attack Actually Works

Walk through the classic case. You're logged into your bank's website - your browser is holding a valid session cookie for it. You then visit a completely unrelated, malicious page in another tab. That page contains something as simple as:

• An auto-submitting HTML form pointed at https://bank.com/transfer?to=attacker&amount=5000
• Or, if the endpoint accepts GET requests for a state change (it shouldn't, but plenty do), something as innocuous-looking as <img src="https://bank.com/transfer?...">

Your browser doesn't know or care that the request originated from a shady page - it just sees a request to bank.com and attaches whatever cookies it's holding for bank.com, exactly the same as if you'd clicked a button on the bank's own site. The bank's server sees a request with a fully valid session cookie and has no way to tell, from the cookie alone, that you never actually intended to send it.

Why Cookies Alone Aren't Enough

The Same-Origin Policy is often assumed to prevent this, and it does prevent something adjacent - it stops the malicious page from readingthe response from bank.com. But CSRF doesn't need to read the response. It only needs the request to be sent and acted on; the attacker in the transfer example doesn't need to see a confirmation page, they just need the money to move. A cookie proves the request came from a browser holding a valid session - it says nothing about whether the user actually meant to send that specific request.

The Defenses That Actually Work

• CSRF tokens (synchronizer token pattern).The server embeds a random, unpredictable token in every form it renders and checks that the same token comes back on submission. A malicious page on another origin has no way to read that token off the real page (Same-Origin Policy blocks that read), so it can't include a valid one in its forged request.
• SameSite cookie attribute. Setting SameSite=Lax (the modern browser default when unset) or SameSite=Strict tells the browser not to attach the cookie on cross-site requests. Lax still allows the cookie on top-level GET navigations (clicking a link), which is why the pattern above of using a mutating GET request is doubly dangerous - it's the one case Lax doesn't stop. Strict closes that gap but breaks the cookie on any cross-site link-in, which has real UX tradeoffs.
• Double-submit cookie pattern.A token is set as both a cookie and required as a request parameter or header; the server checks they match. Useful for stateless APIs where storing a per-session token server-side isn't practical.
• Origin/Referer header checks as defense-in-depth - verifying a state-changing request actually originated from your own domain. Not sufficient alone (these headers can be stripped by proxies or omitted in some configurations) but a solid extra layer alongside a real token.

What Doesn't Actually Protect You

A CAPTCHA on the form doesn't stop CSRF - the forged request is generated by the victim's own browser, not a bot, so nothing about the request looks automated. And none of the defenses above matter if a state-changing action is reachable via a plain GET request in the first place - using GET only for reads and POST/PUT/DELETE for anything that changes state is a prerequisite for CSRF protection to mean anything, not an optional best practice on top of it.

It's worth being explicit that CSRF and Content Security Policy solve different problems entirely, despite the similar-sounding names - CSP restricts what a page is allowed to load and execute to reduce XSS impact, and doesn't address CSRF at all. See our CSP implementation guide if that's the header you're actually looking for.

A security scan checks for the cookie flags and header configuration that indicate whether CSRF protections are in place, alongside the rest of your site's exposed attack surface.