Reject state-changing requests that another site induces through an existing login cookie.
<div class="sd"><div class="sd-head"><strong>CSRF</strong><span id="state">live check</span></div><div class="sd-stage"><div class="tile" id="origin">other site</div><div class="arrow">→</div><div class="tile mono" id="cookie">cookie: yes</div><div class="arrow">→</div><div class="gate" id="gate">TOKEN<br>CHECK</div></div><div class="sd-foot"><span id="caption">checking boundary</span><span>tap to step</span></div></div>.sd{box-sizing:border-box;width:min(94vw,700px);height:min(86vh,318px);padding:clamp(9px,2.3vmin,18px);border:1px solid var(--line);border-radius:14px;background:var(--surface);color:var(--fg);display:flex;flex-direction:column;gap:clamp(6px,1.8vmin,12px);font:600 clamp(12px,2.6vmin,16px)/1.25 var(--font-sans, sans-serif)}.sd *{box-sizing:border-box}.sd-head,.sd-foot{display:flex;align-items:center;justify-content:space-between;gap:8px;min-height:1.25em}.sd-head strong{color:var(--accent);letter-spacing:.06em}.sd-head span,.sd-foot{color:var(--muted)}.sd-foot{font-size:clamp(12px,2.4vmin,14px)}.sd-stage{position:relative;flex:1;min-height:0;display:flex;align-items:center;justify-content:center;gap:clamp(6px,2vmin,18px);overflow:hidden}.sd .mono{font-family:ui-monospace,monospace}.sd .tile{border:1px solid var(--line);border-radius:8px;background:var(--bg);padding:clamp(5px,1.5vmin,10px);text-align:center}.sd .good{color:var(--accent);border-color:var(--accent)}.sd .bad{color:var(--accent-3);border-color:var(--accent-3)}.sd .arrow{color:var(--muted);font-size:clamp(15px,4vmin,28px)}.sd .active{background:color-mix(in srgb,var(--accent) 16%,var(--surface));border-color:var(--accent)}.sd .blocked{background:color-mix(in srgb,var(--accent-3) 13%,var(--surface));border-color:var(--accent-3)}.sd-stage .tile{min-width:21%}.gate{border:2px solid var(--accent-3);border-radius:8px;padding:clamp(5px,2vmin,13px);text-align:center;color:var(--accent-3);transition:all .2s}.gate.pass{border-color:var(--accent);color:var(--accent);background:color-mix(in srgb,var(--accent) 14%,var(--surface))}let n=0;function advance(){n=1-n;document.getElementById('origin').textContent=n?'same site':'other site';document.getElementById('gate').classList.toggle('pass',!!n);document.getElementById('state').textContent=n?'token valid':'token missing';document.getElementById('caption').textContent=n?'request accepted':'cookie alone is insufficient'}advance();setInterval(advance,900);
document.querySelector('.sd').addEventListener('pointerdown',advance);CSRF abuses the browser’s habit of attaching a login cookie automatically. An attacker’s page can induce a state-changing request that a server may mistake for a deliberate user action.
In the demo, a cookie alone is insufficient: the request also needs an accepted origin and CSRF token. SameSite cookies and Origin checks add protection for state-changing requests.
Use these checks for payments, settings changes, deletion, and other state-changing requests with cookie sessions.